Explaining sudden losses of outer radiation belt electrons during geomagnetic storms

Abstract

, can drive storm activity, but several outstanding questions remain concerning dropouts and the precise channels to which outer belt electrons are lost during these events. By analysing data collected at multiple altitudes by the THEMIS, GOES, and NOAA-POES spacecraft, we show that the sudden electron depletion observed during a recent storm's main phase is primarily a result of outward transport rather than loss to the atmosphere.Trapped radiation belt electrons undergo three characteristic types of motion: gyro-motion around magnetic field lines due to any velocity component perpendicular to the field, bounce-motion along field lines between magnetic mirror points due to any velocity component parallel to the field, and drift-motion around the Earth resulting from magnetic gradient and curvature drifts. Associated with each of these oscillatory motions are adiabatic invariants, which are conserved so long as electric and/or magnetic fields do not change on scales similar to those of the associated motions. The first invariant conserves the magnetic moment of the particle and is proportional to the perpendicular momentum squared divided by the local magnetic field strength; the second and third invariants conserve the integral of parallel momentum over one full bounce period and the magnetic flux through a particle's drift orbit, respectively. Magnetospheric changes on timescales much longer than electron drift periods are considered fully adiabatic, that is, they are fully reversible. Initially, it was thought that the observed flux dropouts were fully adiabatic changes in the system. Essentially, electrons moved radially outward (inward) during a storm's main (recovery) phase to conserve their third invariant as Earth's magnetic field was altered by the field produced by an enhanced (weakening) ring current 7,8 , which is a magnetospheric current system resulting from charge-dependent particle drift. As electrons moved radially outward (inward) in the field, their fluxes decreased (increased) for fixed energy as the first adiabatic invariant was also conserved. It was later shown that although this 'Dst effect' (after the disturbance storm time (Dst) geomagnetic index (Kp) used to indicate storm activity) does play a role in the flux dynamics, many flux dropouts do not return to the pre-storm flux level

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

“…In Figure 7c, the magnetopause recovers as the dynamic pressure decreases, and outer radiation belt electrons begin to diffuse in L * to redistribute, so as to flatten the gradient of the number of electrons with the set of M and K along L * . This outward radial diffusion process can be enhanced by ULF wave activities [Loto'aniu et al, 2010;Turner et al, 2012]. Finally, the flattened distribution is given in Figure 7d, and it can be observed as the dropout of electron fluxes.…”

“…The dropout process is schematically summarized in Figure 7, the mechanism of which was previously pointed out by Turner et al [2012]. Figure 7a shows the initial condition.…”

“…Drift loss of electrons occurs when the magnetopause is compressed inward and intersects the drift path of electrons in the outer radiation belt [Shprits et al, 2006;Turner et al, 2012;Tu et al, 2013Tu et al, , 2014.…”

“…It is indicated by a study of Turner et al [2012] that the main cause of the electron dropout during a CIR-related storm is drift loss due to inward motion of magnetopause associated with high solar wind dynamic pressure and the consequent outward radial diffusion. Morley et al [2010] have conducted a superposed epoch analysis to the electron count dropouts associated with 67 CIRs from 2005 to 2008.…”

“…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].…”