We present an artificial neural network (ANN) model that reconstructs > 30 keV electron flux measurements near the geomagnetic equator from low-Earth-orbit (LEO) observations, exploiting the global coherent nature of the high-energy trapped electrons that constitute the radiation belts. To provide training data, we analyze magnetic conjunctions between one of National Oceanic and Atmospheric Administration’s (NOAA’s) Polar Orbiting Environmental Satellites (POES) and National Aeronautics and Space Administration’s (NASA’s) Van Allen Probes. These conjunctions occur when the satellites are connected along the same magnetic field line and allow for a direct comparison of satellites’ electron flux measurements for one integral energy channel, > 30 keV and over 64,000 such conjunctions have been identified. For each conjunction, we fit the equatorial pitch angle distribution (PAD) parameterized by the function JD = C·sinNα. The resulting conjunction dataset contains the POES electron flux measurements, L and MLT coordinates, geomagnetic activity AE index, and C and N coefficients from the PAD fit for each conjunction. We test combinations of input variables from the conjunction dataset and achieve the best model performance when we use all the input variables during training. We present our model’s prediction for the out-of-sample data that agrees well with observations, R2 > 0.80. We demonstrate the ability to nowcast and reconstruct equatorial electron flux measurements from LEO without the need for an in-situ equatorial satellite. The model can be expanded to include existing LEO data and has the potential to be used as a basis of future radiation-belt monitoring LEO constellations.

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 present, for the first time, a plasmaspheric hiss event observed by the Van Allen probes in response to two successive interplanetary shocks occurring within an interval of ~2 hours on December 19, 2015. The first shock arrived at 16:16 UT and caused disappearance of hiss for ~30 minutes. Significant Landau damping by suprathermal electrons followed by their gradual removal by magnetospheric compression opposed the generation of hiss causing the disappearance. Calculation of electron phase space density and linear wave growth rates showed that the shock did not change the growth rate of whistler mode waves within the core frequency range of plasmaspheric hiss (0.1 - 0.5 kHz) during this interval making conditions unfavorable for the generation of the waves. The recovery began at ~16:45 UT which is attributed to an enhancement in local plasma instability initiated by the first shock-induced substorm and additional possible contribution from chorus waves. This time, the wave growth rate peaked within the core frequency range (~350 Hz). The second shock arrived at 18:02 UT and generated patchy hiss persisting up to ~19:00 UT. It is shown that an enhanced growth rate and additional contribution from shock-induced poloidal Pc5 mode (periodicity ∼240 sec) ULF waves resulted in the excitation of hiss waves during this period. The hiss wave amplitudes were found to be additionally modulated by background plasma density and fluctuating plasmapause location. The investigation highlights the important roles of interplanetary shocks, substorms, ULF waves and background plasma density in the variability of plasmaspheric hiss.

We report plasma wave observations of equatorial magnetosonic waves at integer harmonics of the local gyrofrequency of doubly-ionized helium (He++). The waves were observed by Van Allen Probe A on 08 Feb 2014 when the spacecraft was in the afternoon magnetic local time sector near L ≈ 4 inside of the plasmasphere. Analysis of the complementary in-situ energetic ion measurements (1-300 keV) reveals the presence of a helium ion ring distribution centered near 30 keV. Theoretical linear growth rate calculations suggest that the local plasma and field conditions can support the excitation of the magnetosonic waves from the unstable ring distribution. This represents the first report of the generation of magnetosonic equatorial noise via a ring distribution in energetic He++ ions in the near-Earth space plasma environment.

The MMS spacecraft routinely observe electron “microinjections” at energies in the 10s-100s keV range across the nightside magnetosphere at distances <20 Re. Microinjections are typically observed in clusters where multiple dispersed injection signatures are recorded in succession over a short time interval (e.g., ~10 in one hour) and may be related to surface wave activity at the magnetopause. Recent work has shown that microinjections have distinctive features in the angular distributions, where field-aligned distributions are observed near dusk, while trapped distributions are observed near dawn. Due to their recent discovery, the origin and generation of microinjections has yet to be conclusively identified and detailed studies thus far have largely been done on a case-by-case basis. In an effort to elucidate more general properties and characteristics of microinjections, we describe an automated routine designed to identify microinjection signatures in the MMS/FEEPS measurements. The algorithm uses image processing techniques (the radon transform) and is based on a similar method developed to identify whistler-mode chorus elements in Van Allen Probes wave observations (Sen Gupta et al., [2017]). We present an initial set of results from a statistical database of microinjection events obtained from the automated algorithm to further our understanding of this intriguing phenomenon.

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.

We present simulations of the outer radiation belt electron flux during the March 2015 and March 2013 storms using a radial diffusion model. Despite differences in Dst intensity between the two storms the response of the ultra-relativistic electrons in the outer radiation belt was remarkably similar, both showing a sudden drop in the electron flux followed by a rapid enhancement in the outer belt flux to levels over an order of magnitude higher than those observed during the pre-storm interval. Simulations of the ultra-relativistic electron flux during the March 2015 storm show that outward radial diffusion can explain the flux dropout down to L*=4. However, in order to reproduce the observed flux dropout at L*<4 requires the addition of a loss process characterised by an electron lifetime of around one hour operating below L*~3.5 during the flux dropout interval. Nonetheless, during the pre-storm and recovery phase of both storms the radial diffusion simulation reproduces the observed flux dynamics. For the March 2013 storm the flux dropout across all L-shells is reproduced by outward radial diffusion activity alone. However, during the flux enhancement interval at relativistic energies there is evidence of a growing local peak in the electron phase space density at L*~3.8, consistent with local acceleration such as by VLF chorus waves. Overall the simulation results for both storms can accurately reproduce the observed electron flux only when event specific radial diffusion coefficients are used, instead of the empirical diffusion coefficients derived from ULF wave statistics.