Large-Scale Simulations Of Solar Wind Ion Entry And Dayside Precipitation: Dawn-Dusk Asymmetry

We have carried out global magnetohydrodynamics (MHD) simulations together with large-scale kinetic (LSK) simulations to investigate asymmetries in solar wind ion entry and dayside precipitation. Our study found an unexpected dawn-dusk asymmetry in the precipitation of low to middle energy ions for steady southward IMF (B-X = B-Y = 0). We show that the asymmetry results from the direction of the parallel electric field at the magnetopause. In the Northern Hemisphere, the parallel electric field is directed toward Earth in the prenoon sector and away from it in the afternoon sector. Hence, ions are accelerated toward Earth in the prenoon sector and away from it in the afternoon sector. The asymmetry is consistent with results of a statistical study of DMSP low-altitude data showing that ion precipitation from the mantle is predominantly seen over the morning and prenoon sectors. Results suggest that dawn-dusk asymmetries found in solar wind ion entry and dayside precipitation do not require the presence of a B-Y component in the IMF.