Magnetosheath – Interplanetary medium reference frame: Application for a statistical study of mirror type waves in the terrestrial plasma environment

A Magnetosheath – interplanetary medium (MIPM) reference frame is introduced to study various dynamical processes in the region between the planetary bow shock and the magnetosphere. The MIPM reference frame uses recently developed analytical models to determine the bounding magnetopause and bow shock surfaces, and a fractional distance across the magnetosheath. The inputs of the model are the magnetic field and solar wind plasma parameters measured in the interplanetary medium. Angular coordinates of the MIPM reference frame (zenith and clock angles) correspond to those used in the geocentric interplanetary medium (GIPM) frame.The MIPM reference frame is applied then for a statistical study of the frequency of mirror type magnetic field depressions (linearly polarized, relatively large amplitude and symmetric shape) in the terrestrial magnetosheath based on the 10 years long 4 s resolution ISEE-1 magnetic field observations. It was found that these events most frequently occurred in the inner regions of the magnetosheath: close to the magnetopause at larger zenith angles and closer to the middle of the sheath in the subsolar region. Mirror type depressions were least frequently observed in the ‘dawn’ magnetosheath of the MIPM reference frame (downstream of the quasiparallel part of the bow shock), but their relative intensity was the largest in this region. The relative intensity of mirror type depressions was smaller in magnetosheath regions located closer to the bow shock and farther away from the subsolar point. These results suggest that pressure anisotropy raising downstream of quasiperpendicular bow shocks is favorable for the generation of mirror mode waves in addition to magnetic field draping at the magnetopause.