Resonant acceleration and diffusion of outer zone electrons in an asymmetric geomagnetic field

Abstract

[1] The outer zone radiation belt consists of energetic electrons drifting in closed orbits encircling the Earth between $3 and 7 R E . Electron fluxes in the outer belt show a strong correlation with solar and magnetospheric activity, generally increasing during geomagnetic storms with associated high solar wind speeds, and increasing in the presence of magnetospheric ULF waves in the Pc-5 frequency range. In this paper, we examine the influence of Pc-5 ULF waves on energetic electrons drifting in an asymmetric, compressed dipole and find that such particles may be efficiently accelerated through a drift-resonant interaction with the waves. We find that the efficiency of this acceleration increases with increasing magnetospheric distortion (such as may be attributed to increased solar wind pressure associated with high solar wind speeds) and with increasing ULF wave activity. A preponderance of ULF power in the dawn and dusk flanks is shown to be consistent with the proposed acceleration mechanism. Under a continuum of wave modes and frequencies, we find that the drift resonant acceleration process leads to additional modes of radial diffusion in the outer belts, with timescales that may be appropriate to those observed during geomagnetic storms.

Discovery of the action of a geophysical synchrotron in the Earth’s Van Allen radiation belts

“…LANL 1990-095, in the pre-noon local time sector, sees modulation peaking in the 315-500 keV energy channel with clear 180°phase change across energy. As the gradient-curvature drift frequency of the electrons increases with energy, the phase change in the modulation as a function of energy is that expected from what is effectively the response of harmonic oscillators with different (drift) frequencies in response to a coherent single frequency ULF wave driver through drift resonance 6,10,13,14 . This is an expected but previously unreported observational 'smoking gun' signature of ULF drift resonance with relativistic (4B500 keV) electrons in the Van Allen belts 13 .…”

“…Typically, partly because of the paucity of observed discrete frequency modulations in MeV electron flux, the ULF wave-electron interaction is assumed to be stochastic and modelled using radial diffusion theory 3 . Narrow-band coherent ULF waves can accelerate particles proportional to time through drift resonance, whereas diffusive radial transport is slower and acts as the square root of time 10,14 . Determining the importance of coherent ULF acceleration and transport is hence important for understanding radiation belt morphology and dynamics; coherent acceleration being analogous to particle acceleration in synchrotron machines.…”

The Electric and Magnetic Field Instrument Suite and Integrated Science (EMFISIS) on RBSP

“…However, the heart of the radiation belts lies well inside geosynchronous orbit, in the region near 3-5 RE. Theoretical calculations suggest that ULF acceleration mechanisms should be substantially reduced in efficiency at lower L, compared to geosynchronous orbit (Fälthammar and Walt 1969;Elkington et al 2003). On the other hand, local acceleration involving VLF waves, particularly lower-band chorus, becomes most efficient in the region just outside the plasmapause, which corresponds to the radial range 3-5 RE for storm conditions (Summers et al 1998;Meredith et al 2003b).…”

Plasma flow patterns in and around magnetosheath jets

“…In addition, local reconnection may perhaps be triggered at the magnetopause (Hietala et al, 2012). In the magnetosphere, the radiation belt electron population may be modified by magnetopause shadowing (Elkington et al, 2003;Turner et al, 2012). Consequences may even be seen on the ground, in the form of increased ionospheric convection, geomagnetic field variations, and possibly "throat" aurora observations (Hietala et al, 2012;Dmitriev and Suvorova, 2012;Archer et al, 2013b;Han et al, 2017).…”