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

Abstract

Radiation belt electrons undergo frequent acceleration, transport, and loss processes under various physical mechanisms. One of the most prevalent mechanisms is radial diffusion, caused by the resonant interactions between energetic electrons and ULF waves in the Pc4‐5 band. An indication of this resonant interaction is believed to be the appearance of periodic flux oscillations. In this study, we report long‐lasting, drift‐periodic flux oscillations of relativistic and ultrarelativistic electrons with energies up to ∼7.7 MeV in the outer radiation belt, observed by the Van Allen Probes mission. During this March 2017 event, multi‐MeV electron flux oscillations at the electron drift frequency appeared coincidently with enhanced Pc5 ULF wave activity and lasted for over 10 h in the center of the outer belt. The amplitude of such flux oscillations is well correlated with the radial gradient of electron phase space density (PSD), with almost no oscillation observed near the PSD peak. The temporal evolution of the PSD radial profile also suggests the dominant role of radial diffusion in multi‐MeV electron dynamics during this event. By combining these observations, we conclude that these multi‐MeV electron flux oscillations are caused by the resonant interactions between electrons and broadband Pc5 ULF waves and are an indicator of the ongoing radial diffusion process during this event. They contain essential information of radial diffusion and have the potential to be further used to quantify the radial diffusion effects and aid in a better understanding of this prevailing mechanism.

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.…”

“…, where j is the unidirectional flux of local 90° pitch angle electrons, and j 0 is the averaged flux over a time window t 0 centered at the time of measurement. Following Zhao et al (2021), we set t 0 as the drift period of 90° pitch angle electrons at 𝐴𝐴 𝐴𝐴= 3 under a dipole magnetic field (t 0 ∼ 13.3 min for 1.8 MeV electrons and ∼4.3 min for 6.3 MeV electrons). We then perform a running-average on 𝐴𝐴 𝐴𝐴𝐴𝐴 with a 1-min window to filter out variations much faster than electron drift frequencies.…”

“…Following Zhao et al. (2021), we set t 0 as the drift period of 90° pitch angle electrons at $$L=3$$ under a dipole magnetic field ( t 0 ∼ 13.3 min for 1.8 MeV electrons and ∼4.3 min for 6.3 MeV electrons). We then perform a running‐average on $$dj$$ with a 1‐min window to filter out variations much faster than electron drift frequencies.…”

“…Though commonly observed for 10–100s of keV electrons at L ∼ 5–6 (e.g., Claudepierre et al., 2013), fewer drift‐periodic flux oscillations have been reported for multi‐MeV electrons in the outer belt. Among these previous observations, most were made during interplanetary shock impacts and are indicators of prompt radial transport caused by large impulses of the azimuthal electric field; only a few event studies reported multi‐MeV electron drift‐periodic flux oscillations associated with radial diffusion caused by broadband ULF waves (e.g., Su et al., 2015; Zhao et al., 2021). Therefore, a statistical analysis of drift‐periodic flux oscillations of multi‐MeV electrons in the outer belt is needed to further investigate this phenomenon and underlying mechanisms and advance our understanding of multi‐MeV electron dynamics.…”