On the evolutionary aspects of solar coronal holes

Coronal holes (CH) are large, long-lived structures commonly observed in the solar corona as regions of reduced emission in EUV and X-ray wavelengths. They feature a characteristic open magnetic field configuration along which ionized electrons and atoms are accelerated into the interplanetary space. The resulting outflowing plasma is called high seed solar wind stream (HSS; see Cranmer 2009 and references therein). These HSSs are the major cause of minor to moderate geomagnetic activity at Earth (see Richardson 2018 and references therein).

To be able to predict the arrival and impact of those disturbances accurately, their origin and evolution need to be studied in detail. And to do so, it is imperative that CHs are accurately and reliably extracted, thus leading to the development of the Collection of Analysis Tools for Coronal Holes (CATCH). By using the intensity gradient across the CH boundary, it is possible to robustly extract CHs whose properties can then be analyzed. We find that the area of long-living CHs generally evolves by growing to a maximum before decaying. However, the associated magnetic field does not evolve equally. Depending on the CH, we find a correlation, an anti-correlation or even no correlation over the course of its lifetime. Therefore, we believe that the evolution of a CHs magnetic field is primarily driven by the large-scale connectivity changes in the Sun's global magnetic field. Further, we find that the plasma properties within CHs show a significant center to boundary gradient, which may justify the distance-to-boundary parameter used in some solar wind modeling.

To study the evolution of CHs in detail, a 360° view of the Sun is necessary; however, the magnetic far-side of the Sun still eludes. The few snapshots with Solar Orbiter provide only a fragmented picture of the magnetic field on the solar far-side. We found that by using EUV observations of the transition region (specifically using Stereo) it is possible to estimate the magnetic field density of CHs on the solar far side. In addition, we are currently investigating the incorporation of helioseismic observations into synoptic magnetograms to generate a maps that show the magnetic field of the whole Sun at a given time.