Relativistic Electron Enhancements Through Successive Dipolarizations During a CIR‐Driven Storm

Abstract

Relativistic electrons in the Earth's radiation belts are highly dynamic on a variety of timescales during the geomagnetic storm. Using Van Allen Probe spacecraft data, we investigate rapid enhancements of relativistic electrons in the outer radiation belt during a corotating interaction region (CIR) driven storm. Successive dipolarizations associated with 100keV‐MeV electron injections are identified. The evolution of energetic electrons is analyzed in the space of adiabatic invariants (μ, K and L*). Within less than a few hours, the phase space density (PSD) of the relativistic electrons promptly increases corresponding to injections of MeV electrons. The PSD of MeV electrons cumulatively increases by a factor of 4–10 at L* = 4.5–5.8 which is likely due to successive groups of dipolarizations and injections. Both near‐equatorial (small K) and off‐equatorial (large K) energetic electrons increase significantly. The increases in the near‐equatorial electrons are still dominant, suggesting the operation of betatron acceleration. The event study shows that successive dipolarizations associated with the CIR‐driven storm may rapidly affect relativistic electrons of the outer radiation belt over a wide range in the phase space.

Can strong substorm-associated MeV electron injections be an important cause of large radiation belt enhancements?

“…It is noteworthy, however, that in general, for various technical and physical reasons, a local PSD peak is not conclusive evidence for local acceleration (Lejosne et al, 2022). For instance, depending on the preexisting electron spectra and flux radial gradients before injections or large flux drops at higher L's after injections, we may have locally peaked PSD even for injection-driven enhancements (e.g., Sergeev et al, 1998;Green et al, 2004;Xiong et al, 2022).…”

“…Currently, local energization via repeated gyroresonant interactions with whistler-mode chorus waves (e.g., Summers et al, 1998;Horne et al, 2005;Thorne et al, 2013) is considered to be the primary cause of such enhancements. However, while receiving far less attention, rapid injections of relativistic electrons from the magnetotail associated with strong substorm dipolarizations have been suggested to make a significant contribution to radiation belt enhancements from both observations (e.g., Ingraham et al, 2001;Nagai et al, 2006;Nagai, 2012;Dai et al, 2014Dai et al, , 2015Xiong et al, 2022) and modeling (Kim et al, 2000;Fok et al, 2001;Glocer et al, 2011;Sorathia et al, 2018). Despite these efforts, the possibility that injections can be a major contributor to relativistic electron enhancements is mostly not seriously considered by the community relative to wave-driven energization.…”

Geoeffectiveness of Interplanetary Alfvén Waves. I. Magnetopause Magnetic Reconnection and Directly Driven Substorms

“…Generally, CIR-driven magnetic storms are small to moderate in terms of the disturbance storm-time (Dst) index disturbance. However, CIR-driven storms are of longer duration and produce a more severe level of relativistic electrons in the Earth's radiation belt (e.g., Borovsky & Denton 2006;Kataoka & Miyoshi 2006;Xiong et al 2022).…”

Rapid Enhancements of Relativistic Electrons in the Earth's Outer Radiation Belt Caused by the Intense Substorms: A Statistical Study

“…Several other mechanisms can explain rapid enhancements of relativistic electrons in the Earth's outer radiation belt. Previous observations and simulations have shown that relativistic electron injections could lead to rapid enhancements of MeV electrons in the outer radiation belt, which were associated with the intense substorm electric fields or convection electric fields (e.g., Ganushkina et al., 2013; Ingraham et al., 2001; Kim et al., 2000; X. Li et al., 1998; Mithaiwala & Horton, 2005; Tang et al., 2022; Xiong et al., 2022). In addition, an interplanetary (IP) shock acceleration is also an important mechanism, which is caused by IP shock compressions on the dayside (e.g., Blake et al., 1992; Foster et al., 2015; Hudson et al., 2017; Kanekal et al., 2016; Schiller et al., 2016).…”