SOLARNET Conference: The Many Scales of the Magnetic Sun

The EST is a project aimed at studying the dynamic magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. A polarimetrically compensated telescope with integrated multi-conjugated adaptive optics will feed a suite of spectropolarimetric instruments that allows to simultaneously observe the different layers of the solar atmosphere. In this talk, I will present the concept for the light distribution and the instrument suite which intends to significantly improve our capabilities to address critical science cases in our field such as: How does the magnetic field emerge into the photosphere? How does it evolve, interact, and rise in a convectively stable stratification? What is the interplay between the magnetic field, reconnection processes, radiation, waves, and the transport of energy into higher layers?

The phenomena of the active Sun, like flares and coronal mass ejections (CMEs), have significant influence on Earth and our technical civilization. This is usually referred to as "Space Weather". Flares and CMEs accelerate electrons and ions to high energies. These particles are studied both remotely by ground- and space-based telescopes, and in situ by spacecraft. Energetic electrons emit radio waves as they move through the coronal plasma. This plasma emission is observed by radio telescopes, e.g. LOFAR. Since the frequency decreases with plasma density higher in the solar atmosphere, and radio waves below 10 MHz cannot pass Earth's ionosphere, spacecraft are needed to continue observations further into interplanetary space. They are also required for measuring energetic particles and observations of X-ray emission in the corona. Therefore, combining LOFAR and spacecraft data provides new insights into the physical processes in the region where the solar corona turns into the solar wind. Parker Solar Probe (PSP) and Solar Orbiter are two missions currently exploring the inner heliosphere. I'll present LOFAR observing campaigns during PSP and Solar Orbiter perihelia, that cover the Sun and it's surroundings by making use of LOFAR's capability of running multiple observing modes in parallel, and show how they connect the corona with the heliosphere.

The ESA and NASA Solar Orbiter mission started its nominal mission phase in November 2021. Three perihelions and nine remote sensing windows have been successfully completed, providing data from all the instruments on board the spacecraft. The Polarimetric and Helioseismic Imager (SO/PHI), one of Solar Orbiter's remote sensing instruments, is the first magnetograph to observe the Sun from outside the Sun-Earth line. Most of the data acquired by the High Resolution Telescope (SO/PHI-HRT) has been placed on the Solar Orbiter Archive (SOAR) and are available to the whole scientific community. The excellent polarimetric sensitivity of the instrument and the absence of aberrations induced by the Earth’s atmosphere provide an ideal scenario to study the photospheric dynamics and magnetism of the Sun. The talk will introduce the SO/PHI-HRT instrument and describe the data that has been released so far. The processing of the data will also be shortly discussed. We will then present some of the scientific cases that have been addressed, either with standalone observations performed by the instrument, or with data combined with observations made by other Solar Orbiter instruments, or with data gathered by instruments on other spacecrafts or on ground.

Quasi-static modelling of the Sun’s corona using the magneto-frictional approximation makes predictions of how its large-scale magnetic structure might vary over the solar cycle. Unlike the traditional potential field source surface model, this approach is able to probe the effects of low coronal electric currents and the corresponding free magnetic energy. In particular, ongoing footpoint shearing and flux cancellation lead to the concentration of free energy within closed-field regions, naturally forming sheared arcades and magnetic flux ropes. I will show model results for Cycle 24 driven by magnetogram data from SDO/HMI, with the aim of comparing to earlier results from a decade ago for Cycle 23. In particular, we will consider how solar flux ropes and their eruptions vary over the solar cycle, at least according to this model.

Robbe Vansintjan - The SOLARNET project and the Solar Virtual Observatory (SVO)
Fabio Duvan Lora Clavijo - Effect of the Hall term on the energy release and the magnetic reconnection rate in solar flares
Merlin Mendoza - Association between Magnetic Pressure Difference and Movement of Solar Pores
Bartosz Dabrowski - Fine structures of a solar bursts observed with LOFAR
Markus Roth - SolarLab as Pathfinder towards SPRING
Frederic Schuller - Pointing stability of Solar Orbiter and its instruments
Ziwen Huang - Imaging and spectroscopic observations of EUV brightenings using EUI and SPICE on board of SOLAR ORBITER

The Fibre-Resolved opticAl and Near-ultraviolet Czerny-Turner Imaging Spectropolarimeter (FRANCIS) was developed as a low-budget prototype integral field unit for the upcoming Indian National Large Solar Telescope. At its core, FRANCIS is a fibre-fed Czerny-Turner spectrograph that is designed to work within the spectral range of 350-700 nm, hence enabling high cadence and spatially resolved studies of spectral lines in the near-ultraviolet. In the summer of 2022, FRANCIS was shipped to the Dunn Solar Telescope in the Sacramento Peak mountains, New Mexico, USA, where it was commissioned and underwent science verification. The first-light observations are very encouraging, with the final data products suitable for mainstream scientific exploitation. Here, I will overview the history of the instrument, provide a snapshot of its strengths and associated weaknesses, indicate a timescale for future upgrades, as well as provide samples of its spectral imaging capabilities. At present, FRANCIS is available as a common user instrument at the Dunn Solar Telescope, so I will also highlight how interested parties can become involved with future observing campaigns.

The SunDish project aims to map and monitor the Sun at high radio frequencies with the Italian Sardinia (64-m) and Medicina (32-m) Radio Telescopes in the K-band range (18-26 GHz), up to 100 GHz in perspective. Since the two instruments were not originally designed to observe the Sun, a new solar radio imaging system was implemented. To date we acquired more than 350 solar maps with resolutions in the 0.7-2 arcmin range, filling the observational gap in the field of chromospheric imaging at these frequencies to date.

Observations of the brightness temperature of the solar atmosphere in the radio band can map plasma processes that produce free-free emission in the local thermodynamic equilibrium and gyromagnetic phenomena, providing a probe of physical conditions in a wide range of atmospheric layers both for quiet and active regions.

As a first early science result of the project, we present the first catalog of radio continuum solar imaging observations with Medicina 32-m and SRT 64-m radio telescopes including the multi-wavelength identification of active regions, their brightness and spectral characterization. The interpretation of the observed emission as thermal bremsstrahlung components combined with gyro-magnetic variable emission pave the way to the use of our system for long-term monitoring of the Sun. Single-dish radio imaging represents an ideal technique to perform accurately calibrated solar observations. We obtained obtained unprecedented and very precise calibrated measurements of the Quiet Sun in the K-band affected by a mean relative error of 3%.

Through systematic monitoring of the Sun, our system can provide accurate measurement of the brightness temperature of the radio-quiet Sun component; characterization of the flux density, spectral properties and long-term evolution of dynamical features (active regions, coronal holes, loop systems, streamers, and the coronal plateau); prediction of powerful flares through the detection of peculiar spectral variations in the active regions, as a valuable forecasting probe for the Space Weather hazard network.

The mechanism for the storage and release of magnetic energy in coronal mass ejections (CMEs) remains a major unsolved problem of Heliophysics. Thus, observations of the coronal magnetic field before, during, and after the eruption are crucial both for scientific progress and space weather predictions. We use the newly built Upgraded Coronal Multichannel Polarimeter (UCoMP) instrument from the Mauna Loa Solar Observatory (MLSO) to study the magnetic and thermodynamic morphology of CME precursors and eruptions. With this coronagraph's 20-cm aperture and field of view extending from 1.05 to 2.0 solar radii, we determine the magnetic field orientation of pre- and post-CME coronal loops using Stokes Q and U parameters as derived from the linearly polarized radiation observed from the Fe XIII 1074 nm spectral line. To ascertain the density, temperature, and ionization distribution of the hot plasma, we compare and contrast the structures seen at 1074 nm with the coronal features observed by UCoMP at the Fe X 637 nm line, the Fe XV 706 nm line, the Fe XI 789 nm line, and the Fe XIII 1079 nm line. In this study, we use several CME precursors and eruptions as examples for showcasing the unique capabilities of UCoMP, which will inform the future of ground-based coronograph polarimeter observations that will eventually be performed with the COronal Solar Magnetism Observatory (COSMO).

A new era of solar physics commences with observations of the quiet Sun using the 4-metre Daniel K. Inouye Solar Telescope/Visible Spectropolarimeter (DKIST/ViSP). We present full-Stokes observations at a spatial resolution of 0.1’’, taken during DKIST’s cycle 1 Operations Commissioning Phase, in the Fe I 630.1/630.2 nm and Ca II 854.2 nm lines, allowing us to examine small-scale magnetism in the photosphere and chromosphere. We focus on the photospheric diagnostic and use the Stokes Inversion based on Response functions (SIR) code to invert the Fe I line pair. This allows us to characterize the thermodynamic, kinematic and, crucially, magnetic properties of the pervasive small-scale magnetic features present in the data in the photosphere – namely, constraining their magnetic field strengths, filling factors, and magnetic inclinations. We reveal small-scale magnetic structures, including magnetic bi-poles/loops, in unprecedented detail.

To infer the physical conditions in the solar atmosphere, it is necessary to record not only the spatial distribution of the emerging radiation, but also its spectral and polarimetric properties. Traditionally, solar instrumentation has accomplished this by recording slices through the spatio-spectral domain, and scanning in the missing dimension. A Microlensed Hyperspectral Imager (MiHI) is an instrument that is able to observe a spatio-spectral cube instantaneously, thus eliminating the need to scan. This increases the photon efficiency of the observations, and with the aid of state of the art cameras allows for the acquisition of very high cadence data. Combined with image restoration, high resolution spectra with a spatial resolution close to the diffraction limit of the telescope can be obtained at a cadence of less than one second, in all four Stokes parameters. The talk covers the basic principle of operation, data extraction and restoration, and some examples of data recorded at the Swedish Solar Telescope.

