Global‐Scale ULF Waves Associated With SSC Accelerate Magnetospheric Ultrarelativistic Electrons

Abstract

We study electron behavior in the outer radiation belts during the 16 July 2017 storm sudden commencement (SSC), in which prompt intensification of ultrarelativistic electron fluxes was observed at around L = 4.8 by Van Allen Probe B immediately after an interplanetary shock. The electron fluxes in multiple energy channels show clear oscillations in the Pc5 frequency range, although the oscillation characteristics are quite different in different energy channels. At energies above ∼1 MeV, the oscillation periods were very close to the electron drift period, which resembles an energy spectrogram evolution expected for an energetic particle injection event and its drift echoes. At lower energies, however, the oscillation periods hardly depended on the energy: They were very close to the ultralow frequency (ULF) wave period derived from electric field measurements (about 250 s according to wavelet analysis). These complex signatures are consistent with the picture of drift resonance between electrons and short-lived ULF waves with low azimuthal wave numbers. Good agreement between the observations and numerical simulations confirms that shock-induced global-scale ULF waves can efficiently accelerate outer belt ultrarelativistic electrons up to 3.4 MeV over a time scale shorter than 1 hr.

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

“…Furthermore, in situ measurements have shown that there can be drift resonant interactions with corresponding PSD enhancement of particles by these low‐frequency waves. For instance, Hao et al () in this collection show the outer belt ultrarelativistic electron enhancement (from Relativistic Electron‐Proton Telescope (REPT) measurements) associated with the storm sudden commencement of the 16 July 2017 geomagnetic storm. These authors explain and reproduce the prompt electron acceleration response (from 2 to 3.4 MeV in less than 1 hr) to the shock‐induced ULF wave in the Pc5 frequency range using a generalized drift resonance theory.…”

Drift‐Bounce Resonance Between Charged Particles and Ultralow Frequency Waves: Theory and Observations

“…These ULF waves, especially their poloidal-mode branches with electric field oscillations in the azimuthal direction, can accelerate or decelerate charged particles as they follow magnetic gradient and/or curvature drift orbits around Earth. In other words, this process provides an important energy source for particle acceleration and transportation in the Van Allen radiation belts Liu et al, 2016;Mann et al, 2016;Sarris et al, 2017;Hao et al, 2019;Degeling et al, 2019). This resonant process is called drift resonance (Southwood and Kivelson, 1981;Southwood and Kivelson, 1982;, during which particles can have a net energy gain (or loss, depending on the phase difference) from the ULF waves.…”

Simulation of Prompt Acceleration of Radiation Belt Electrons During the 16 July 2017 Storm

“…The simulated energization and drift phase bunching due to the bipolar electric field that accompanies the dayside compression are found to be consistent with VAP observations, reproducing the energy-dependent drift echoes and observed spectra. It is clear from direct comparison of simulated and observed drift echoes that the energization is prompt, dominated by the initial bipolar electric field impulse rather than continuous acceleration by the lower-amplitude damped ULF oscillations, as suggested by Hao et al (2019). Thus, the mechanism proposed by Li et al (1993) for the March 1991 injection holds for much weaker events, commonly seen in the VAPs era.…”

“…An oscillatory electric field typically follows the primary bipolar electric field impulse due to the magnetopause compression as seen in Figure 2. Hao et al (2019) showed that a persistent m = 1 oscillation can also be effective at accelerating lower-energy electrons when a finite damping time allows for interaction of drift resonant electrons with the ULF wave over multiple drift periods. Hao et al (2019) also show in their supporting information Figure S2 that an idealized dipolar azimuthal electric field impulse only, without ringing ULF oscillations, can produce the observed acceleration at higher energies as modeled here with direct MHD test particle simulations.…”