Relativistic electron dropouts by pitch angle scattering in the geomagnetic tail

Abstract. Relativistic electron dropout (RED) events are characterized by fast electron flux decrease at the geostationary orbit. It is known that the main loss process is non adiabatic and more effective for the high energy particles. RED events generally start to occur at midnight sector and propagate to noon sector and are correlated with magnetic field stretching. In this paper, we discuss this kind of event can be caused from pitch angle diffusion induced when the gyro radius of the electrons is comparable to the radius of curvature of the magnetic field and the magnetic moment is not conserved any more. While this process has been studied theoretically, the question is whether electron precipitation could be explained with this process for the real field configuration. This paper will show that this process can successfully explain the precipitation that occurred on 14 June 2004 observed by the low-altitude (680 km) polar orbiting Korean satellite, STSAT-1. In this precipitation event, the energy dispersion showed higher energy electron precipitation occurred at lower L values. This feature is a good indicator that precipitation was caused by the magnetic moment scattering in the geomagnetic tail. This interpretation is supported by the geosynchronous satellite GOES observations that showed significant magnetic field distortion occurred on the night side accompanying the electron flux depletion. Tsyganenko-01 model also shows the magnetic moment scattering could occur under the geomagnetic conditions existing at that time. We suggest the pitch angle scattering by field curvature violating the first adiabatic invariant as a possible candidate for loss mechanism of relativistic electrons in radiation belt.