More than 20 years ago most photospheric MHD simulations focused primarily on small patches of granulation with moderate numerical resolution. Over the past 2 decades advances in computing infrastructure have enabled photospheric MHD simulations to cover most magnetic environments from quiet Sun to active Sun. In this talk I review these recent developments and focus primarily on simulations that cover the extreme ends of the spectrum: (1) Small-scale dynamo simulations of quiet Sun magnetism and (2) Simulations of active region scales. (1) Recent research (observations and modeling) supports the view that the origin of small-scale magnetism is mostly due to a small-scale dynamo that operates independently from the large-scale dynamo responsible for the solar cycle. Recent simulations have shown that the saturation field strength and structure of the resulting magnetic field in the photosphere depend critically on the contributions from deep and shallow recirculation within the strongly stratified convection zone. Therefore, the small-scale dynamo is not restricted to the photosphere and involves a wide range of scales. Recent research suggests that small-scale magnetic fields may play a critical role for convection zone dynamics. Outstanding questions concern the role of the magnetic Prandtl number, which will require in the future simulations with better resolution (< 1km) to be addressed properly. (2) Active region-scale simulations have provided a comprehensive picture of how sunspot fine structure from umbral dots to penumbral filaments arises from overturning magneto convection. Larger-scale simulations have addressed the connection to the moat region and studied sunspot decay. However, the presence of penumbra in simulations is found to be critically dependent on boundary conditions and the subsurface structure of sunspots. To date none of these simulations have provided penumbra with the “robustness” solar observations indicate, and no simulations of flux emergence have led to the formation of a penumbra as part of the spot formation process (although certain aspects are captured). Ultimately progress may require a more comprehensive simulations flux emergence and active region formation that are fully coupled to large-scale dynamo simulations.

The Daniel K. Inouye Solar Telescope is currently acquiring first science data as part of its Operations Commissioning Phase. High-resolution, fast-cadence imaging in the chromospheric Ca II K filter of the Visible Broadband Imager reveals signatures of bright arches emanating radially from the locations of G-band bright points. The G-band bright points denote small-scale magnetic elements harboring strong magnetic fields (~kGauss). Comparison with 3-D simulations suggest that the traveling bright arches are acoustic waves or shock fronts, triggered by the movements of magnetic elements. Inspecting several examples of the events we indeed see the photospheric bright points changing their appearance and position, indicating either a horizontal movement, perhaps swaying, or rotation and seem to be the source location for the bright arches.

The Fe I 6173 Å line is widely used to observe the solar photosphere by many instruments. This includes the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO) and the Polarimetric and Helioseismic Imager on board the Solar Orbiter. During analysis, this line is often assumed to have formed in Local Thermodynamic Equilibrium (LTE) conditions. However, the UV overionization effects acting on the iron species has a strong influence on the Fe I abundance, which affects the strength of this line, resulting in departures from LTE. By synthesizing the Stokes profiles from snapshot of a three-dimensional magnetohydrodynamic simulation of the solar photosphere, we have carried out a detailed investigation on the nature and strength of the non-LTE (NLTE) effects. We find that both intensity and polarization profiles can be strongly affected by the NLTE effects, and that these effects survive even when the profiles are averaged spatially or sampled on a coarse wavelength grid such as that used by the SDO/HMI and other magnetographs. Based on the nature of departures from LTE, treating the 6173 Å line in LTE will likely result in an overestimation of temperature and an underestimation of the magnetic field strength.

Extreme ultraviolet (EUV) observations of the quiet sun corona have revealed extended regions of reduced emission. These coronal voids are on average around 30% less intense than the quiet Sun and can have sizes corresponding to several supergranular cells. At least the larger of these voids can exist for many hours with their boundaries to the quiet Sun remaining more or less unchanged. The aim of our study is to understand what causes the reduced emission through an investigation of the underlying magnetic features in the photopshere.

At least two scenarios could explain why the voids appear dark:
(1) The coronal voids could be magnetic structures that are (locally) open similar to coronal holes and appear dark for the same reason as those do. (2) they could also be areas of lower (magnetic) heating due to lower magnetic field strength in the photosphere below, when compared to the typical quiet Sun.

To distinguish between the two scenarios, we combine quiet Sun data from the high-resolution EUV-channel at 17.4 nm of the Extreme Ultraviolet Imager (EUI), showing coronal plasma close to 1 MK, and magnetograms obtained with the high-resolution telescope of the Polarimetric and Helioseismic Imager (PHI), both onboard Solar Orbiter.

Our results show the average unsigned magnetic field inside the voids to be reduced by $\sim$ 25% with respect to the entire observed quiet Sun region. Specifically, there is little or no network structure visible inside the coronal voids. Observed flux imbalances are within the range of flux imbalances found in quiet Sun areas of similar size. Hence, the flux imbalances we find in the voids are not significant. Consequently, it is highly unlikely that these voids are true miniature versions of coronal holes.
Instead, we find that in general the unsigned magnetic flux in the photosphere below the voids is significantly smaller than in the quiet Sun, and the vast majority of stronger magnetic patches is located outside the voids. The weaker photospheric field below the voids will produce a Poynting flux smaller than in the surrounding quiet Sun. Consequently, the coronal part above will be heated less and the voids will appear darker in coronal emission.

We started a subsequent study into the temporal evolution of coronal voids on time scales from hours to several days. Our initial results for a time scale of a few hours indicate that at least larger structures persist, and their outer boundaries remain mostly unchanged during this time.

Solar events such as flares and coronal mass ejections are the most energetic phenomena affecting interplanetary space in timescales ranging from minutes to a few days. There has been, in recent years, a strong interest in characterizing the processes that inject magnetic energy and helicity in solar active regions (ARs) and the mechanisms that destabilize their magnetic structure leading to the energy released during those events. We analyze the magnetic evolution of AR NOAA 11476 that produced a series of minifilament confined eruptions of the “surge” type accompanied by M-class flares. In a previous work, we found that these events are associated to the presence of a small rotating bipole that emerged in the middle of the globally bipolar AR magnetic configuration. The bipole rotation preceded and concurred in time with the observed ejections. Magnetic flux cancellation is also identified along the polarity inversion line of the bipole where the minifilaments were recurrently formed and ejected. Here we combine the analysis, for the full AR and the rotating bipole, of the evolution of a series of magnetic parameters computed from SDO/HMI vector magnetograms with estimations of the magnetic energy and magnetic helicity injection obtained using the Differential Affine Velocity Estimator for Vector Magnetograms (DAVE4VM) method. This procedure is based on the determination, from vector magnetograms, of the affine velocity field constrained by the induction equation. Our results provide a series of relations between the studied parameters that define proxies of the magnetic evolution of the AR that, eventually, can be used as possible precursors of active events.

This study investigates the photospheric and chromospheric flow field in active region NOAA 13096, which consists of pores, sunspots with partial penumbrae, and light bridges. The improved High-resolution Fast Imager (HiFI+) and GREGOR Infrared Spectrograph (GRIS) acquired high-resolution data at the 1.5-meter GREGOR solar telescope at Observatorio del Teide, Izaña, Tenerife, Spain. The Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) complements the GREGOR data with synoptic full-disk continuum images and line-of-sight (LOS) magnetograms. Background-Subtracted Activity Maps (BaSAMs) were used to locate areas with enhanced activity, for which Local Correlation Tracking (LCT) provides horizontal proper motions and GRIS LOS velocities and full-Stokes polarimetry. We present first results of the three-dimensional photospheric and chromospheric flow and magnetic fields scrutinizing the (inverse) Evershed effect in a complex active region.

We have exploited seeing-free and high-quality observations of several small magnetic pores from the High Resolution Telescope (HRT) of the Polarimetric and Helioseismic Imager (PHI) on board the Solar Orbiter spacecraft, during its first close perihelion in March 2022 (at a distance of 0.5 au from the Sun). Only such small magnetic pores, captured at stable observing conditions, could manifest (large) area fluctuations, on the order of 30% variations, that is an indication of (non-linear) fast sausage (surface) modes. Furthermore, cross correlations between perturbations in intensity, area, line-of-sight velocity, and magnetic fields have provided us with additional wave signatures, that together with applications of modern analysis techniques, such as the Spectral Proper Orthogonal Decomposition (SPOD), we have been able to characterise the underlying MHD wave modes in more details.

We present the results obtained from applying the Self Organizing Map technique to a spectral dataset of monochromatic images acquired by the Interferometric Bidimensional Spectropolarimeter at the Dunn Solar Telescope, along the Ca II 854.2 nm and Hα 656.28 nm lines. This technique allowed us to segment the photospheric and chromospheric penumbra fine structure, revealing the presence of the Evershed flow in the chromosphere along the portions of the penumbral filaments near the umbra. In addition, the flow velocity as a function of distance from the spot center in the peripheral regions of the super-penumbra was consistent with the inverse-Evershed flow. These signatures, along with slight differences in the chromospheric plasma velocities measured along the two spectral lines, provided us with the opportunity to combine the Evershed and inverse-Evershed flows in the same context, in accordance with the uncombed model of the sunspot penumbra.

