On the storm‐time evolution of relativistic electron phase space density in Earth's outer radiation belt

Abstract

[1] We report on internal, magnetospheric processes related to markedly different storm-time responses of phase space density (PSD) in invariant coordinates corresponding to equatorially mirroring, relativistic electrons in Earth's outer radiation belt. Two storms are studied in detail, selected from a database of 53 events (Dst min < À40 nT) during the THEMIS era thus far (December 2007-August 2012. These storms are well covered by a number of in situ THEMIS spacecraft and complemented by additional ground-based and in situ observatories, and they epitomize the divergent behaviors that the outer radiation belt electrons can exhibit during active periods, even during otherwise similar Dst and auroral electrojet (AE) profiles. From our statistical results with the full database, the changes in the radial profile peak in PSD reveal notably consistent behavior with prior studies: 58% of geomagnetic storms resulted in PSD peak enhancements, 17% resulted in PSD peak depletions, and 25% resulted in no significant change in the PSD peak after the storm. For the two case studies, we examined the PSD at multiple equatorial locations (using THEMIS), trapped and precipitating fluxes from low-Earth orbit (using POES), and chorus, hiss, EMIC, and ULF waves (using THEMIS spacecraft, ground observatories, and the GOES spacecraft). We show that (1) peaks in PSD were collocated with observed chorus waves outside of the plasmapause during the most active periods of the PSD-enhancing storm but not during the PSD-depleting storm, providing evidence for the importance of local acceleration by wave-particle interactions with chorus; (2) outer belt dropouts occurred following solar wind pressure enhancements during both storms and were consistent with losses from magnetopause shadowing and subsequent outward radial transport; during the PSD-enhancing storm, this revealed how the outer belt can replenish itself seemingly independently of the remnant of the pre-existing belt leftover after a dropout, which in this case resulted in a double-peaked outer belt distribution; (3) slow decay in PSD was associated with corresponding locations in L* and enhanced wave amplitudes of plasmaspheric hiss; (4) precipitation loss associated with wave-particle interactions with hiss and EMIC waves appeared to be significantly more important during the PSD-depleting storm than the PSD-enhancing storm; and (5) PSD transport during the recovery phase of both storms and throughout the PSD-enhancing storm was consistent with ULF-wave-driven radial diffusion away from maxima in PSD; this indicates the importance of ULF waves in redistributing outer belt PSD after local acceleration occurs. We conclude that these source, transport, and loss processes, individually well characterized by previous studies, do indeed appear to act in concert, leading to predominance of local acceleration in one case and loss in another. These processes can therefore conspire toward optimal source or loss of outer belt electrons under suitable external drivers, and the conditions ...

Approximate analytical solutions for the trapped electron distribution due to quasi‐linear diffusion by whistler mode waves

“…Neglecting mixed diffusion (which can be important too) [see Albert , ] and assuming an initially cold distribution without high‐energy electrons (for instance, just after dropouts) [e.g., see Turner et al , , and references therein], the energy broadening of the electron distribution F ( E , t ) in the presence of quasi‐linear energy diffusion by quasi‐parallel whistler mode waves is given approximately by [ Horne et al , ; Balikhin et al , ] where with E in MeV.…”

First results from CSSWE CubeSat: Characteristics of relativistic electrons in the near‐Earth environment during the October 2012 magnetic storms

“…Recently, the paradigm for explaining the enhancement of the electron radiation belt has shifted from the almost exclusive use of the theory of radial diffusion to a greater emphasis on the role of waves in the in situ heating of radiation belt electrons. The waves are produced by the injection of plasma‐sheet electrons into the inner magnetosphere [ Horne and Thorne , ; Shprits et al ., ; Chen et al ., ; Kasahara et al ., ; Bortnik and Thorne , ; Yoon , ; Turner et al ., ; Reeves et al ., ]. Though it has become generally accepted that both mechanisms can energize radiation belt electrons, their relative contributions remain uncertain.…”

On the cause and extent of outer radiation belt losses during the 30 September 2012 dropout event

“…However, it has now been shown that dropouts can also occur independent of geomagnetic storms [e.g., Morley et al, 2010] and that the adiabatic effects alone cannot explain the magnitude of loss observed during dropouts [e.g., Kim and Chan, 1997;Li et al, 1997]. The clearest evidence that outer belt dropouts are driven by true losses from the system (i.e., not just adiabatic effects) have resulted from studies of events, revealing that distributions of electron phase space density (PSD) in adiabatic invariant coordinates, which remove most of the ambiguity due to purely adiabatic effects, also undergo outer belt dropouts [e.g., Turner et al, 2013]. Here we use the definition of dropouts described in Turner et al [2012b], which includes both adiabatic effects and nonadiabatic losses.…”