Testing the key processes that accelerate outer radiation belt relativistic electrons during geomagnetic storms

Abstract

Since the discovery of the Earth’s radiation belts in 1958, it has always been a challenge to determine the dominant physical mechanisms, whether local acceleration by chorus or inward radial diffusion, that leads to outer radiation belt relativistic electron flux enhancements. In this study, we test a chain of processes with several potential successive steps that is believed to accelerate outer belt relativistic electrons. By performing correlation analysis of different part of this chain, including the geomagnetic condition, evolution of source and seed electron fluxes, chorus wave activity, and maximum fluxes (jmax) of relativistic electrons, we aim to identify the critical steps that lead to acceleration of MeV electrons. Based on 5-years of Van Allen Probes observations, our results confirm the repeatable response of both source and seed electrons to the storms, showing a significant flux enhancement during the main phase of storms, followed by either a gradual decay or flux persistence at a stable level. However, it is the intense and prolonged occurrence of substorms that contributes to the long-lasting existence of both source and seed electrons, which is also strongly associated with the jmax of relativistic electrons. The significant correlation (Correlation Coefficient, CC∼0.8) between the seed electron fluxes and jmax reveal that the prolonged and pronounced seed electrons are the prerequisite for the significant flux enhancement of relativistic electrons regardless of the acceleration mechanism. The slightly smaller CC (∼0.5–0.7) between source electron fluxes and jmax of relativistic electrons indicates that while local acceleration by chorus wave plays an important role to accelerate relativistic electrons to jmax, other mechanisms such as inward radial diffusion are still needed in this process. The CC between the source electrons and the chorus wave amplitude increases with increasing levels of substorms, showing (CC)max of ∼0.8, which further supports the crucial role of chorus waves in accelerating the relativistic electrons during intense substroms.

A Statistical Study on the Acceleration Conditions of Ultrarelativistic Electrons in the Earth's Outer Radiation Belt During Geomagnetic Storms

“…To test the physical mechanisms that are primarily responsible for the acceleration of the outer radiation belt electrons to their peak fluxes, Hua et al. (2023) examined the correlation among various chains of events during electron acceleration. In this study, substorm activity occurring during the early recovery stage is more intense than that occurring during both the early and later recovery stages (Figure 7).…”

Dependence of Electron Flux Dropouts in the Earth's Outer Radiation Belt on Energy and Driving Parameters During Geomagnetic Storms

“…There were 110 storm events selected in this study, which are the same as those in Hua, Bortnik, Chu, et al. (2022) and Hua, Bortnik, Spence, and Reeves (2023). Similar to previous studies (Boynton et al., 2016, 2017), the dropouts are automatically selected based on the following steps and criteria: We first bin the electron fluxes into the grids of 0.1 L × 6 hr UT in four different MLT regions: 00–06, 06–12, 12–18, and 18–24.…”

“…Since Van Allen Probes had a highly elliptical orbit period of ∼9 hr, the time bin size of 6 hr here ensures at least one available measurement in each bin for the most of the time. This bin size has also been widely adopted to investigate the outer belt electron dynamics in previous studies (e.g., Drozdov et al., 2019; Hua, Bortnik, Spence, & Reeves, 2023; Hua et al., 2019; Turner et al., 2015). A decrease of flux by a factor >4 in 6 hr (the electron flux in the previous time step, j ( t − 1), being at least 4 times larger than the current one, j ( t ), that is, $$\frac{j(t-1)}{j(t)}\ge 4$$), or a decrease by a factor >4 in 12 hr while the flux decay by at least a factor of 1.5 in previous two successive time steps ($$\frac{j(t-2)}{j(t)}\ge 4$$, $$\frac{j(t-2)}{j(t-1)}\ge 1.5$$, and $$\frac{j(t-1)}{j(t)}\ge 1.5$$). Here, the flux decrease factor is adapted from the studies of Boynton et al.…”

“…Consequently, there are more electron injections and potentially stronger chorus wave activity under such intense and continuous substorms (Hua, Bortnik, Spence, & Reeves, 2023). Since the most efficient energy range of pitch angle scattering of electrons due to chorus waves is from tens to ∼100 keV, with the typical loss time scale varying from tens of minutes to hours Ni et al, 2016), the quick electron loss at tens of keV as marked by the triangles could be driven by locally interacting with chorus waves.…”

“…These outer belt electrons range from tens of keV to several MeV, and can exhibit highly dynamic behavior, including loss, transport, and acceleration processes. Their flux change can reach several orders of magnitude with timescales varying from hours to days, especially during strong storm or substorm activity (Baker et al., 2004, 2019; Hua, Bortnik, Spence, & Reeves, 2023; Reeves et al., 2016; Turner et al., 2015, 2019). Among these processes, fast and strong decreases in electron fluxes, also called dropouts (Green et al., 2004; Onsager et al., 2002; Turner, Morley, et al., 2012, and references therein), have attracted extensive attention due to their essential importance for radiation belt modeling and prediction.…”