Elemental Abundances at Coronal Hole Boundaries as a Means to Investigate Interchange Reconnection and the Solar Wind

Abstract

The origin of the slow solar wind is not well understood, unlike the fast solar wind which originates from coronal holes. In-situ elemental abundances of the slow solar wind suggest that it originates from initially closed field lines that become open. Coronal hole boundary regions are a potential source of slow solar wind as there open field lines interact with the closed loops through interchange reconnection. Our primary aim is to quantify the role of interchange reconnection at the boundaries of coronal holes. To this end, we have measured the relative abundances of different elements at these boundaries. Reconnection is expected to modulate the relative abundances through the first ionization potential (FIP) effect. For our analysis we used spectroscopic data from the extreme ultraviolet imaging spectrometer (EIS) on board Hinode. To account for the temperature structure of the observed region we computed the differential emission measure (DEM). Using the DEM we were able to infer the ratio between coronal and photospheric abundances, known as the FIP bias. By examining the variation of the FIP bias moving from the coronal hole to the quiet Sun, we have been able to constrain models of interchange reconnection. The FIP bias variation in the boundary region around the coronal hole has an approximate width of 30-50 Mm, comparable to the size of supergranules. This boundary region is also a source of open flux into interplanetary space. We find that there is an additional $\sim$ 30$\%$ open flux that originates from this boundary region.