The Rapid Responses of Magnetosonic Waves to the Compression and Expansion of Earth's Magnetosphere

Abstract

AbstractUnder different solar wind dynamic pressures, we observed the magnetosonic (MS) wave amplification and attenuation associated with the compression and expansion of the Earth's magnetosphere. By analyzing the wave and particle variations recorded by the twin Van Allen Probes, we found that the magnetospheric compression or expansion can alter the keV proton phase space density distribution in velocity space and thus affects the MS wave intensity in upper band (f > 50 Hz) in the dawnside magnetosphere (magnetic local time ~ 4.0–7.9 and L ~ 5.7–3.0). During the magnetospheric compression period, the reduction of the 0.1 to 2 keV protons and the enhancement of the 3 to 7 keV protons form a positive phase space density gradient in their velocity space (i.e., the proton ring distribution), and meanwhile, the upper band MS waves are significantly amplified in the proton ring distribution region. During the subsequent magnetospheric expansion period, the enhancement of the 0.1 to 2 keV protons and the reduction of the protons above 3 keV produce a negative phase space density gradient in their velocity space, and meanwhile, the upper band MS waves are rapidly damped. These observations demonstrate that the intensity of the upper band MS waves in the dawnside magnetosphere is highly variable during the change in solar wind pressure.