The Effect of Thermal Nonequilibrium on Helmet Streamers

Solar loops in which the coronal heating scale is short compared to the loop length are known to be susceptible to thermal nonequilibrium (TNE). We investigate the effects of this process on the largest loops in the corona, those of a helmet streamer. Our numerical study uses a 2.5D MHD code that includes the full magnetic field dynamics as well as the detailed plasma thermodynamics. The simulation model is axisymmetric, consisting of an equatorial streamer belt and two polar coronal holes. As in previous 1D loop studies, we find that TNE occurs in coronal loops with sufficiently large length, but in contrast to these 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. Simulations are preformed using different scale heights for the heating and different numerical resolution in order to determine the dependence of our findings on these important parameters. We conclude that TNE in streamers may explain several puzzling observations, such as the ubiquitous blueshifts observed at the edges of active regions. We also discuss the implications of our results for the solar wind.