The short-term stability and tilting motion of a well-observed low-latitude solar coronal hole

Abstract

Context. Our understanding of the solar magnetic coronal structure is tightly linked to the shape of open field regions, specifically coronal holes. A dynamically evolving coronal hole coincides with the local restructuring of open to closed magnetic field, which leads to changes in the interplanetary solar wind structure. Aims. By investigating the dynamic evolution of a fast-tilting coronal hole, we strive to uncover clues about what processes may drive its morphological changes, which are clearly visible in extreme ultraviolet (EUV) filtergrams. Methods. Using combined 193 Å and 195 Å EUV observations by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory and the Extreme UltraViolet Imager on board the Solar Terrestrial Relations Observatory-Ahead, in conjunction with line-of-sight magnetograms taken by the Helioseismic and Magnetic Imager, also on board the Solar Dynamics Observatory, we tracked and analyzed a coronal hole over 12 days to derive changes in morphology, area, and magnetic field. We complemented this analysis by potential field source surface modeling to compute the open field structure of the coronal hole. Results. We find that the coronal hole exhibits an apparent tilting motion over time that cannot solely be explained by solar differential rotation. It tilts at a mean rate of ∼3.2°  day−1 that accelerates up to ∼5.4°  day−1. At the beginning of May the area of the coronal hole decreased by more than a factor of three over four days (from ∼13 × 109 km2 to ∼4 × 109 km2), but its open flux remained constant (∼2 × 1020 Mx). Furthermore, the observed evolution is not reproduced by modeling that assumes the coronal magnetic field to be potential. Conclusions. In this study we present a solar coronal hole that tilts at a rate that has yet to be reported in literature. The rate exceeds the effect of the coronal hole being advected by either photospheric or coronal differential rotation. Based on the analysis we find it likely that this is due to morphological changes in the coronal hole boundary caused by ongoing interchange reconnection and the interaction with a newly emerging ephemeral region in its vicinity.