On Energetic Electron Dynamics During Geomagnetic Quiet Times in Earth's Inner Radiation Belt due to Atmospheric Collisional Loss and CRAND as a Source

Abstract

To investigate the role of atmospheric collisions and cosmic ray albedo neutron decay (CRAND) in the dynamics of energetic electrons in the Earth's inner radiation belt during geomagnetic quiet times, a drift‐collision‐source model that includes azimuthal drift, pitch angle diffusion from elastic collision, energy loss from inelastic collision, and a CRAND source is developed. In the model, the bounce‐averaged pitch angle diffusion coefficients and energy loss rates are calculated based on scattering of electrons with neutrals given by the NRLMSISE‐00 model and with ions and electrons given by International Reference Ionosphere (IRI) 2012 model. The electron source rate from CRAND follows the recently developed drift‐source model in Xiang et al. (2019). For 304‐keV quasi‐trapped electrons at L = 1.25, simulation results with CRAND show good agreement with Detection of Electro‐Magnetic Emissions Transmitted from Earthquake Regions satellite observations, confirming that CRAND is the main source for these quasi‐trapped electrons, in contrast to the previous understanding that these quasi‐trapped electrons were formed by wide‐angle scattering of the trapped populations. For trapped electrons, 153, 304, and 509 keV at L < 1.3, the simulation results with only azimuthal drift and atmospheric collisions show a much quicker decrease than observations, while simulation results including a CRAND source are generally comparable to the observations, suggesting that CRAND is an important source of trapped hundreds of kiloelectron‐volt electrons at L < 1.3 during quiet times and provides a baseline for the electron flux even during active times as well. Furthermore, these results suggest that actual radial diffusion rates in the inner belt are lower than previous estimates in which CRAND contributions were not considered.

Very-Low-Frequency transmitters bifurcate energetic electron belt in near-earth space

“…It is clearly shown that the combined scattering due to VLF transmitter waves, lightning-generated whistlers, plasmaspheric hiss, and magnetosonic waves can systematically reproduce the key features of the formation and evolution of the bifurcated electron belt. However, the energetic electron flux decay at L <~1.8 is not well captured by the simulation, which is likely attributed to loss by atmospheric collision 17,41 . Pitch angle scattering by VLF transmitter waves dominates the decay of tens of keV electrons at L~1.8-2.4 during the 14-day period.…”

“…It is noteworthy that the energetic electron flux decay at L-shell of 1.5-1.7 is not well captured by the simulations. At these low L-shells, the typical cyclotron resonant electron energies due to VLF transmitter waves are above 100 keV, and losses by an atmospheric collision can potentially play an important role in modulating the electron dynamics 17,41 . Moreover, we performed simulations by using the statistical wave amplitude profile of VLF transmitter waves during the northern hemisphere winter, the results of which are presented in Supplementary Fig.…”

Dynamics of Energetic Electrons in the Slot Region During Geomagnetically Quiet Times: Losses Due to Wave‐Particle Interactions Versus a Source From Cosmic Ray Albedo Neutron Decay (CRAND)

“…Therefore, the conclusion in Albert et al (2020) that energy diffusion is a significant source of energetic electrons in the slot region may not be applicable during extended quiet times. Different from the source due to energy diffusion, which is strongly dependent on the boundary condition and the energy spectrum, the CRAND source is relatively constant and can be quantitatively determined by the flux level of quasi‐trapped electrons (Xiang et al, 2019, 2020; Zhang, Li, et al, 2019). Based on the drift‐diffusion‐source model, we demonstrated that CRAND is an important source of relativistic electrons in the slot region during extended quiet times.…”

“…The wave information used to calculate <D αα > is summarized in Table 1. Collisions between electrons and atmospheric species can also induce PA diffusion. Generally, atmospheric scattering dominates the loss of relativistic electrons at L < 1.3 (e.g., Walt, 1964), while wave-particle interactions become more important at higher L. In this study, we have calculated the bounce-averaged PA diffusion coefficients from atmospheric scattering following formulas in previous studies (Cunningham et al, 2018;Selesnick, 2012;Xiang et al, 2020) with inputs from the NRLMSISE-00 model (Picone et al, 2002) and the International Reference Ionosphere (IRI) 2012 (Bilitza et al, 2014).…”

“…We assume that the dynamics of trapped electrons in Earth's slot region (2 < L < 3) are controlled by azimuthal drift, PA diffusion, and a CRAND source. The drift‐diffusion‐source model, for a given L shell and kinetic energy, is governed by the following equation (Xiang et al, 2020), where J is the electron flux, α eq is the equatorial PA, ω d is the bounce‐averaged drift frequency (assumed independent for numerical simplicity since the variation with α eq are within a factor of 1.5; Tu et al, 2010), ϕ is the drift phase or geomagnetic longitude, G 0 ( α eq ) = vτ b /2 for bounce period τ b and electron speed v , and t is time. S e is the electron source rate from CRAND: where q 0 is 1 × 10 −12 cm −3 sec −1 ster −1 and φ ( E ) is the neutron β‐decay spectrum (Selesnick, 2015).…”