Paper detail

How Can Active Region Plasma Escape into the Solar Wind from below a Closed Helmet Streamer?

Recent studies show that active-region (AR) upflowing plasma, observed by the EUV-Imaging Spectrometer (EIS), onboard Hinode, can gain access to open field-lines and be released into the solar wind (SW) via magnetic-interchange reconnection at magnetic null-points in pseudo-streamer configurations. When only one bipolar AR is present on the Sun and it is fully covered by the separatrix of a streamer, such as AR 10978 in December 2007, it seems unlikely that the upflowing AR plasma can find its way into the slow SW. However, signatures of plasma with AR composition have been found at 1 AU by Culhane et al. (2014) apparently originating from the West of AR 10978. We present a detailed topology analysis of AR 10978 and the surrounding large-scale corona based on a potential-field source-surface (PFSS) model. Our study shows that it is possible for the AR plasma to get around the streamer separatrix and be released into the SW via magnetic reconnection, occurring in at least two main steps. We analyse data from the Nançay Radioheliograph (NRH) searching for evidence of the chain of magnetic reconnections proposed. We find a noise storm above the AR and several varying sources at 150.9 MHz. Their locations suggest that they could be associated with particles accelerated during the first-step reconnection process and at a null point well outside of the AR. However, we find no evidence of the second-step reconnection in the radio data. Our results demonstrate that even when it appears highly improbable for the AR plasma to reach the SW, indirect channels involving a sequence of reconnections can make it possible.