Solar flares sometimes lead to coronal mass ejections that directly affect the Earth's environment. However, a large fraction of flares, especially on solar-type stars, are confined flares. What are the differences in physical properties between confined and eruptive flares? For the first time, we quantify thermodynamic and magnetic properties of hundreds confined and eruptive flares of GOES class C5.0 and above, 480 flares total. We first analyze large flares of GOES class M1.0 and above observed by the SDO, 216 flares total, including 103 eruptive and 113 confined flares, from 2010 until 2016 April; we then look at the entire dataset above C5.0 of 480 flares. We compare GOES X-ray thermodynamic flare properties, including peak temperature and emission measure, and active-region and flare-ribbon magnetic field properties, including reconnected magnetic fluxes and peak reconnection rates. We find that confined and eruptive flares have similar distributions of reconnection flux. Confined flares, however, have larger peak magnetic reconnection rates, are more compact, and occur in larger active regions than eruptive flares. These findings suggest that confined flares are caused by reconnection within more compact, stronger, lower lying magnetic-field regions in larger ARs that reorganizes smaller fractions of ARs. This reconnection proceeds at faster rates and ends earlier, leading potentially to more efficient flare particle acceleration in confined flares.

The chromosphere is a dynamic and complex layer where all the relevant physical processes happen on very small spatio-temporal scales. A few spectral lines that can be used as chromospheric diagnostics, give us convoluted information that is hard to interpret without realistic theoretical models. What are the key ingredients that these models need to contain? In this review, we present the recent numerical models and how they compare to observations. We will discuss the formation and properties of chromospheric structures at different spatial scales and the challenges we meet when modeling the solar chromosphere.

Spectropolarimetric reconstructions of the photospheric vector magnetic field are intrinsically limited by the 180-degree-ambiguity in the orientation of the transverse component. So far, the removal of such an ambiguity has required assumptions about the properties of the photospheric field, which makes disambiguation methods model-dependent.

The successful launch and operation of Solar Orbiter have made the removal of the 180-ambiguity possible solely using observations of the same location on the Sun obtained from two different vantage points. To that purpose, the Stereoscopic Disambiguation Method was recently developed and successfully tested on numerical simulations.

Here we present the first application of the SDM to data obtained by the High Resolution Telescope (HRT) onboard Solar Orbiter during the March 2022 campaign, when the angle with Earth was 27 degrees. The method is successfully applied to remove the ambiguity in the transverse component of the HRT vector magnetogram solely using observations (from HRT and from the Helioseismic and Magnetic Imager-HMI), for the first time.

The SDM is proven to provide observation-only disambiguated vector magnetograms that are spatially homogeneous and consistent.
While we present here an application combining PHI with HMI data, the same method can be applied to magnetic information from Solar Orbiter together with any Earth-bound observatory.

Open magnetic flux in the photosphere, especially at the poles, presents an important boundary condition for modeling the heliospheric field. The field extrapolated from polar observations is roughly 50% lower than the in-situ measurements (“open flux problem”). Here we present our efforts to characterize biases in the open flux inference and that way explain the discrepancy above. We use a state-of-the-art plage-like MURaM MHD simulation of the solar photosphere to model polarized spectra of magnetically sensitive Fe I 630 nm doublet lines, observed by the Hinode SOT/SP instrument. We mimic the realistic observation at the disk center and the pole by calculating emergent intensity, applying instrumental effects (spatial and spectral PSF, binning), and adding the photon noise. We then apply different inversion codes built upon different physical assumptions to these synthetic observations and compare results to the original simulation.

At the disk center, a significant fraction (20, down to 50%) of open flux gets lost for the telescopes with apertures smaller than 1m. At the pole, the magnetic field disambiguation becomes a critical part: the assumption of the radial field underestimates flux by around 30% while the inclusion of the telescope PSF further exacerbates the problem. The more in-depth investigation that accounts for stray light still yields significant discrepancies between the original and inferred intrinsic properties of the magnetic field. These systematic errors in field diagnostics may be reduced by high-resolution or out-of-ecliptic observations.

For a long time, the study of the evolution of active regions in the solar photosphere has been limited by the transit time of the active regions over the solar disk as seen from Earth. Since its launch in February 2020, ESA/NASA's Solar Orbiter spacecraft provides us, from time to time, with the possibility to see the solar far side. The Polarimetric and Helioseismic Imager (SO/PHI), one of its ten instruments, delivers data of the solar photosphere in intensity and vector magnetic field. In February 2021, during Solar Orbiter’s first superior conjunction, the full disk telescope of SO/PHI acquired the first polarimetric data of the photospheric solar far side. We combine this data with data from the Helioseismic and Magnetic Imager (HMI) flying on board NASA’s Solar Dynamic Observatory. This enables an almost 360º view of the Sun and allows, for the first time, the tracking of active regions over a full solar rotation almost without interruption, from the near-Earth side to the far side. We study the evolution of four active regions in intensity and LOS magnetograms while they rotate from the near-Earth side, seen by HMI, to the solar far side, into the field-of-view (FOV) seen by SO/PHI. Three of the active regions decay on the solar far side and do not show any signal in intensity when reaching the FOV of SO/PHI. One active region crosses the disk as seen from Earth while appearing to decay. However, new flux emerges on the solar far side leading to the development of pores and a small sunspot before the region reaches the limb as seen in SO/PHI and reappears on the near-Earth side. We will present the longest almost uninterrupted study of the evolution of the magnetic field of active regions, achieved so far. This demonstrates the uniqueness of combining Solar Orbiter and near-Earth side observations to continuously study the evolution of active regions from their emergence to their decay.

While it is being conjectured that a chromospheric canopy plays a role in penumbra formation, it has been difficult to find observational evidence of the connectivity between the photosphere and the chromosphere.

We investigate the existence of a chromospheric canopy as a necessary condition for the formation of a penumbra and aim to find the origin of the inclined magnetic fields. Spectropolarimetric observations of NOAA AR 12776 from the GRIS@GREGOR instrument were analyzed. Atmospheric parameters were obtained from the deep photospheric Ca I 10839 Å line (VFISV inversion code), the mostly photospheric Si I 10827 Å line (SIR inversion code) and the chromospheric He I 10830 Å triplet (HAZEL inversion code).

In the deepest atmospheric layers, we find that the magnetic properties (inclination and field strength distribution) measured on the sunspot sector with fully fledged penumbra are similar to those measured on the sector without penumbra. Yet, in higher layers, magnetic properties are different. In the region showing no penumbra, almost vertical chromospheric magnetic fields are observed. Additionally, thin filamentary structures with a maximum width of 0.1 arcsec are seen in photospheric high-resolution TiO-band images in this region.

The existence of a penumbra is found to be discriminated by the conditions in the chromosphere. This indicates that a chromospheric canopy is a necessary condition for the formation of a penumbra. However, our results demonstrate that inclined fields in the chromospheric canopy are not needed for the development of inclined fields in the photosphere. We question the `fallen-magnetic-flux-tubes' penumbra formation scenario and favor a scenario, in which inclined fields emerge from below the surface and are blocked by the overlying chromospheric canopy.

Fallon Konow - A New Instrument for Synoptic Space Weather Observations
Meetu Verma- Tales of Two Pores – Observed with GREGOR IFU and HiFI
José Roberto Canivete Cuissa - Vortices and Alfvénic pulses in the simulated solar atmosphere
Arooj Faryad - Automatic detection of Ellerman bombs in Halpha using COCOPLOTs
Nikolina Milanović - Thermal structuring and evolution of coronal bright points
Matteo Cantoresi - Scaling properties of Coronal Holes photospheric magnetic fields
Philip Lindner - Decay of a photospheric transient filament at the boundary of a pore and the chromospheric response

The penumbra is an intriguing part of a sunspot where the coupling between magnetic field and plasma is characterized by peculiar physical conditions. Although it has been the focus of many observational and theoretical studies, the processes involved in the formation and decay of a sunspot penumbra are still not fully understood. I will give a brief summary of recent observations leading valuable contributions to better understanding these processes.

We will review the recent observational results that show the key role played by the vertical component of the magnetic field (B$_\mathrm{ver}$) in the inhibition of convection in the solar photosphere. In sunspots, only regions with B$_\mathrm{ver}$ stronger than a critical value of approximately 1.8 kG (the critical value is dependent on the data used, definition of the umbral boundary, inversion scheme, etc.) are stable against convection and the intensity boundaries of stable umbrae can be equally outlined by this critical B$_\mathrm{ver}$. In regions with B$_\mathrm{ver}$ weaker than the critical value, more vigorous modes of magneto-convection take over. This behavior is observed during the formation of penumbra and light bridges and during the decay of sunspots and pores. We will compare these observations to analyses of sunspots simulations and to theoretical models describing the stability of overturning convection in the presence of magnetic fields and we will discuss the repercussion in the life cycle of magnetic structures.

