Acceleration and loss of relativistic electrons during geomagnetic storms

Abstract

[1] We analyze the response of relativistic electrons to the 276 moderate and intense geomagnetic storms spanning the 11 years from 1989 through 2000. We find that geomagnetic storms can either increase or decrease the fluxes of relativistic electrons in the radiation belts. Surprisingly, only about half of all storms increased the fluxes of relativistic electrons, one quarter decreased the fluxes, and one quarter produced little or no change in the fluxes. We also found that the pre-storm and post-storm fluxes were highly uncorrelated suggesting that storms do not simply ''pump up'' the radiation belts. We found that these conclusions were independent of the strength of the storm (minimum Dst) and independent of L-shell. In contrast, we found that higher solar wind velocities increase the probability of a large flux increase. However, for all solar wind velocities both increases and decreases were still observed. Our analysis suggests that the effect of geomagnetic storms on radiation belt fluxes are a delicate and complicated balance between the effects of particle acceleration and loss.

Relativistic electron response to the combined magnetospheric impact of a coronal mass ejection overlapping with a high‐speed stream: Van Allen Probes observations

“…The latter usually resulting in stronger storms (typically Dst < −100 nT) with a shorter recovery phase as compared to CIRs [Tsurutani et al, 2006]. While the electron response to IP drivers is a balance between energization and loss processes operating within the magnetosphere [Reeves et al, 2003], when there are increased fluxes associated with CIR-driven storms, they tend to occur at higher L shells, i.e., radial distances.…”

“…The relativistic electron flux variability on relatively short timescales, e.g., days, are due to a combination of energization and loss processes acting within the radiation belts, resulting in increased, reduced, or unchanged population levels [Reeves et al, 2003]. The energization of electrons to relativistic energies in the radiation belts is considered to be due to physical processes falling into two large classes, namely, radial transport [e.g., Schulz and Lanzerotti, 1974;Elkington, 2006] and in situ wave-particle energization [Thorne, 2010;Summers et al, 2007;Reeves et al, 2013] (for a recent review, see Millan and Baker [2012]).…”

“…Generally, CMEs are predominant around solar cycle maxima, while the HSS occur during the descending phase of the solar cycle and are associated with corotating interaction regions, CIRs, resulting from coronal holes (for recent reviews, see Webb and Howard [2012] and Richardson and Cane [2012]). While the association between geomagnetic activity and solar drivers has been examined over solar cycles [Richardson et al, 2001;Richardson and Cane, 2012], the relationship between geomagnetic storms and relativistic electron populations is more complex in that not all geomagnetic storms result in enhanced electron fluxes; it is the dynamic balance between energization and loss that determines flux levels [Reeves et al, 2003]. For example, using long-term observations by the low-altitude spacecraft, SAMPEX, Kanekal [2006] performed a superposed epoch analysis of about 20 HSS-and CME-driven events and observed that HSS-driven relativistic electrons (⪆2.0 MeV) appear to decay faster than the CME-driven populations, even though higher solar wind speeds persisted for longer periods during HSS-driven electron enhancements.…”

Identifying the radiation belt source region by data assimilation

“…For a review, see the studies of Friedel et al [2002], Brautigam and Albert [2000], and Green and Kivelson [2004]. Reeves et al [2003] show that the net effect of geomagnetic activity on radiation belt dynamics is a delicate balance of acceleration, transport, and losses that can lead to either increased or decreased fluxes or to almost no changes at all.…”

Relativistic electron flux dropouts in the outer radiation belt associated with corotating interaction regions

“…It has been suggested that the dynamic variations of the outer radiation belt are the result of a complicated balance between loss and acceleration processes [e.g., Reeves et al, 2003;Meredith et al, 2003;Summers et al, 2004]. There are currently three types of loss mechanisms to explain the electron flux dropout in the outer radiation belt: adiabatic loss or "Dst effect" [Kim and Chan, 1997]; precipitation loss to the atmosphere via pitch angle scattering with plasma waves Abel and Thorne, 1998;O'Brien et al, 2003;Voss et al, 1998]; and drift loss to the magnetopause and outward radial diffusion [Brautigam and Albert, 2000;Miyoshi et al, 2003;Shprits et al, 2006;Ohtani et al, 2009;Matsumura et al, 2011;Turner et al, 2012;Tu et al, 2014].…”