SOLARNET Conference: The Many Scales of the Magnetic Sun

Coronal bright points (CBPs) are systems of small, 1–3 MK hot loops, with lengths of about 10–20 Mm, and lifetimes of many hours. They are connected to bipolar magnetic field concentrations in the photosphere. CBPs are prominent and widespread in the quiet solar corona and coronal holes, best observed in the extreme-ultraviolet (EUV) and X-rays. Investigating their thermal structure will provide important constraints on the processes heating the plasma confined in the CBPs.

We report on the thermal characteristics of CBPs using quiet-Sun observations from the EUV spectrometer SPICE onboard the Solar Orbiter. One unique feature of SPICE is its simultaneous coverage of a broad temperature range from coronal plasma to the low transition region.

Overall, we find that the differential emission measure (DEM) in the CBPs increases at all temperatures when compared to the quiet Sun. Furthermore, the peak of the DEM in the corona shifts from $\log T \approx 6.2$ in the quiet Sun, to about $\log T \approx 6.4$ in the CBPs. At the low-temperature end, in the low transition region below $\log T \approx 5.2$, the negative slope of the DEM is rather similar to that of the quiet Sun. This implies that, in response to the additional heating in the CBP, the density increases by the same factor at all temperatures below $\log T \approx 5.2$. In the upper transition region, above temperatures of $\log T \approx 5.2$, the situation is quite different. Here, the increase of the DEM towards the corona (up to $\log T \approx 6.3$) is significantly shallower than in the quiet Sun.

In previous studies, the slope of the DEM curve between its minimum in the transition region and its maximum in the corona has been interpreted in terms of the waiting-time distribution of heating events. If the heating frequency is high, such that the coronal plasma has no time to cool, then the temperature distribution in the coronal part will be narrow, and thus the gradient of the DEM quite steep. If the waiting time between heating pulses is longer, then the coronal part can cool, and more plasma will be found also at temperatures well below 1 MK. Hence, the gradient of the DEM between $\log T \approx 5$ and 6 would be much smaller. Our finding, that the gradient of the DEM in this range in the CBP is very shallow, would suggest that the heating in the CBP would occur in larger, less frequent events as compared to the quiet Sun. To further test if this scenario applies, we will complement SPICE data with coronal observations from the imager EUI onboard the Solar Orbiter, that provide information on the coronal emission at very high spatial and temporal cadence.