Publication Date:
2013-03-22
Description:
[1] Abstract : 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 〈 - 40 nT ) during the THEMIS era thus far ( Dec . 2007 - Aug . 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 towards optimal source or loss of outer belt electrons under suitable external drivers , and the conditions resulting in wave growth for these acceleration and loss mechanisms are therefore an important area of future research .
Print ISSN:
0148-0227
Topics:
Geosciences
,
Physics
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