Electron Acceleration in the Heart of the Van Allen Radiation Belts

Abstract

The Van Allen radiation belts contain ultrarelativistic electrons trapped in Earth's magnetic field. Since their discovery in 1958, a fundamental unanswered question has been how electrons can be accelerated to such high energies. Two classes of processes have been proposed: transport and acceleration of electrons from a source population located outside the radiation belts (radial acceleration) or acceleration of lower-energy electrons to relativistic energies in situ in the heart of the radiation belts (local acceleration). We report measurements from NASA's Van Allen Radiation Belt Storm Probes that clearly distinguish between the two types of acceleration. The observed radial profiles of phase space density are characteristic of local acceleration in the heart of the radiation belts and are inconsistent with a predominantly radial acceleration process.

Relativistic electron response to the combined magnetospheric impact of a coronal mass ejection overlapping with a high‐speed stream: Van Allen Probes observations

“…The peaked structure is evident for electrons with higher values of the first adiabatic invariant, , with PSD profiles at lower values showing a smoother monotonic behavior. The latter behavior is indicative of radial diffusion [Green and Kivelson, 2004;Reeves et al, 2013]. Although, as Green and Kivelson [2004] point out, such localized peaks in PSD may arise due to losses at higher L shells, that does not seem to be the case on 11 November.…”

“…In order to differentiate between the physical processes of radial transport and in situ energization, it is necessary to transform the flux and pitch angle measurements into phase space density (PSD), f , defined as f (W, , r) = j(W, , r)∕p 2 , where j is the measured differential directional flux and is a function of the particle energy W, pitch angle , spatial location r, and particle momentum p. In order to understand the physical process of electron energization, the electron PSD must be transformed and expressed in terms of the three adiabatic invariants of particle motion in the geomagnetic field, viz., , K, and L * [Schulz and Lanzerotti, 1974] which correspond to gyrational motion about the magnetic field, bounce motion along the field lines, and drift motion around the Earth, respectively. The full details of the PSD calculations for electrons measured by MagEIS and REPT, including the geomagnetic field models used and quantitative estimates of uncertainties on the PSD values, are described in the supplementary materials of Reeves et al [2013]. Differentiation between radial transport and in situ modes of energization is obtained by examining the radial profiles of the PSD; energization predominantly by in situ processes results in a local "bump" in the radial profiles of the PSD, whereas radial transport leads to a monotonic increase in PSD with increasing L * [e.g., Reeves et al, 2013 andreferences therein andKivelson, 2004].…”

“…The full details of the PSD calculations for electrons measured by MagEIS and REPT, including the geomagnetic field models used and quantitative estimates of uncertainties on the PSD values, are described in the supplementary materials of Reeves et al [2013]. Differentiation between radial transport and in situ modes of energization is obtained by examining the radial profiles of the PSD; energization predominantly by in situ processes results in a local "bump" in the radial profiles of the PSD, whereas radial transport leads to a monotonic increase in PSD with increasing L * [e.g., Reeves et al, 2013 andreferences therein andKivelson, 2004]. "lower" energy (see quantitative description below) electrons than the bottom row.…”

“…The relativistic electron flux variability on relatively short timescales, e.g., days, are due to a combination of energization and loss processes acting within the radiation belts, resulting in increased, reduced, or unchanged population levels [Reeves et al, 2003]. The energization of electrons to relativistic energies in the radiation belts is considered to be due to physical processes falling into two large classes, namely, radial transport [e.g., Schulz and Lanzerotti, 1974;Elkington, 2006] and in situ wave-particle energization [Thorne, 2010;Summers et al, 2007;Reeves et al, 2013] (for a recent review, see Millan and Baker [2012]). These processes are thought to act on a "seed" population of low energy (approximately hundreds of keV) produced as a result of substorm injections [Baker et al, 1998;Boyd et al, 2014].…”

Theoretical analysis on lower band cascade as a mechanism for multiband chorus in the Earth’s magnetosphere

“…[1][2][3][4][5] These waves are believed to be a dominant source of "killer electrons" (∼MeV) in Earth's magnetosphere, 3,5,6 which is a major threat to astronauts and operating satellites. Furthermore, they are also the primary contributor of strong diffuse auroral precipitation into the Earth's atmosphere, 2,7 resulting in enhanced chemical changes of the atmosphere.…”