Mini-filament eruptions are one of the most common small-scale transients in the solar atmosphere. They are small-scale analogs to solar filaments. They are associated with coronal jets that represent transient, collimated plasma ejections along open fields or far-reaching coronal loops. One particular type is blowout jets. These often come along with erupting loops or twisted filaments at the base of the jets. Consequently they have a more dynamic base and also often a wider spire than standart jets. Recently, a mini-filament eruption model has been put forward, in which a successful mini-filament eruption drives a blowout jet, while a partial or failed mini-filament eruption causes a standard jet. During blowout jets, many blobs have been observed and drawn much attention as they could provide evidence of the magnetic reconnection process, because these blobs are odten identified with plasmaoids in the current sheet. However, it is still difficult to observationally clarify the relation between the moving plasma blobs and magnetic reconnection. The clarification is mainly limited by the low spatio-temporal resolution of previous data. Here, with a combination of 174 Å images at high spatial resolution and high cadence taken by the Extreme Ultraviolet Imager on board Solar Orbiter and images of the Atmospheric Imaging Assembly on board Solar Dynamics Observatory, we investigate in detail an erupting mini-filament over a weak magnetic field region on 2022 March 4. During the eruption, two bright ribbons clearly appeared underneath the erupting mini-filament as it quickly ascended. Subsequently, some dark materials blew out when the erupting mini-filament interacted with the outer ambient loops, thus forming a blowout jet characterized by a widening spire. At the same time, a chain of small bright blobs with diameters of 1–2 Mm are intermittently expelled from the interaction region between the erupting mini-filament and ambient loops and propagated along the post-eruption loops toward the footpoints of the erupting fluxes at a speed of ~ 100 km/s. They have short lifetimes, varying from 30 s to 90 s. They also caused a semi-circular brightening structure. We suggest that these fast-moving blobs are evidence for magnetic reconnection. We performed a differential emission measure (DEM) analysis and found that various brightenings show plasma that is heated during the mini-filament eruption, e.g. two bright ribbons, footpoints of the ambient loops, and the interaction region. Based on the features aforementioned, we suggest that the mini-filament eruption first experiences internal reconnection that occurs underneath the erupting mini-filament and then external reconnection between the mini-filament and the ambient loops. The transfer of mass and magnetic flux from the erupting mini-filament to the ambient corona is mainly due to the external reconnection.

The Interferometric Bidimensional Spectrometer (IBIS) at the Dunn Solar Telescope (DST) was employed to capture high spatial resolution spectral scans of a sunspot in the H-alpha line. These scans allowed for the examination of super-penumbral fibrils, including their transverse motions both parallel and perpendicular to the line of sight. Near ubiquitous transverse oscillations of the fibrils have been observed and statistics gathered on their properties, which are consistent with previous investigations. Further examination of cases demonstrating well-resolved fibril oscillations were performed, which allowed us to track their propagation velocities and associated energy flux along the length of the fibril. Additionally, analysis of the H-alpha line width has been performed, which is known to correlate with plasma temperature. This allowed for a unique investigation of whether the energy damped from the transverse fibril oscillations provides evidence of thermalisation of the local plasma. Here, I will report on the energy damping and associated fluctuations in the H-alpha line width, which we believe is evidence of MHD wave dissipation in the form of plasma thermalisation.

A quiescent prominence was observed on October 22, 2013 quasi-simultaneously and nearly co-spatially in the Lyman series of hydrogen by SoHO/SUMER and in the Mg II h&k lines by IRIS. Such spectroscopic observations comprising multiple lines offer an excellent opportunity for diagnostics of prominence plasma and the dynamics of its fine structures. However, it also invokes significant challenges when compared with synthetic spectra provided by radiative transfer modelling. Only a few similar coordinated datasets of Lyman and Mg II h&k observations were ever obtained in prominences. We present here the first spectroscopic analysis of this unique dataset. Moreover, for the first time, we assess the influence of noise on the statistics of profile characteristics. We focus on the following characteristics – the number of distinct peaks, integrated line intensity, center-to-peak ratio describing the depth of the reversal of two-peaked profiles, and the asymmetry of peaks. There occur many profiles of both Lyman and Mg II lines which except of two major peaks, contain also lower peaks – hereafter called as minor peaks. Nature of these, possibly spurious minor peaks is uncertain. They may be either due to noise or vigorous dynamics of the prominence. We show that noise has a negligible effect on the integrated intensities of all lines but it significantly affects the classification of spectral profiles using the number of distinct peaks, the reversal depth and also the peak asymmetry. We also demonstrate that by taking into account an influence of noise, we can assess which characteristics of particular lines are suitable for diagnostics of different properties of prominence. For example, we show that the minor peaks of the Lyman line profiles are mostly caused by noise, which means that only the dominant peaks should be used for statistical analyses or comparisons with synthetic spectra. On the other hand, for Mg II k&h, distinction between multi-peaked profiles with low peaks and profiles with deep reversals lies mainly in the dynamics of multiple fine structures located along a line of sight. The complex, multi-peaked profiles are observed in places where multiple fine structures with different line-of-sight velocities are crossing the line of sight while the profiles with deep reversals likely correspond to instances when we observe single fine structure or multiple fine structures but with similar line-of-sight velocities. Such results lead to a conclusion that if we are interested in the diagnostics of dynamics of prominence fine structures, the best approach is to use a combination of profile asymmetry in the Lyman lines together with the complex profiles of Mg II h&k lines. On the other hand, if we want to diagnose the temperature and pressure of individual prominence fine structures, we need to focus on deeply reversed Mg II h&k line profiles in combination with the Lyman lines and to analyse the depth of central reversal and the integrated intensity.

Abrupt and permanent photospheric magnetic field changes have been observed in many flares. It is believed that such changes are related to the reconfiguration of magnetic field lines, however, the real origin is still unclear. I will present the analysis of 37 flares observed by the Solar Dynamics Observatory (SDO). The characteristics of the magnetic field changes in the flare events are determined using HMI high-cadence vector magnetogram (135 s), whereas the ribbon properties are derived from co-aligned AIA 1600 Å images. We find that pixels swept up by ribbons do not always exhibit permanent changes in the field. However, when they do, ribbon emission typically occurs several minutes before the start of field changes. The changes in the properties of the field show no relation to the size of active regions but are strongly related to the flare-ribbon properties. We find that the duration of permanent changes in the field is strongly coupled with the duration of the flare. Our results suggest that changes in photospheric magnetic fields are caused by a combination of two scenarios: contraction of flare loops driven by magnetic reconnection and coronal implosion.

The solar ultraviolet (UV) and extreme ultraviolet (EUV) radiations which are major contributors to the solar spectral irradiance (SSI) below 4000 Å, significantly affect the composition and the thermal structure of the Earth's atmosphere. The Magnesium II core-to-wing ratio (also known as the Mg II index) is one of the best proxies for solar activity and UV-SSI variability. To date, the Mg II index has been studied, assuming Sun-as-a-star using low (11 Å) and medium spectral resolution (1 Å) data. On the other hand, Mg II h and k line pairs being optically thick, are highly sensitive to the thermodynamical properties of the line-forming region and the viewing angle. Therefore, in this study, we explore the center-to-limb variation (CLV) in the Mg II line and continuum intensities in various solar features like quiet Sun (QS), plage, inter-plage, sunspot umbra, and penumbra using the high spatial and spectral resolution data from the Interface Region Imaging Spectrometer (IRIS). Our study indicates that plages with the lowest magnetic field density show the highest limb darkening as opposed to the QS, whereas the umbra and penumbra show a combination of both limb darkening and brightening depending on their photospheric magnetic field densities. This would provide us an opportunity to understand the importance of the spatially resolved Mg II index on the SSI studies.

The solar chromosphere and corona are heated by the dissipation of magnetic energy that emerges from the photosphere. The outward flux of magnetic energy, or Poynting flux, is therefore important to understand in both qualitative and quantitative terms. It can be computed by taking a cross product of electric and magnetic fields, and in ideal MHD conditions it can be expressed in terms of magnetic field and plasma velocity. There are existing estimates of Poynting flux in active regions and plages, but the quiet Sun (QS) remains a challenging target due to resolution effects and polarimetric noise. However, with upcoming DKIST capabilities, these estimates will become more feasible than ever before.

Here we present our findings from studying QS Poynting flux in Sunrise/IMaX observations and MURaM simulations. Since this problem has not been comprehensively approached before, and due to the aforementioned challenges of QS observations, we explore and evaluate multiple methods for computing Poynting flux. For transverse velocities, we try two inversion methods – a classic correlation tracking approach FLCT and a neural network based method DeepVel – and show DeepVel to be the more suitable method in the context of small-scale QS flows. Further, we investigate the effect of magnetic field azimuthal ambiguity on Poynting flux estimates, and we describe a new method for azimuth disambiguation. Finally, we use two methods for obtaining the electric field. The first method relies on idealized Ohm's law, whereas the second is a state-of-the-art inductive electric field inversion method PDFI_SS.

We compare the resulting Poynting flux values with theoretical estimates for chromospheric and coronal energy losses and find that some, but not all, of Poynting flux estimates are sufficient to match the energy losses. Using MURaM simulations, we show that photospheric Poynting fluxes vary with optical depth, and that there is an observational bias that results in underestimated Poynting fluxes due to unaccounted shear term contribution.

​Continuous vector magnetic-field measurements by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO) allowed us to run data-driven simulations of solar eruptions and perform statistical studies of magnetic-fields for many flares. In this talk I will review new aspects of flare magnetism discovered using SDO data, including progress in data-driven simulations and statistical studies of magnetic-reconnection fluxes, their rates, magnetic fluxes of flare dimmings, and magnetic-field changes during flares. I will summarize how these results, along with statistical studies of coronal mass ejections (CMEs), have improved our understanding of flares and the flare/CME feedback relationship. Finally, I will highlight future directions to improve the current state of understanding of solar-flare magnetism using observations and simulations.

Solar flares originate from active regions (ARs) hosting complex and strong bipolar magnetic fluxes. Forecasting the probability of an AR to flare and defining reliable precursors of intense flares, i.e., X- or M-class flares, are extremely challenging tasks in the space weather field.

