On the Association Between Electron Flux Oscillations and Local Phase Space Density Gradients

Abstract

Electron flux oscillations are often observed in electron energy channels in the range of 10 keV to 100 keV. They are observed in the form of flux fluctuations with periods corresponding to the drift periods of electrons around the Earth and can be produced in the magnetosphere as a result of drift resonant interactions in association with broadband ultra‐low frequency (ULF) waves. They are observed in particular during quiet times and are indicators of ongoing drift‐resonant processes and resulting radial transport caused by ULF waves. In this study we investigate the association of the appearance of such flux oscillations with the local phase space density. In particular, it is shown that flux oscillations appear when nonzero phase space density gradients are present and furthermore that the observed amplitude of flux oscillations is dependent on radial phase space density gradients, with steeper gradients enabling the observation of flux oscillations with higher‐amplitude. Via statistical observations from the Van Allen probes twin‐spacecraft mission while at apogee at L* ∼5 to ∼5.6 it is found that, during quiet times, electron flux oscillations are more likely to diminish for electron energy of ∼400 keV, and associated phase space density is more likely to be zero for the corresponding first adiabatic invariant values of ∼300 MeV/G. It is also concluded that flux oscillations could be used as indicators of phase space density gradients.

Statistics of Multi‐MeV Electron Drift‐Periodic Flux Oscillations Using Van Allen Probes Observations

“…Future work using satellites with coverage of higher latitudes, such as Arase, will provide a better view of the distribution of drift-periodic flux oscillations at a broader range of latitudes and shed light on a deeper understanding of the latitudinal dependence of this phenomenon. Some recent event studies on flux oscillations suggested the potentially important role of local PSD radial gradient (e.g., Hartinger et al, 2020;Sarris et al, 2021;Zhao et al, 2021). Thus, we also explore the role of electron PSD radial gradient using the statistical approach.…”

“…Some recent event studies on flux oscillations suggested the potentially important role of local PSD radial gradient (e.g., Hartinger et al., 2020; Sarris et al., 2021; Zhao et al., 2021). Thus, we also explore the role of electron PSD radial gradient using the statistical approach.…”

Drift Phase Structure Implications for Radiation Belt Transport

“…Figure 1 illustrates this process: the particles carry their initial phase-space density with them as they move in L. Drift phase structure arises because the displacement is phase-dependent. The size of the drift phase structure depends on the displacement in L and the local phase space density gradient (for a statistical investigation of this phenomenon, see Sarris et al, 2021). As noted by Hartinger et al (2020) if there is no radial gradient in the phase space density, radial transport will not result in drift phase structures, including drift echoes; this is common in the outer zone at ∼MeV energies.…”

“…Drift phase structure arises because the displacement is phase‐dependent. The size of the drift phase structure depends on the displacement in L and the local phase space density gradient (for a statistical investigation of this phenomenon, see Sarris et al., 2021). As noted by Hartinger et al.…”

Van Allen Probes Observations of Multi‐MeV Electron Drift‐Periodic Flux Oscillations in Earth's Outer Radiation Belt During the March 2017 Event

“…Using a test-particle simulation, they showed that such energy-dependent flux oscillations can be well explained by the resonant interactions caused by m = 1 broadband ULF waves. In addition, Sarris et al (2020Sarris et al ( , 2021 showed that the amplitude of such flux oscillations is well correlated with the particle detector's energy channel width and the radial gradient of electron PSD.…”