Electron holes in the outer radiation belt: Characteristics and their role in electron energization

Abstract

Van Allen Probes have detected electron holes (EHs) around injection fronts in the outer radiation belt. Presumably generated near equator, EHs propagate to higher latitudes potentially resulting in energization of electrons trapped within EHs. This process has been recently shown to provide electrons with energies up to several tens of keV and requires EH propagation up to rather high latitudes. We have analyzed more than 100 EHs observed around a particular injection to determine their kinetic structure and potential energy sources supporting the energization of trapped electrons. EHs propagate with velocities from 1000 to 20,000 km/s (a few times larger than the thermal velocity of the coldest background electron population). The parallel scale of observed EHs is from 0.3 to 3 km that is of the order of hundred Debye lengths. The perpendicular to parallel scale ratio is larger than one in a qualitative agreement with the theoretical scaling relation. The amplitudes of EH electrostatic potentials are generally below 100 V. We determine the properties of the electron population trapped within EHs by making use of the Bernstein‐Green‐Kruskal analysis and via analysis of EH magnetic field signatures. The density of the trapped electron population is on average 20% of the background electron density. The perpendicular temperature of the trapped population is on average 300 eV and is larger for faster EHs. We show that energy losses of untrapped electrons scattered by EHs in the inhomogeneous background magnetic field may balance the energization of trapped electrons.

A Census of Plasma Waves and Structures Associated With an Injection Front in the Inner Magnetosphere

“…The properties of phase space holes observed by the Burst 1 data are now compared with phase space hole properties derived from Burst 2 observations, specifically with Vasko, Agapitov, Mozer, Artemyev, Drake, and Kuzichev (), where the amplitude, phase velocity, magnetic spike signature, parallel width, and potential depth of 100 electron phase space holes were examined using Burst 2 data. Vasko, Agapitov, Mozer, Artemyev, Krasnoselskikh, and Bonnell () used the Vasko, Agapitov, Mozer, Artemyev, Drake, and Kuzichev () results to define typical electron hole properties from which they derived diffusion coefficients describing the diffusive scattering of electrons by electron phase space holes. The diffusion coefficients computed by Vasko, Agapitov, Mozer, Artemyev, Krasnoselskikh, and Bonnell () were based on electron hole parameters as determined using amplitude‐biased Burst 2 data.…”

“…This identification consists simply of integrating the E ∥ electric field waveform in time to estimate the shape of the electric potential associated with each solitary wave Φ( t ) = . A symmetric phase space hole will have a near‐Gaussian potential profile indicative of no net potential drop (see Vasko, Agapitov, Mozer, Artemyev, Drake, & Kuzichev, , and references therein). The hole velocity v ϕ is assumed constant during the brief period of observation, so it has no effect on the shape of Φ( t ).…”

“…Following (Vasko, Agapitov, Mozer, Artemyev, Drake, & Kuzichev, ), the potential depth and parallel spatial scale of a phase space hole are estimated using a Gaussian best fit to the measured potential profile Φ( t ) of the form describing an idealized phase space hole potential profile: . Again, the Gaussian fit must satisfy the good fit criteria described earlier.…”

“…Figure summarizes the observed phase space hole properties. To facilitate comparison between the Burst 1 (unbiased) and Burst 2 (triggered) data, this figure shows the same quantities, in the same format, with the same horizontal axes as Figure 3 from Vasko, Agapitov, Mozer, Artemyev, Drake, and Kuzichev () (except Figure 6c, where the axis is defined by Vasko, Agapitov, Mozer, Artemyev, Drake, & Kuzichev, , does not apply). The vertical axes of all panels in Figure (except Figure c) show the fraction of all 1,084 identified phase space holes with velocity estimates in each histogram bin.…”

“…Using the Van Allen Probes high‐resolution electric and magnetic field time series data (burst data), researchers have demonstrated that this broadband power is due to waves and structures such as electron‐acoustic solitons (Agapitov et al, ; Mozer et al, ; Vasko, Agapitov, Mozer, Bonnell, et al, ), ion‐acoustic double layers (Malaspina et al, ), electron phase space holes (Malaspina et al, ; Vasko, Agapitov, Mozer, Artemyev et al, ; Vasko, Agapitov, Mozer, Artemyev, Krasnoselskikh, & Bonnell, ), nonlinear whistler mode waves (Gao et al, ; Mozer et al, ), and kinetic Alfvén waves (Chaston et al, , ; Malaspina, Claudepierre, et al, ).…”