SOLARNET Conference: The Many Scales of the Magnetic Sun

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.