Yearly variations of magnetosheath ion density dawn-dusk asymmetry and its impact on viscous plasma transport

Abstract:

The local and global properties of the magnetosheath regulate the mass and momentum which enters the magnetosphere. One pronounced feature of the magnetosheath is the presence of dawn-dusk asymmetries which are to some extent, dependent on upstream solar wind conditions. The present poster focuses exclusively on the dawn-dusk asymmetry of ion density in close vicinity to the dayside magnetopause. We study the yearly variation of the magnitude of this asymmetry over the THEMIS era between 2008 and 2015. During the 3 years surrounding 2009 (solar minimum) we observe a strong dawn-favoured asymmetry (~20%) and then a reduced value for the years following 2011. From a statistical standpoint, we attempt to identify the physical mechanisms responsible for this yearly variation. We also investigate the underlying mechanism generating the asymmetry from both an MHD and kinetic perspective. The susceptibility of the magnetopause to the Kelvin-Helmholtz Instability (KHI) depends also on the density gradient of the adjacent magnetopause plasma. Therefore, we show local MHD and Hall-MHD simulations of the KHI based on initial conditions determined statistically from yearly THEMIS observations. These experimental and simulated results are discussed in combination with our previous studies, and are used to determine the magnetospheric impact of magnetosheath dawn-dusk asymmetries and their temporal variability.