In this work, we focus on two metrics as flare precursors, the unsigned flux R and the novel topological parameter D, representing the complexity of a solar active region. Both metrics are based on the automatic recognition of magnetic polarity inversion lines (PILs) in identified SDO/HMI ARs. We use both a heuristic approach and a supervised machine-learning method to validate the effectiveness of these metrics to predict the occurrence of X- or M-class flares in a given solar AR during the following 24 hr period. Moreover, we revise the statistics of CME-flare relationship on Solar Cycle 24 using the GOES database and a CME database recently released, developed using the Drag-Based Model (DBM) to assess the quality of such a database. In particular, we exploit the R and D parameters to classify flaring regions in different classes, studying the CME-flare relationship for those classes over Solar Cycle 24.

It is often observed that the Sun produces repetitive coronal mass ejections (CMEs) with similar morphological features originating from the same source region. This kind of CMEs is known as homologous CMEs and their occurrence is majorly controlled by the storage of free magnetic energy in the active region corona. The stored energy is catastrophically released during flares, accompanied by successful eruption of plasma materials, to be observed as CMEs. Therefore, it is important to study the homologous flare-CME events in order to understand their association in terms of sustained energy build-up in the corona. In this study, we select three homologous flare-CME events of successively increasing flaring intensities (M2.0, M2.6, X1.0) originated in NOAA AR 12017 during 2014 March 28-29. We analyze the detailed evolution of the photospheric line-of-sight magnetogram from ≈1 day before the first event and encompassing all the three events. Our observations reveal significant phases of emergence and cancellation of magnetic flux in and around the flaring region, which strongly supports the idea of ‘tether-cutting’ model as a plausible triggering mechanism of the eruptions. We also analyze the build-up and release of free magnetic energy in the active region corona, which temporally correlates with a prolonged phase of magnetic flux emergence. The importance of this study lies in the investigation of the intrinsic coupling of magnetic fields and associated processes from the photosphere to corona that resulted into the repetitive build-up and eruption of magnetic flux ropes. Interestingly, the homologous flux rope eruptions led to broad CMEs (angular width ≈100° to 360°), in spite of their origin from compact eruption-source site. We investigate this apparent observational paradox in the framework of generalized ‘magnetic-arch blowout’ scenario, which satisfactorily explains the observed phenomena.

Coronal Mass Ejections (CMEs) are in the center of interest for solar - terrestrial research. They are interesting both from the fundamental plasma physical and space weather perspectives. The integral part of CMEs are huge magnetic flux ropes where magnetic field lines wind about a common axis. They form when magnetic energy first accumulates in the solar corona and then lose its balance. After being released from the Sun, CMEs expand and propagate outwards into outer corona and interplanetary space. Their initial flux rope structure can change dramatically due to interactions with the ambient solar wind and with other eruptions. This presentation will review the key mechanism of how CMEs are believed to form and erupt from the Sun, and physical processes behind their early evolution. Understanding their nascent phase and evolution are paramount for reliable space weather forecasting. The status and challenges related to coronal modelling of CMEs are also discussed.

Of the six remote-sensing instruments aboard Solar Orbiter, the Spectrometer/
Telescope for Imaging X-rays (STIX) is the one dedicated
to the study of solar flares. It performs imaging spectroscopy
in the hard X-ray regime, which provides key physical diagnostics on
both the hot thermal plasma as well as on the accelerated energetic
electrons. During its operation since launch in 2020, which now includes
the first year of the nominal mission phase, STIX has detected
over 25,000 solar flares. The first scientific results based on these novel
observations will be discussed. In particular, we will focus on studies
that use STIX jointly with other observational assets, such as the
other remote-sensing instruments on Solar Orbiter, various X-ray instruments
on other spacecraft, and in-situ particle detectors.

During the 8th call of the SOLARNET Trans-National Access Programme we were funded for a flare observing campaign at GREGOR (Tenerife). This campaign was supported by additional instruments, such as the IRIS and Hinode satellites, and ground-based instruments at the Astronomical Institute of the Czech Academy of Sciences in Ondřejov (CZ), and at the Białków Observatory of the University of Wroclaw (PL).  

Our goal was to observe solar flares during their initial evolutionary stages and at different atmospheric layers in order to understand their structure and the triggering processes. 

On May the 4th a M5.7 flare was captured by the GREGOR Infrared Spectrograph (GRIS) and by the improved High-resolution Fast Imager (HiFI+) multi-wavelength imaging filtergraph. In this work we focus on the analysis of the temporal evolution of HiFI+ images and IRIS dataset, with a special emphasis on the study of the continuum enhancement. After a proper co-alignment with respect to the IRIS dataset, we are able to present a multi-wavelength evolution of the flaring solar atmosphere at selected positions of interest.

In the era of Solar Orbiter and Parker Solar Probe, we have now an unprecedented view on eruptive events of the Sun, from their onset to their propagation in the heliosphere. Thanks to the proximity of the probes to the Sun for at least part of their orbit, we are currently acquiring more and more key data on the structuring and the propagation of solar eruptive events, as well as on the acceleration of particles at each stage of the eruption and propagation.

I will report on an on-going modelling effort of solar eruptive events using the versatile PLUTO code within the STORMGENESIS project. I will present a full MHD effort to model idealised and realistic and eruptive active regions, based on the initiation of the magnetic with both non-linear force-free and non force-free extrapolations. I will present very recent result on the modelling of the eruption of active region #12241, which is well reproduced when using a non force-free initiation. Coupled to charged test-particles under the gyrokinetic approximation, and to the heating and cooling associated with magnetic reconnection and radiative losses, I will present first results on the prediction for thermal and non-thermal emission from this active region.

In a second part, I will show how the same simulation framework, now using adaptive mesh refinement, can be used to model the propagation of coronal mass ejections from their onset to their propagation up to 1 AU in a realistically structured solar wind. These results, published recently in Regnault et al. (2023), allow to highlight the generic features of CMES and the simulation results compare very well with superposed-epochs analysis of hundreds of CMEs. In addition, these simulations point towards various rotation patterns of the CMEs that depend on their size and that happen during the initial stage of their propagation within the Alfvén surface. These results warrant further analyses, coupling the two set of simulations presented here to provide a complete picture of the energetics and of thermal and non-thermal emission of solar eruptive events.

A multi-wavelength analysis is conducted to study flare signatures both in the
low and higher corona. LOFAR, STIX/Rhessi, and AIA data provide an extensive
picture about different aspects of flare characteristics. The active region and
its thermal evolution are studied through Differential Emission Measure (DEM)
reconstructions from AIA data. Flare-accelerated electrons and their radio
traces are observed with STIX and LOFAR, respectively. These are in principal
all associated with the energy release during the flare process. We focus on
fluctuations of the radio sources and the thermal evolution of the active
region, and their possible relations. Solar magnetic fields are a binding
element between low and high corona, accelerated electrons, and heated flare
loops. They are included in the analysis via a Potential Field Source Surface
(PFSS) model.

Solar prominences consist of a thin thus low-collisional and partially ionized plasma.
Therefore, the coupling of ions and neutral atoms is much weaker than in photosphere,
and ions are affected by the Lorentz-force, while neutral atoms are not.
To investigate this issue, we performed simultaneous observations of the spectral lines
of several ions and neutral atoms with THEMIS at the Observatorio del Teide, Tenerife.
The selected spectral lines are optically thin, so that we probe also the inner part
of the prominence. We find that lines of ions are broadened compared to such from
neutral atoms, and Sr II 4216 is even broader than Fe II lines. Lines from neutral
metals (Mg I b2, Na I D1) appear broader than He I 5016, a line from the singlet
system. Doppler velocities also differ among the observed lines and show a typical
sequence. Probably the situation is more complicated than just having one ionized
and one neutral plasma component.

EUI is a set of (E)UV imaging telescopes that observe the solar corona from >10 Rsun scales down to pixel scales of 100 km. EUI has been embarked on the ESA/NASA Solar Orbiter mission on a 10-year mission, while the imaging cadence can be as fast as 2s. This wide range of spatial and temporal scales allows to observe the scale invariance of events, from the largest to the smallest flares, and from the largest eruptions to the smallest jets. Besides identifiable events, also the background spatial structuring shows scale invariance over several orders of magnitude. In this presentation we will present the newest contributions of EUI at the smallest scales (picoflares, picojets) and discuss limitations on how much further down this coronal scale invariance can extend.

The X1.6 flare observed on 22 October 2014 was one of the strongest flares observed in the complex active region NOAA 12192. We report on observations acquired in the Fe I 617.30 nm and Ca II 854.2nm line profiles by the Interferometric BIdimensional Spectropolarimeter (IBIS) immediately after the peak of the X1.6 flare. We focused on the chromospheric signals and studied the temporal variation of the line of sight (LOS) magnetic field, using the Weak-Field Approximation (WFA). Our results show that stepwise variations occurred at specific locations along the eastern flare ribbon, during the tail of the flare impulsive phase. This result was validated by the STiC inversion of the pixels used for the WFA analysis. We also investigated the photospheric magnetic field obtaining that the LOS magnetic field did not exhibit the same stepwise changes observed in the chromosphere in the selected pixels. Finally, we found no clear evidence of changes along the polarity inversion line around a magnetic polarity intrusion, which is almost cospatial to the area affected by the initial energy release.

