Pitch Angle Isotropy of Relativistic Electron Microbursts as Observed by SAMPEX/HILT: Statistical and Storm‐Time Properties

Abstract

AbstractObservations of relativistic electron precipitation events from Earth's inner magnetosphere to the upper atmosphere provide essential information about the source/loss processes and dynamics of Earth's radiation belts. Equatorial electron pitch angle isotropy can be an indicator of electron precipitation into the atmosphere, with isotropic populations resulting in a full bounce loss cone (BLC) and anisotropic populations resulting in an emptier BLC. Here we study electron microbursts, which are ˜100 millisecond duration streams of relativistic electrons thought to be scattered into the BLC by particle‐wave scattering. By measuring the ratios of >1 MeV electron flux across different detector rows in the Heavy Ion Large Telescope (HILT) instrument onboard the Solar, Anomalous, and Magnetospheric Particle Explorer (SAMPEX) satellite, we quantify the pitch angle isotropy of an electron population. Applying this calculation to microburst events, we have determined the dependence of microburst pitch angle distributions on burst magnitude, magnetic local time (MLT), L‐Shell, and storm phase. We found that high flux magnitude microbursts are typically highly isotropic, while lower flux magnitude microbursts have varying degrees of anisotropy. Furthermore, microbursts are most isotropic as compared to the background population in the afternoon/evening sector as well as initial and main phases of geomagnetic storms. These results have important implications for microburst scattering mechanisms as well as quantification of precipitation driven by this process.