Multi‐MeV Electron Dynamics Near the Inner Edge of the Outer Radiation Belt

Abstract

A variety of dynamic behavior of multi‐MeV electrons near the inner edge of the outer radiation belt has been revealed by detailed analysis of Van Allen Probes data. This study presents multi‐MeV electron flux and phase space density (PSD) using Van Allen Probes data during two strong geomagnetic storms to reveal their energy‐dependent dynamics in the outer belt, with dynamics occurring on timescales of hours to ∼1 week. Enhancements are shown down to L = 2.6, where ∼3 MeV electron populations are enhanced by an order of magnitude during one storm of study. Study of a second comparable storm shows rapid depletion of electron populations up to ∼7 MeV in the region 2.6 ≤ L ≤ 3. We also identify a local electron PSD peak at L ≈ 3 that slowly accumulates during quiet time and is rapidly depleted during an intense storm. Possible contributors to these dynamics are discussed.

On the Dynamics of Ultrarelativistic Electrons (>2 MeV) Near L* = 3.5 During 8 June 2015

“…During the period after the March 2015 storm through June 2015, Hogan et al. (2021) show the development of the local minimum at L * = 3.5 discussed here. During this period and before the 8 June event studied in detail here (of Dst min = −67 nT), other moderate storms in terms of Dst are present: 10–11 April (Dst min = −85 nT), 15–19 April (Dst min = −88 nT), and 11–13 May (Dst min = −82 nT).…”

“…Hogan et al. (2021) reported an energy‐dependent local minimum in multi‐MeV electron PSD that forms over a long‐term period from March to June 2015 near L * = 3.5, lower than where EMIC wave‐induced minima have been reported before with event studies. PSD minima in this L * region can be difficult to find with automatic detection algorithms such as those used by Drozdov et al.…”

“…This study analyzes a moderate storm on 8 June 2015 during which Hogan et al. (2021) reported the formation of a local minimum in PSD in March–June 2015 at L * = 3.5, lower than where EMIC wave‐induced local minima had been reported before and lower than where the more common H+ and He+ EMIC waves are generally observed. We investigate the physical mechanism responsible for this local minimum by analysis of multi‐MeV electron measurements, wave observations by the spacecraft, and consideration of wave–particle interaction theory for the local plasma environment.…”

“…PSD increases at higher L * values, suggesting no readily apparent effects of magnetopause shadowing on the trapped particle populations at L * = 3.5. The loss feature is shown to persist in pass‐averaged PSD through the ∼2‐day period after the initial loss observation shown, and the resulting local minimum in PSD exists until a strong storm on 21–23 June 2015 in daily‐averaged PSD (Hogan et al., 2021). The persistence of this feature with Dst recovery indicates that the process is not adiabatic and the Dst effect is not causing these dynamics.…”

“…Hogan et al. (2021) reported daily‐averaged PSD between 26 March and 20 June 2015 and showed the development of a local minimum in PSD near L * = 3.5. During this period, the time of greatest deepening of the observed minimum was during a moderate geomagnetic storm on 8 June 2015, where the Dst min reached −67 nT.…”