THEMIS Observations of Electron Pitch Angle Distributions in the Pc5 Compressional Waves

Abstract:

Magnetospheric Pc5 compressional waves are 150 ~ 600s periodic magnetic field oscillations observed in the Earth’s magnetosphere. Since they were first observed in 1960s, their characters such as generation mechanisms, plasma environment and distributions have been widely studied. Previous studies show that they can be usually excited by drift mirror instability. However, because of the lack of the high-resolution data the particle distributions in the waves is less studied. In this work we use THEMIS A data from 2007 to 2016 to investigate the electron distributions in the compressional waves. We find that in the mirror structures the electrons have different kinds of pitch angle distributions (PADs): 1. Typical mirror trapped profile (maximum flux in the magnetic valley at 90° pitch angle); 2. Mirror trapped ‘donut’ shape (butterfly PADs in the magnetic trough region); 3. Flied-align (maximum flux at the 0, 180° pitch angles) and modulated by the waves. Each type shows different energy dependence, which means for a certain wave event different energy range can have different PAD types. These three types of PADs tend to occur at 5~32keV, 0.2~5 keV and >32keV energy ranges, respectively. We attribute the formation of last two types of PADs to the betatron cooling process. Other possible factors including drift-shell splitting and whistler wave interaction inside the magnetic field trough region are also considered.