Rapid acceleration of electrons in the magnetosphere by fast‐mode MHD waves

Abstract

During major magnetic storms, enhanced fluxes of relativistic electrons in the inner magnetosphere have been observed to correlate with ULF waves. The enhancements can take place over a period of several hours. In order to account for such a rapid generation of relativistic electrons, we examine the mechanism of transit‐time acceleration of electrons by low‐frequency fast‐mode MHD waves, here the assumed form of ULF waves. Transit‐time damping refers to the resonant interaction of electrons with the compressive magnetic field component of the fast‐mode waves via the zero cyclotron harmonic. In terms of quasi‐linear theory, a kinetic equation for the electron distribution function is formulated incorporating a momentum diffusion coefficient representing transit‐time resonant interaction between electrons and a continuous broadband spectrum of oblique fast‐mode waves. Pitch angle scattering is assumed to be sufficiently rapid to maintain an isotropic electron distribution function. It is further assumed that there is a substorm‐produced population of electrons with energies of the order of 100 keV. Calculations of the acceleration timescales in the model show that fast‐mode waves in the Pc4 to Pc5 frequency range, with typically observed wave amplitudes (ΔB = 10–20 nT), can accelerate the seed electrons to energies of order MeV in a period of a few hours. It is therefore concluded that the mechanism examined in this paper, namely, transit‐time acceleration of electrons by fast‐mode MHD waves, may account for the rapid enhancements in relativistic electron fluxes in the inner magnetosphere that are associated with major storms.