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

Observations 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. Plain Language Summary: Understanding the fundamental mechanisms that drive electron loss from Earth's radiation belts is essential for the success of various human operations. During geomagnetically active periods, increased electron fluxes around Earth can damage orbiting satellites, cause GPS signal errors, and black out ground‐based power grids. When electrons resonate with plasma waves in the magnetosphere, sub‐second bursts of electrons can be generated. These loss mechanisms, called microbursts, often cause electrons previously trapped in the radiation belts to stream into the atmosphere. Although past studies have quantified total microburst‐induced electron loss for given time periods, assumptions were made regarding how directly electrons were scattered into the atmosphere. This study uses differential electron count rates across the detector rows of the HILT instrument onboard the SAMPEX satellite to develop a qualitative measurement of the distribution of electrons within microbursts relative to the local magnetic field lines. By applying this measurement, called the isotropy index, to past calculations of electron loss, a more accurate electron loss rate associated with microbursts could be determined. Furthermore, the spatial and storm‐time dependence of the microburst isotropy index is detailed in the results section. Key Points: The pitch angle distribution of electrons within MeV microbursts is explored using 1993 data from SAMPEX/HILTLow flux magnitude microbursts have varying levels of anisotropy and high flux magnitude microbursts are consistently close to full isotropyMicroburst isotropy also shows trends with respect to local time and storm phase [ABSTRACT FROM AUTHOR]