SOLARNET Conference: The Many Scales of the Magnetic Sun

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.