Determination of the Equatorial Electron Differential Flux From Observations at Low Earth Orbit

Abstract

Variations in the high-energy relativistic electron flux of the radiation belts depend on transport, acceleration, and loss processes, and importantly on the lower-energy seed population. However, data on the seed population is limited to a few satellite missions. Here we present a new method that utilizes data from the Medium Energy Proton/Electron Detector on board the low-altitude Polar Operational Environmental Satellites to retrieve the seed population at a pitch angle of 90 ∘ . The integral flux values measured by Medium Energy Proton/Electron Detector relate to a low equatorial pitch angle and were converted to omnidirectional flux using parameters obtained from fitting one or two sin N functions to pitch angle distributions given by three and a half years of Van Allen Probes data. Two methods to convert from integral to differential flux are explored. One utilizes integral and differential flux energy distributions from the AE9 model, the second employs an iterative fitting approach based on a Reverse Monte Carlo (RMC) method. The omnidirectional differential flux was converted to an equatorial pitch angle of 90 ∘ , again using statistical pitch angle distributions from Van Allen Probe data. We validate the resulting 90 ∘ flux for 100-to 600-keV electrons against measurements from the Van Allen Probes and show an average agreement within a factor of 4 for L* > 3.7. The resulting data set offers a high time resolution, across multiple magnetic local time planes, and may be used to formulate event-specific low-energy boundary conditions for radiation belt models.Prior to 2012, the National Oceanic and Atmospheric Administration (NOAA) Polar Operational Environmental Satellites (POES) were operational. The POES satellites carry the Space Environment Monitor (SEM-2) Medium Energy Proton and Electron Detector (MEPED), which measures >30-, >100-, and >300-keV electron data at a 2-s resolution (Evans & Greer, 2004). Launched in May 1998, NOAA15 was the first POES satellite to carry SEM-2. Since then, a further four POES satellites have been launched and, to date, three POES satellites are still sampling data. POES MEPED measurements are therefore available for more than the last 19 years, providing a wealth of information on seed population electrons. Operating in a ∼98.5 ∘ inclination low Earth orbit, these satellites sample the electron flux across a broad range of the magnetic coordinate L* (Roederer, 1970). During quiet conditions, POES coverage can extend from L* < 1.3 to L* > 8.5; a larger L* range than possible with the Van Allen Probes. Due to the orbit, the POES satellites offer very rapid measurements of the radiation belts and, with up to five satellites sampling the region, provide data in multiple magnetic local time (MLT) planes. However, one of the major limitations of the POES data set for studying the seed population is that the electron channels of MEPED only supply integral flux. Additionally, measurements are taken near the bottom of magnetic field lines, and the electron flux sampled is cons...

Medium Energy Electron Flux in Earth's Outer Radiation Belt (MERLIN): A Machine Learning Model

“…Therefore, at higher magnetic latitudes, the satellites sample only a part of the equatorial pitch angle distribution. However, using the appropriate pitch angle models, it is possible to reconstruct the values of equatorial flux from MERLIN predictions (e.g., using a methodology of Allison et al., 2018). Further directions for the present study include, first, a more refined analysis of the feature importance using the appropriate permutation and Shapley value methods and the corresponding feature selection.…”

Particle Dynamics in the Earth's Radiation Belts: Review of Current Research and Open Questions

“…Precomputed empirical models for electron pitch angle distribution can be useful for initial and boundary conditions, analytical estimates, etc. PSD models are legion in the literature (e.g., Vampola, ; Horne, Meredith, et al, ; Gannon et al, ; Xudong et al, ; Zhao et al, , ; Chen et al, ; Ni et al, ; Shi et al, ; Allison et al, , ). For instance, Denton et al (2015, Denton et al, ) derived an empirical model of particle fluxes in the energy range ~1 eV to ~40 keV at geosynchronous orbit based on a total of 82 satellite years of observations (between 1990 and 2007) made by LANL/GEO data.…”

Variability of Quasilinear Diffusion Coefficients for Plasmaspheric Hiss

“…In addition to influencing the electron pitch angle range, which can be lost from the radiation belt region due to scattering from plasmaspheric hiss, variations in the f pe / f ce ratio will affect electron pitch angle distributions. Cap top pitch angle distributions (Allison et al, ; Zhao et al, ) are formed from hiss wave scattering due to the gap in D αα that arises between the cyclotron and Landau resonances (Lyons et al, ; Meredith et al, ). Figures g–i shows that variations in the plasma to gyrofrequency ratio alter the pitch angle range of this D αα gap and, as such, will influence the width of the resulting cap top of pitch angle distributions.…”