Sun-as-a-star observations provide a valuable link between resolved solar observations and disk-integrated stellar observations, as it gives a unique insight into how well-defined solar activity affects the average spectrum. This activity can affect the integrated spectrum in complex ways, and therefore the values of for example reference spectra and exoplanet characterizations. It is therefore important to understand the magnitude and timescale of these variations. We aim to contribute to this field by focusing on the most rapidly changing type of activity: solar flares. We present the first Sun-as-a-star detection of solar flares with TNG/HARPS, and discuss its effects on the integrated spectrum. When combined with observations made with SST/CRISP&CHROMIS and SDO/AIA, we are able to point to several evolutionary features of the flares in the HARPS data, as well as discuss their imprints on activity indices and RV measurements.

Solar prominences consist of cool and dense plasma that is suspended in the corona, surrounded by hotter and less dense coronal material. As predecessors of coronal mass ejections, solar prominences are important drivers of space weather, but their exact formation mechanism is still unknown. We use the radiative magnetohydrodynamic code MURaM to simulate the formation and dynamics of a prominence in the solar atmosphere. MURaM includes the relevant physical processes to realistically simulate the solar photosphere, chromosphere and corona.

We create a stable dipped magnetic field configuration in a 3D box of size 80 x 30 x 10 Mm and let it evolve. In the course of the simulation, a dense plasma seed ejected from the chromosphere randomly settles into a magnetic dip and is cooled by radiative losses. The resulting pressure drop drives a strong flow of plasma into the feature and builds up a cool, long-lasting structure in the solar corona. This prominence is very dynamic but stable due to the stability of the underlying magnetic field. Its properties and dynamics are comparable to certain observations of real prominences. In this contribution, we present the formation mechanism and properties of the simulated prominence.

Recent ultraviolet observations performed by the IRIS satellite have shown the occurrence of small-scale eruptive phenomena, characterized by significant increase of the integrated line intensity (up to a factor of 100 in Si IV lines) and of the line width (up to ± 100 km s$^{-1}$), likely due to magnetic reconnection. Some of these events also exhibit counterpart in all the SDO/AIA channels.
We report on a series of similar events observed during secondary flux emergence in an arch filament system in active region NOAA 12585. We use high-resolution observations acquired by SST/CRISP relevant to the photosphere and the chromosphere to characterize the magnetic field configuration and the phenomena occurring in the low atmosphere. IRIS and SDO/AIA observations reveal the atmospheric response and the presence of heating and plasma ejection activity in the upper atmosphere.

On May 30, 2020, the Solar Orbiter High-Resolution Imager operating in 174 A being for the first time approximately at a half AU to the Sun, registered a large number of short, small-scale heating events, often called campfires, with rich morphology and smaller space-time characteristics than nanoflares. We estimate the thermal energy of the heating events from increases in the emission measure and temperature using methods assessing thermal energy in flares. We compare these with the characteristics of X-ray flares, in particular, those recorded by the X-ray spectrometer/telescope onboard Solar Orbiter. We found campfires emit thermal energy in picoflares range. The additional previously unaccounted energy input of is about 1% to the total required to sustain quiet solar corona power. The observed frequency distribution would have to continue to the femtoflares range to fulfill the observed heating requirement. We explain the existence of solar flares below the expected nanoflare threshold as follows. They originate in small coronal loops of 200-5000 km, smaller than considered by Parker. Some picoflares show only a partial energy release along a single loop.

One-dimensional (1D) models of solar flares often generate hot upflows from the chromosphere (chromospheric evaporation) due to heating by energetic particle beams (Allred et al. 2005, 2015, Druett et al. 2017, Polito et al. 2023). The beams of particles are injected at the top of the models using distributions consistent with X-ray emission in observations assumed to result from Bremsstrahlung emission from energetic particle beams impacting the ambient plasma. To date, 2D and 3D flare models produce evaporation as a result of thermal conduction and the delivery of kinetic energy to the lower atmosphere from the outflows of reconnection jets (e.g. Ruan et al. 2020). In this study we evaluate and explore the reasons for this difference compare the differences between the findings of 1D and multi-dimensional flare models. We report the first simulation of chromospheric evaporation in a multi-dimensional model caused by beam electrons.

The build-up of magnetic energy during flux emergence, which powers flares and coronal mass ejections (CME), requires the presence of significant amounts of volume electric currents. Measuring these electric currents requires knowledge of the magnetic field vector in three dimensions in the solar atmosphere. However, systematic high-quality spectropolarimetric observations are constrained at the photosphere. Thus, only the vertical component of the electric current density, Jz, can be routinely calculated by means of differentiation of the photospheric magnetic field vector. In this work, a different method, relying on image segmentation and taking into account observational and numerical errors, is employed to determine the total unsigned non-neutralized (net) electric currents, INN,tot, injected to the corona. This quantity deviates from Jz and is more closely associated to the development of strongly sheared polarity inversion lines (PIL). It is found that INN,tot is at least one order of magnitude higher for flare productive active regions and its temporal evolution from emergence to decay exhibits intricate structure, owing to the interaction between opposite magnetic polarities and the formation of PILs. Further relevant quantities facilitate the distinction between imminent eruptive and confined major flares and they are strongly correlated with the kinematic characteristics of subsequent CMEs. These quantities are examined during the emergence and pre-eruptive evolution of active regions and their association with the magnetic complexity of active regions.

Xudong Sun- Formation and Destruction of Kilogauss Bald Patches in a Delta-Spot Light Bridge
Sibaek Yi- A New Method of Coronal Force-Free Field Reconstruction and Its Application to an Active Region
Salvo Guglielmino - Multi-instrument analysis of an eruptive M1.2 flare observed by Metis and STIX onboard Solar Orbiter
Tino L. Riethmüller - The potential of many-line inversions and an outlook to its application to data recorded with state-of-the-art instrumentation
Sanghita Chandra - Investigating Chromospheric Fine Structures using MURaM
Sofya Belov - Numerical simulations of oscillations in solar corona excited by vortex shedding
Xiang Li - Nature of small-scale Ca II H brightenings associated with low magnetic flux in quiet Sun region

Observational precursors of small-scale solar flares provide a basis information for future operational systems for large events forecasting. We present the results of observational comparative study of the active flaring (AFR) and non-flaring regions (ANFR). We study power spectral density (PSD) of the intensity signal in the level of solar chromosphere and corona, through 2D-spatial maps of the power estimated for seven active regions, using intensity data from Atmospheric Imaging Assembly (AIA) on board Solar Dynamics Observatory (SDO). We selected six active regions with pore or sunspot feature, and magnetic class of α, β, γ (according to the Hale' magnetic classification system). The selected data are analysed in four wavelengths: 1700Å and 193Å, 171Å, and 131Å. This very fast method of analysis based on 2D-Fourier Power Maps (FPM) and the Wavelet Power Spectrum (WPS) of individual pixels spatially resolved in the flare location.

Understanding the physical processes in the solar chromosphere requires comprehensive models which can be compared to observations. These models include the treatment of magnetic fields, heat conduction and radiative transfer (RT). However, usually models fail to reproduce observed properties of chromospheric spectral lines. The MgII h&k lines form in the middle to upper chromosphere and are observed by the IRIS satellite mission with high precision. The forward modeled spectral lines appear on average to have too narrow line widths and larger peak asymmetries than the observations.
We use the recently developed chromospheric extension of the radiative MHD code MURaM to simulate an enhanced network (EN) region. From the resulting modeled atmospheres we use the RT codes RH1.5D and Multi3D to synthesize the MgII h&k lines in 1D and 3D RT. We find that the line width as well as the peak separations from the forward modeled spectra are on average larger and in better agreement to observations. We discuss the physical properties such as velocity gradients and density stratification in the chromosphere of the modeled atmosphere that lead to the improved synthetic spectral profiles.

Measurements of solar irradiance and surface magnetic flux sufficiently accurate as to allow assessment of their variability are only available for roughly half a century. Variability on longer time scales is evidenced by proxies. Upon emergence, the magnetic field on the Sun manifests itself as dark (sunspots and pores) and bright (faculae/plage and network) features. The combined effect of their evolution and the solar rotation is the modulation of the solar irradiance. Models relating the variability of the solar irradiance to the evolution of the solar surface magnetic flux account for almost all of the measured variability. Reconstructions of past variability are, however, thwarted by the lack of reliable long-term proxies of solar surface magnetism, in particular those describing bright magnetic features (faculae and network). We will review our recent efforts to improve understanding of the long-term changes in solar magnetic activity and irradiance. These include recovery and analysis of suitable proxies of past solar magnetic activity, examination of the link between sunspot and facular emergence and evolution, as well as modelling efforts to tighten constraints on the long-term solar variability.

Solar coronal holes (CHs) are the regions with low coronal emissions and an unbalanced magnetic flux, which led to the common expectation that their magnetic structures are mostly ``open'', or, in other words, extending to infinity. The objective of this study is to examine the CH magnetic structures to verify this expectation. The coronal holes are identified from the synoptic maps constructed using the AIA 193 images and HMI line-of-sight magnetograms from 2010 June to 2020 February. The magnetic structures of the CHs are constructed using the Potential Field Source Surface (PFSS) model. As a comparison, the data from a thermodynamic magnetohydrodynamic (MHD) model are also used. The results from both models reveal that there is a significant percentage of closed field lines extending beyond the CH boundaries, leading to an unbalanced magnetic flux within the boundaries. The boundary-crossing field lines tend to be located slightly closer to the CH boundaries, and are more likely to be found in the lower latitudes during active times. The results also show that more than 50% (17%) of PF (MHD) CHs do not contain open magnetic field lines. The CHs without open field lines are often smaller and less unipolar than those with open field lines.

