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, that focus on studying electron microbursts. High resolution electron data from FIREBIRD-II consists 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 understanding of the scattering mechanism behind electron microbursts by investigating their spectral properties and their relationship to 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 is 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 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.

In this study, by simulating the wave-particle interactions, we show that sub-relativistic/relativistic electron microbursts form the high-energy tail of pulsating aurora (PsA). Whistler-mode chorus waves that propagate along the magnetic field lines at high latitudes cause precipitation bursts of electrons with a wide energy range from a few keV (PsA) to several MeV (relativistic microbursts). The rising tone elements of chorus waves cause individual microbursts of sub-relativistic/relativistic electrons and the internal modulation of PsA with a frequency of a few Hz. The chorus bursts for a few seconds cause the microburst trains of sub-relativistic/relativistic electrons and the main pulsations of PsA. Our simulation studies demonstrate that both PsA and relativistic electron microbursts originate simultaneously from pitch angle scattering by chorus wave-particle interactions.

Microbursts are an impulsive increase of electrons from the radiation belts into the atmosphere and have been directly observed in low Earth orbit and the upper atmosphere. Prior work has estimated that microbursts are capable of rapidly depleting the radiation belt electrons on the order of a day, hence their role to radiation belt electron losses must be considered. Losses due to microbursts are not well constrained, and more work is necessary to accurately quantify their contribution as a loss process. To address this question we present a statistical study of > 35 keV microburst sizes using the pair of AeroCube-6 CubeSats. The microburst size distribution in low Earth orbit and the magnetic equator was derived using both spacecraft. In low Earth orbit, the majority of microbursts were observed while the AeroCube-6 separation was less than a few tens of km, mostly in latitude. To account for the statistical effects of random microburst locations and sizes, Monte Carlo and analytic models were developed to test hypothesized microburst size distributions. A family of microburst size distributions were tested and a Markov Chain Monte Carlo sampler was used to estimate the optimal distribution of model parameters. Finally, a majority of observed microbursts map to sizes less then 200 km at the magnetic equator. Since microbursts are widely believed to be generated by scattering of radiation belt electrons by whistler mode waves, the observed microburst size distribution was compared to whistler mode chorus size distributions derived in prior literature.

We compute quasilinear diffusion rates due to pitch-angle scattering by various mechanisms in the Earth’s electron radiation belts. The calculated theoretical lifetimes are compared with observed decay rates and we find excellent qualitative agreement between the two. The overall structure of the observed lifetime profiles as a function of energy and is largely due to plasmaspheric hiss and Coulomb scattering. The results also reveal a local minimum in lifetimes in the inner zone at lower energy (~50 keV), attributed to enhanced scattering via ground-based VLF transmitters, and a reduction in lifetimes at higher and energy (>1 MeV), attributed to enhanced EMIC wave scattering. In addition, we find significant quantitative disagreement at <3.5, where the theoretical lifetimes are typically a factor of ~10 larger than the observed, pointing to an additional loss process that is missing from current models. We discuss potential factors that could contribute to this disagreement.

We explore the hypothesis that electron precipitation curtains such as those observed by the AeroCube-6 satellite pair can be produced by electron microbursts. Precipitation curtains are latitudinal structures of stable precipitation that persist for timescales of 10s of seconds or longer. The electrons involved have energies of 10s-100s of keV. The microburst formation hypothesis states that a source region in the equatorial region produces a series of very low frequency chorus wave emissions. Each of these emissions in turn produces a microburst of electron precipitation, filling the drift and bounce loss cone on the local field line. Electrons in the drift loss cone remain on the field line and bounce-phase mix over subsequent bounces while also drifting in azimuth. When observed at downstream azimuths by a satellite equipped with an integral energy sensor, no bounce phase structure remains, or, equivalently, the same time profile is present when two such satellites pass by many seconds apart. The spatial structure that remains reflects the pattern of microburst sources. Statistical studies of where and when curtains occur have indicated that some, but not all, curtains could be caused by microbursts. We use test particle tracing in a dipole magnetic field to show that spatially stationary source regions generating periodic microbursts can produce curtain signatures azimuthally downstream. We conclude that one viable explanation for many of the curtains observed by the AeroCube-6 pair is the accumulation of drift-dispersed microburst electron byproducts in the drift loss cone.

Curtain precipitation are a recently discovered stationary, persistent, and latitudinally narrow electron precipitation phenomenon in low Earth orbit. Curtains are observed over consecutive passes of the dual AeroCube-6 CubeSats while their in-track lag varied from a fraction of a second to 65 seconds, with dosimeters that are sensitive to > 30 keV electrons. This study uses the AeroCube-6 mission to quantify the statistical properties of 1,634 curtains observed over three years. We found that many curtains are narrower than 10 kilometers in the latitudinal direction with 90\% narrower than 20 kilometers, corresponding to a few hundred kilometer radial size at the magnetic equator. We examined the magnetic local time and geomagnetic dependence of curtains. We found that curtains are observed in the late-morning and pre-midnight magnetic local times, with a higher occurrence rate at pre-midnight, and curtains are observed more often during times of enhanced Auroral Electrojet. We found a few curtains in the bounce loss cone region above the north Atlantic, whose electrons were continuously scattered for at least 6 seconds. Such observations suggest that continuous curtain precipitation may be a significant loss of > 30 keV electrons from the magnetosphere into the atmosphere, possibly scattered by a parallel direct current electric field.

We use measurements from NASA’s Van Allen Probes to calculate the decay time constants for electrons over a wide range of energies (30 keV - 4 MeV) and values ( = 1.3 - 6.0) in the Earth’s radiation belts. Using an automated routine to identify flux decay events, we construct a large database of lifetimes for near-equatorially-mirroring electrons over a 5-year interval. We find long lifetimes (~100 days) in the inner zone that are largely independent of energy, contrasted with shorter, energy-dependent lifetimes (~1-20 days) in the slot region and outer zone. We compare our lifetime calculations with prior empirical estimates and find good quantitative agreement. The comparisons suggest that some prior estimates may overestimate electron lifetimes between ≈ 2.5-4.5 due to instrumental effects and/or background contamination. Previously reported two-stage decays are explicitly demonstrated to be a consequence of using integral fluxes.