The Energy Spectra of Electron Microbursts Between 200 keV and 1 MeV

Abstract

This study investigates the energy spectrum of electron microbursts observed by the Focused Investigations of Relativistic Electron Burst Intensity, Range, and Dynamics II (FIREBIRD‐II, henceforth FIREBIRD) CubeSats. FIREBIRD is a pair of CubeSats, launched in January 2015 into a low Earth orbit, which focuses on studying electron microbursts. High‐resolution electron data from FIREBIRD‐II consist of 5 differential energy channels between 200 keV and 1 MeV and a >1 MeV integral channel. This covers an energy range that has not been well studied from low Earth orbit with good energy and time resolution. This study aims to improve the understanding of the scattering mechanism behind electron microbursts by investigating their spectral properties and their relationship with the equatorial electron population under different geomagnetic conditions. Microbursts are identified in the region of the North Atlantic where FIREBIRD only observes electrons in the bounce loss cone. The electron flux and exponential energy spectrum of each microburst are calculated using a FIREBIRD instrument response modeled in GEANT4 (GEometry ANd Tracking) and compared with the near‐equatorial electron spectra measured by the Van Allen Probes. Microbursts occurring when the Auroral Electrojet (AE) index is enhanced tend to carry more electrons with relatively higher energies. The microburst scattering mechanism is more efficient at scattering electrons with lower energies; however, the difference in scattering efficiency between low and high energy is reduced during periods of enhanced AE.

Characteristics of Electron Microburst Precipitation Based on High‐Resolution ELFIN Measurements

“…Previous studies have provided statistics of mainly >1 MeV microbursts (Douma et al, 2017(Douma et al, , 2019Lorentzen et al, 2001). The recent FIRE-BIRD CubeSats have also mainly measured only the upper part (>250 keV) of the microburst energy spectrum (Johnson et al, 2021), although their lowest energy channel (∼200 keV) has been operating during the first 6 months in space (Crew et al, 2016;Johnson et al, 2021). However, 150 keV and >1 MeV microbursts are not always correlated (Blake et al, 1996), which could imply either altogether different wave-scattering mechanisms at low and high energies or the occasional appearance of more oblique chorus waves at higher latitudes (Lorentzen et al, 2001).…”

Observation of an Electron Microburst With an Inverse Time‐Of‐Flight Energy Dispersion

“…In energy, we describe the uncertainty with three assumptions: no uncertainty, electrons uniformly distributed within each energy channel range, and electrons exponentially distributed within each energy channel range. While we tested all three assumptions, due to the exponentially‐falling microburst energy spectrum (Johnson et al., 2021), the exponential energy channel assumption is the most realistic. For the linear fit prior we assumed y ‐intercept ∼ Normal ( μ = 0, σ = 50) with units of [ms], and slope ∼ Normal ( μ = 0, σ = 5) with units [ms/keV].…”

Achievements and Lessons Learned From Successful Small Satellite Missions for Space Weather‐Oriented Research

“…This information provides constraints on the physical scattering process and on total radiation belt loss due to microbursts. Measuring electron microbursts with high energy and time resolution (Johnson et al, 2021), FIREBIRD has helped us determine what plasma conditions contribute to the processes that scatter electrons from the magnetosphere into the Earth's upper atmosphere. While the first two FIREBIRD science objectives constrain the physical processes that generates relativistic electron microburst precipitation, the final objective quantifies the geoeffectiveness and overall space weather impact.…”