Variations of the solar magnetic field are the main sources of several manifestations of the solar activity (e.g. flares, CMEs, solar wind). A way to quantify such variations is by means of a physical index, the Ca II K index, which has been proven to be a great proxy for the line-of-sight unsigned magnetic flux density along all phases of the cycle, and not only when sunspots are present.
In this respect, an assessment of the long-term behaviour of the solar wind is of paramount importance to understand the nature of its relationship with the global magnetic field of the Sun and with its cycle-to-cycle variations. Here, we study the variations of physical proxies of the solar activity (i.e. Ca II K index, Mg II index), on solar cycle time-scales, and how they are related to variations of solar wind parameters. The available time series allow to evaluate this relation over the last five solar cycles (from 1965 to now).
By adopting a climatological approach, the results show that there is a delayed response of the solar wind with respect to the solar activity, but such response is not the same for the different solar cycles. The analysis offers the possibility to deepen the understanding of the processes that link the global dynamo to solar variability and to the properties of the solar wind near the Earth.

The solar magnetic field is generated and sustained by the combined action of turbulent convection and differential rotation. This dynamo process can sometimes lead to magnetic cyclic variabilities, like the solar 11 years cycle. Traces of magnetic cycles have also been detected in other solar-like stars, ranging from a few years to a few tens of years. How is the solar cycle period controlled?
Recent 3D numerical simulations of solar-like stars show that different regimes of differential rotation can be characterized with the Rossby number (advection over Coriolis force). In particular, “anti-solar” differential rotation (fast poles, slow equator) may appear for high Rossby number (slow rotators). If large-scale flows transitions occur during the solar main sequence, we may wonder how the magnetic generation will be impacted.

We present a numerical multi-D study to understand the magnetic field generation and evolution of solar-like stars under various differential rotation regimes, in a Sun in time context. We find that short and local cycles near the surface are favoured for small Rossby numbers (fast rotators), while long and global cycles appear for intermediate (solar-like) Rossby numbers. Slow rotators (high Rossby number) are found to produce statistically steady dynamo with no cyclic activity in most cases considered. We further evaluate energy transfers in these models and quantify that dynamos can be powered by up to 3% of the stellar luminosity, part of it being then available to power surface events. In this regard, future Solar Orbiter observations outside the ecliptic will provide fantastic constraints on solar polar caps magnetic field, and thus on the type of dynamo that our Sun is most likely to operate.

Emission in the Ca II H & K spectral lines is sensitive to the changes in the stellar magnetic activity and is a good proxy for studying stellar magnetic fields. This emission is most commonly characterized via the S-index, representing the flux in the Ca II H & K line cores relative to the flux in the nearby continuum. The sources of stellar Ca II emissions are yet to be investigated in detail. Empirical and physics-based models have shown faculae/plage to be the main source of Ca II emissions and thus neglect the contribution from spots. This is because the solar surface coverage by spots is much smaller than those by faculae. Further, spots are assumed to be equally dark in the Ca II line cores and the continuum so that they do not affect the S-index. While this is a good approximation for old less-active stars such as the Sun, for young active stars with large surface coverages by spots, the role played by spots might become significant.

Here we use high quality sunspot observations in the Ca II H line from the Swedish Solar Telescope to determine the Ca II flux from chromospheric counterparts of sunspots. We find that spots are brighter than the surrounding quiet regions in the chromosphere. Using the sunspot flux computed from observations in our physics-based model which also accounts for the effect of faculae, we compute S-index for four solar activity cycles. We find that sunspots lead to an increase in the S-index. However, this increase is quite small to have a significant impact on the observed solar Ca II emissions. Further, we compute S-indices considering cases of stars which are more active than the Sun. Previous studies have indicated that surface coverages of spots increase much faster with increasing activity than the coverages by faculae. We show that with increasing activity and hence spot area coverages, S-index increases steeply suggesting that Ca II emissions of active stars have a significant contribution from spots.

The solar radiation in the cores of the Mg II h & k spectral lines strongly correlates with solar magnetic activity and global variations of magnetic fields with the solar cycle. This work provides a data-driven model of temporal evolution of the solar full-disk Mg II h & k profiles over the solar cycle. Based on selected 76 IRIS near-UV full-Sun mosaics covering almost the full solar cycle 24, we find the parameters of double-Gaussian fits of the disk-averaged Mg II h & k profiles and a model of their temporal evolution parameterized by the Bremen composite Mg II index. The Markov Chain Monte Carlo algorithm implemented in the IDL toolkit SoBAT is used in modeling and predicting temporal evolution of the Mg II h & k peak-to-center intensity ratio and the Bremen Mg II index. The relevant full-disk Mg II h & k calibrated profiles with uncertainties and spectral irradiances are provided as an online machine-readable table. To facilitate utilization of the model corresponding routines, written in IDL, are made publicly available at GitHub.

Solar dynamo models can explain the production and maintenence of the solar magnetic fields on two different spatial scales: the large-scale solar dynamo, manifested in the cycle-dependent appearance of solar active regions, and the small-scale solar dynamo, driven by the turbulent convective motions close to the solar surface. Disentangling these two mechanisms observationally is extremely challenging, but provides important input for the dynamo models. Highly stable long-term observations with sufficient magnetic sensitivity are required.

The Helioseismic Imager on board the Solar Dynamics Observatory (SDO/HMI) has measured the solar magnetic field for more than one solar cycle. In ths work, we present the results of these 13-year long magnetic field measurements in the most quiet patches of the solar surface after applying a combination of a temporal and spatial averaging algorithm and a careful selection process. The results provide strong evidence for a solar-cycle modulation in even the most quiet patches when they include network fields. In contrast, the modulation is absent in the internetwork fields, where only the interior of the supergranular cells is included in the analysis.

We interpret the absence of a solar-cycle modulation in the internetork fields as a consequence of them being generated by the small-scale (or fluctuating) dynamo acting on scales smaller than a supergranular cell. The solar cycle modulation when including the network fields can be explained by tangling of the solar-cycle dependent large-scale sub-surface field.

The total and spectral solar irradiance, TSI and SSI, respectively, vary on many timescales from sub-daily variations to annual, decadal, centennial timescales. The variations of the SSI are also wavelength dependent, with larger relative variations in the EUV/UV and smaller variations in the visible and IR. These irradiance variations allow us to understand, on the one hand, the physics of the Sun as a star and, on the other hand, the key energy source for the Earth's climate system. In this talk I will review the available irradiance measurements, composite datasets, models and how the underlying physics, which drives the irradiance variations, is implemented.

The Sun’s large-scale magnetic field undergoes periodic reversals due to the activity of the dynamo within the solar interior. The evolution of the Sun’s magnetic field has a clear impact on the structure of the corona and solar wind above. Here we show results from the spherical harmonic decomposition of the Sun’s photospheric magnetic field, using WSO, SOHO/MDI and SDO/HMI synoptic magnetograms that span sunspot cycles 21-24, plus cycle 25 to present. From this, trends in the magnetic field components (notably the lowest order, dipole, quadrupole and octupole) are extracted and compared between cycles. In addition, we perform potential field source surface modeling of the coronal magnetic field, in order to make inferences about the solar wind sources and likely coronal rotation rate. These results are compared against scattered whitelight observations of the corona from MLSO/K-Cor and SOHO/LASCO-C2. The role of nested flux emergence and poleward surges of magnetic flux will be discussed in this context, with future links to heliospheric missions like Solar Orbiter and Parker Solar Probe.

Differential rotation is a key aspect of the solar dynamo model and is well-established in the Sun's interior and photosphere. However, our understanding of the chromosphere and upper atmosphere's differential rotation is limited and has yielded contradictory results in previous studies. To address this, we utilized a century-long dataset of Ca II K images (1907-2007) from the Kodaikanal Solar Observatory (KoSO) to investigate the chromosphere's rotation profile during different solar cycles. By employing the image correlation analysis on sets of consecutive day images, we found that the chromosphere rotates 1.52% faster than the photosphere but with less differential rotation compared to the photosphere values. To validate our method and results, we applied the same technique to MDI white light data and other Ca II K data sources (Meudon and PSPT/Rome) and obtained consistent results. Moreover, we examined the solar cycle dependence of differential rotation parameters and investigated the north-south asymmetry of the solar rotation profile.

Recently, there has been consideration of two possible methods for the nonlinear modulation of solar cycle amplitudes. The first mechanism, known as Tilt Quenching (TQ), involves a negative feedback relationship between the cycle amplitude and the mean tilt angle of bipolar active regions relative to the azimuthal direction. The second mechanism, known as Latitude Quenching (LQ), involves a positive correlation between the average emergence latitude of active regions and the cycle amplitude.
Here we study the effects of quenching parameters on the Surface Flux Transport (SFT) models and the effects of implementing the surface inflows into the SFT models on the two different observable nonlinearities: TQ and LQ
Our SFT models are grid-based. We have methodically investigated the extent of nonlinearity caused by TQ and LQ, as well as their combination in the presence of surface inflows.
A preliminary result is that there is a linear dependence of quenching parameters on the dipole moment. This is expected from the model. A second finding is that the effects of introducing surface inflows to the SFT models will produce lower values of dipole moments for the different cases of quenching in the solar cycle.

