Loss time scales of plasma sheet electrons in the morning side: Analysis based on THEMIS observations

One of the dominant loss processes of electrons in the inner magnetosphere is pitch angle scattering by plasma waves. Electrons scattered into the loss cone precipitate into the atmosphere and contribute to diffuse auroral emissions. However, there is still much controversy on dominant scattering mechanism of plasma sheet electrons because electrons resonate with both electrostatic electron cyclotron harmonic (ECH) waves and whistler-mode waves. The purpose of this study is to investigate what waves are mainly responsible for the loss of the plasma sheet electrons. We estimated loss time scales of the plasma sheet electrons from the THEMIS observations, and compared them with the theoretical loss time scales due to the pitch angle scattering by whistler-mode chorus. We have derived global distributions of the average phase space density (PSD) in the first adiabatic invariants of 50 and 100 eV/nT, using the electron data obtained from the electrostatic analyzer (ESA) on board the THEMIS satellites for 2 years. The electron loss time scales were estimated, based on the PSD distributions, from spatial gradients of the PSD along the drift paths that are calculated from the UNH-IMEF electric field and TS04 magnetic field models. The theoretical loss time scales were evaluated from the pitch angle diffusion coefficients due to a typical spectrum of whistler-mode chorus waves. We also estimated the required wave amplitudes that can explain the loss time scales based on PSD distributions and compared them with the average chorus wave amplitudes obtained from the THEMIS wave measurements. The result showed that the required wave amplitudes are roughly consistent with the observed chorus amplitudes. These investigations strongly suggest that whistler-mode chorus is responsible for the loss of the plasma sheet electrons and contributes to the generation of diffuse auroras in the morning side. We will extend the estimation of the loss time scales to lower and higher first adiabatic invariants to investigate the energy dependence of the electron loss processes.