Electron loss rates from the outer radiation belt caused by the filling of the outer plasmasphere: the calm before the storm Page: 4 of 31
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The outer portions of the plasmasphere build up and drain away with changes in
geomagnetic activity [e.g. Chappell et al., 1971; Carpenter et al., 1993; Elphic et al., 1997;
Sandel and Denton, 2007], the build up occurring when geomagnetic activity is weak and the
drainage occurring when geomagnetic activity strengthens. During recurrent high-speed-
stream-driven storms, the outer plasmasphere goes through an episodic cycle of slowly
building up in density and then rapidly draining [Borovsky et al, 1998a; Borovsky and
Steinberg, 2006; Borovsky and Denton, 2008a]. During recurrent high-speed-stream-driven
storms the number density of the outer electron radiation belt at geosynchronous orbit also
goes through an episodic cycle [Borovsky et al., 1998a].
Prior to most high-speed-stream-driven storms, intervals of unusually calm geomagnetic
activity occur [Borovsky and Steinberg, 2005,2006]. During these "calms before the storms",
the outer plasmasphere build up out to large radii in the dipolar portions of the
magnetosphere. It is anticipated that when the outer plasmasphere is present, plasma waves
that reside in the plasmasphere can pitch-angle diffuse radiation-belt electrons into the
atmospheric loss cone [e.g., Smith et al., 1974; Kelley et al., 1975; Albert, 2004; Summers et
al., 2004; Shprits and Thorne, 2004], wherein the electrons are lost from the magnetosphere.
Indeed, the number density of the outer radiation belt decreases with time in the days prior to
high-speed-stream-driven storm onset [e.g. Fig. 13 of Borovsky et al., 1998a].
Unlike high-speed-stream-driven storms wherein calms tend to occur prior to storm
onset, calms do not tend to occur prior to coronal-mass-ejecta-driven storms [Borovsky and
Denton, 2006]. In general, magnetic-cloud-driven (CME-driven) geomagnetic storms do not
show a substantial loss of radiation-belt electrons before the storm (cf. Fig. 10 of Borovsky
and Denton [2008b]).
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Borovsky, Joseph E & Denton, Michael H. Electron loss rates from the outer radiation belt caused by the filling of the outer plasmasphere: the calm before the storm, article, January 1, 2009; [New Mexico]. (https://digital.library.unt.edu/ark:/67531/metadc927261/m1/4/: accessed April 23, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.