The Solar Mean Magnetic Field (SMMF) is the average value of the line-of-sight (LOS) magnetic field considered over the visible hemisphere of the Sun. Various synoptic observations of the solar magnetic field have shown that the SMMF has a periodicity of ~ 27 days, and an amplitude that varies according to the solar cycle, from ±0.2 G during solar minima to ±2 G during solar maxima. It has also been shown that the SMMF has a good correlation with the Interplanetary Magnetic Field (IMF). Nevertheless, the origin of the SMMF is still a controversial subject. Till date, SMMF measurements have always been carried out using photospheric spectral lines only. In this regard, we utilised full-disk LOS magnetograms from the Vector Spectromagnetograph (VSM) (one of the instruments in SOLIS - Synoptic Optical Long-term Investigations of the Sun), calculated the SMMF as the disk-averaged magnetic field in each magnetogram, and compared the SMMF at photospheric and chromospheric heights. Our comparison showed that the SMMF at the chromosphere is a factor of 0.6 times the SMMF at the photosphere, with a correlation coefficient of 0.80; indicating that a considerable fraction of the SMMF might be propagating outwards from the photosphere to the chromosphere. This brings us one step closer to understanding the connection between the SMMF and the IMF. Further, to understand the origin of the SMMF, which is a magnetic flux imbalance, we are studying the full-disk Stokes I and Stokes V data from VSM, which are disk-averaged to create Stokes profiles. Any localised source of the SMMF should reflect in the Stokes V profile as a rotationally modulated doppler shift. Our initial analyses show that there is a periodicity in the Stokes V doppler shift. This periodicity is being analysed along with associated magnetograms, to understand the source of the SMMF.

Francesco Berrilli - The Multiscale Magnetic Signature of the Solar Turbulent Magneto-Convection
Andrei Gorobets - Co-existence of self-similarity and anomalous scaling in photospheric turbulent small-scale magnetic fields
Malcolm Druett - Comparing the evolution of mottles and fibrils in a radiation-MHD simulation
Carsten Denker - Instruments for High-resolution Solar Observations
Saida Milena Diaz Castillo - Signatures of an impulsive jet in a small-scale magnetic tornado: Spectro-polarimetric observations of a dynamic photospheric small-scale magnetic vortex associated with a chromospheric swirl

The change in the distribution of the magnetic field in the quiet Sun photosphere over the solar cycle could be expected to cause systematic variations in the photospheric thermodynamic structure. This could be due to changes in the dynamics of the convective overshoot or radiative properties. To look for such changes, we use high spatial and spectral resolution spectropolarimetric observations obtained by Hinode SOT and apply the Spectropolarimetric Inversions Based on Response Functions (SIR) code to infer physical conditions in the lower solar photosphere. By using multiple homogeneous datasets acquired over 15 years, we analyze the distributions of the inferred parameters in the granules and intergranules of magnetic and non-magnetic spectra throughout the solar cycle. The final goal of our study is to characterize changes in the inferred atmospheric parameters, notably, the temperature structure, the line-of-sight velocities, and magnetic properties.

Contemporary solar physics deals with increasing volumes of high-dimensional data from observations and realistic theoretical models alike. Machine Learning (ML) is rapidly integrated into solar and heliophysics research, facilitating dimensionality reduction, visualization and analysis. We developed a pipeline, which employed carefully selected unsupervised ML algorithms for classification and cluster analysis, to extract information regarding the physical properties of the solar atmosphere contained in the large variety of spectral profiles. The pipeline is tested on the synthetic spectra of the Fe I $\lambda\,7090.38$ Å photospheric absorption line, computed with the CO$^5$BOLD radiation hydrodynamics code. This line is also part of the observing setup for the Fast Multi-line Universal Spectrograph (FaMuLUS) camera system at the Vacuum Tower Telescope (VTT). CO$^5$BOLD snapshot time-series serve as a simulation of high-resolution, fast-cadence solar observations, thus confronting the pipeline with the scenario of dynamic solar feature evolution. This project aims to deliver a robust classification scheme with minimal user interaction, which also prepares the spectral dataset for further analysis, such as spectral inversions. Successful classification allows quick identification of structure and dynamics in the region of interest, as well as diagnostics of the ambient plasma based on spectra line parameters.

Many solar spectral lines have poorly determined atomic parameters, such as the transition probability or the central wavelength, especially in the near-ultraviolet (NUV) range. We present a novel approach to exploit high-angular-resolution spectropolarimetric observations of the Sun to obtain precise atomic parameters of spectral lines, required for the proper analysis of solar and stellar observations. The inversion-based approach requires that unique (global) values of atomic parameters must represent very well spectral line profiles originating from very different solar atmospheres within an observed field of view. To test this so-called global method, we analyzed synthetic spectra computed from magneto-hydrodynamic models of the solar atmosphere and compared the results to the previously used methods. The method is general enough to be applied to the spectral lines from the NUV to the infrared wavelengths. The global method is able to retrieve reliable estimates of atomic parameters even for weak and blended spectral lines, thus extending the list of spectral lines with reliable atomic parameters used to analyze solar and stellar spectra.

Solar spectropolarimetry is entering the realm of big data. Current and future telescopes will produce data at a rate that will make it hard to store in a single machine and even harder to operate on the data. Thankfully, in the last decade, machine learning has experienced an enormous advance, thanks to the open possibility of training very deep and complex neural networks. In this
contribution I show options to explore to deal with the big data problem and also how deep learning can be used to efficiently solve difficult problems in Solar Physics. I will focus on how differentiable programming (aka deep learning) is helping us to have access to velocity fields in the solar atmosphere, correct for the atmospheric degradation of spectropolarimetric data and carry out fast 3D inversions of the Stokes parameters to get physical information of the solar atmosphere.

A solar magnetic flux tube, like any physical system, is capable of supporting a variety of resonances, which constitute its natural response to external forcing. Apart from a few single low order eigenmodes in small scale magnetic structures, several simultaneous resonant modes were not found until recently. These eigenmodes are associated to small but coherent oscillations and require the application of particular spatial filtering techniques to be detected.
In this talk I will report and discuss about the detection and physical implications of large scale coherent oscillations in a sunspot, with a spectrum significantly different from p-modes.

Physics-informed neural networks (PINNs) provide a novel approach for numerical simulations, tackling challenges of discretization and enabling seamless integration of noisy data and physical models (e.g., partial differential equations). In this presentation, we highlight the new opportunities for solar magnetic field modelling that are enabled through physics-informed machine learning.
We present a novel approach for coronal magnetic field extrapolation using physics informed neural networks. The neural network is optimized to match observations of the photospheric magnetic field vector at the bottom-boundary, while simultaneously satisfying the force-free and divergence-free equations in the entire simulation volume. We demonstrate that our method can account for noisy data and deviates from the physical model where the force-free magnetic field assumption cannot be satisfied.
We utilize meta-learning concepts to simulate the evolution of the active region 11158. Our simulation of 5 days of observations at full cadence, requires about 12 hours of total computation time. The derived evolution of the free magnetic energy and helicity in the active region, shows that our model captures flare signatures, and that the depletion of free magnetic energy spatially aligns with the observed EUV emission. With this we present the first method that can perform realistic magnetic field extrapolations in quasi real-time, which enables advanced space weather monitoring.
We conclude with an outlook on our ongoing work where we extend this approach to create a new class of MHD simulations, that can flexibly incorporate additional observational constraints and perform fast computations.

We present results of generative deep learning as applied to a large database of solar images and discuss challenges in training and validation, in particular with distributed training on GPU supercomputers. Our dataset is based on SDO EUV data with high resolution and with excellent coverage since 2010. This dataset is thus well suited to study the application of advanced machine learning techniques that require large amounts of data for training, such as deep learning approaches. We focus on diffusion type models and generative adversarial deep learning (GANs). We address the potential of data augmentation techniques for improved learning and image quality and the opportunities for latent space structure exploration and control. The challenges in reaching the highest resolutions with good image fidelity are analyzed. Potential application downstream that can make use of such generated images are briefly discussed and the need for a community-driven, physics-based basis to establish evaluation criteria for generative models will be emphasized.

Observations of plasma motions in the low corona are often limited to magnetic field lines originating in active regions, which are ideal for spatial domain enhancements across individual extreme ultraviolet (EUV) images to see loops, flares, and other bright activity contrasted against dim background features.

The quiet Sun is essentially all dim background features, which requires advanced image processing and ideal observation parameters to emphasize the temporal domain in order to visualize faint, fine-scale plasma flows. We utilize time-normalized optical flow (TNOF) on large sets of high cadence EUV data by reducing instrumental noise to a high degree and then emphasizing the minor brightness variations indicative of plasma motion. Maps of plasma flow paths are produced via optical flow tracking algorithms by the computer vision method of Lucas-Kanade and the underlying velocity field is estimated with line integral convolution.

To test the effectiveness of the TNOF approach, we have applied this method to an EUV case study of data from EUI 174 and AIA 171 on 29 March 2022. This date marked a near-perpendicular line of sight orientation between the two spacecraft, had similarly short observation intervals, and provided the opportunity to compare contrast enhanced plasma features off-limb with temporally enhanced on-disk plasma motion.

In this case study, we generated movies and flow paths that show TNOF succeeds at qualitatively outlining plasma flow along magnetic field lines from both Solar Orbiter’s and SDO’s point of view which are in general agreement with potential field models. Additionally, detailed velocities of plasma motion within coronal loops, overall velocity trends, and a new quasi-magnetic flow trend within the quiet Sun are presented.