Plasmaspheric virtual resonances in the inner magnetosphere

Abstract:

Low-latitude Pi2 pulsations have been attributed to the fast mode waves trapped in the Earth's plasmasphere. The conventional idea of this scenario requires somewhat rigid boundaries, the ionosphere and plasmapause, where waves can be reflected off of and get trapped. Sometimes, however, the concept of the plasmapause is inapplicable for this description when the Earth's magnetosphere is devoid of a sharp density drop or if the plasmasphere is too small to confine such a low frequency waves in the Pi2 range. We have conducted 3D MHD simulations in dipole coordinates to understand the generation of Pi2 pulsations at various plasmaspheric conditions. We start from the case at which the plasma density drops rapidly that forms the well-defined plasmapause to where the plasma density decreases monotonically, representing geomagnetically disturbed and quiet time, respectively. Our simulations show the characteristic modes excited in the inner magnetosphere regardless of the shape of the plasmapause. Our results are favorable to the plasmaspheric virtual resonance as a generation mechanism of the low-latitude Pi2 pulsations and able to explain the occurrence of Pi2 pulsations during geomagnetic quiescence when the plasmasphere can reach the saturated state.