Driving of Reconnection in the Heliospheric Current Sheet by Thermal Nonequilibrium

Observations have shown that the coupling between the corona and heliosphere is intrinsically dynamic with quasi-periodic density structures ubiquitously seen in the Heliospheric Current Sheet (HCS) that maps down to the top of the streamer belt. These structures have been identified by several authors as due to magnetic reconnection that produces magnetic islands in the HCS. Such islands have important implications for understanding the origins of heliospheric energetic particles and plasma/field variability. A key feature of the density structures is their quasi-periodicity, on time scales of one to two hours. We propose that the mechanism responsible for the periodicity is thermal nonequilibrium (TNE), a process by which solar coronal loops undergo quasi-periodic cycles of heating and cooling due to the spatial localization of coronal heating near the loop base. Since the requirement for TNE onset is that the loop length is large compared to the scale of the heating, it is most likely to occur on the largest coronal loops, those near the open-closed boundary. We use 2.5D MHD numerical simulations to investigate the effect of TNE in the corona and heliosphere of an axisymmetric helmet streamer and polar coronal holes. As in the many 1D loop studies, we find that TNE occurs in coronal loops with sufficiently large length, but in contrast to previous studies, we find that the process also drives substantial magnetic dynamics, especially near the top of the streamer where the plasma beta becomes of order unity. From the simulation results we determine predictions for spectroscopic and imaging observations of both the hot and cool helmet streamer plasma and the solar wind near to the streamer stalk. We conclude that TNE occurring in the largest closed loops in the corona may explain several puzzling observations of the corona and wind, such as the ubiquitous blue shifts observed at the edges of active regions, and the quasi-periodic solar wind blobs. This work was supported by the NASA Living With a Star Program.