A Statistical Study of the Dawn-Dusk Asymmetry of Ion Temperature Anistrophy and Mirror Mode Occurrence in the Terrestrial Dayside Magnetosheath Using THEMIS Data

We present a statistical study of ion temperature anisotropy and mirror mode activity in the Earth\u27s dayside magnetosheath using 6 years of Time History of Events and Macroscale Interactions during Substorms (THEMIS) observations focusing on the quantification of dawn-dusk asymmetry as a function of upstream conditions and distance from the magnetopause. Our statistical data show a pronounced dusk favored asymmetry of T⊥/T∥ which drives a similar asymmetry of mirror mode activity. T⊥/T∥ decreases with increasing solar wind Alfvén Mach number, whereas mirror mode occurrence increases. In both cases, the relative asymmetry between the dawn and dusk flanks decrease with increasing Alfvén Mach number. In addition, during the transition from low/moderate MA, there was a shift in our data set from dips to peaks, suggesting that the magnetosheath strongly favors peaks during intervals of higher Mach number. We also observed more mirror modes and larger asymmetry during atypical inward ortho-Parker spiral interplanetary magnetic field compared to the more statistically relevant Parker spiral configuration. Our results are consistent with previous experimental studies of mirror modes and to some extent, numerical models