The importance of electrostatic ion‐cyclotron instability for quiet‐time proton auroral precipitation

Authors:
  • MAM. Ashour‐Abdalla
    M. Ashour‐Abdalla
  • RTRichard Mansergh Thorne
    Richard Mansergh Thorne
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Abstract
Even during geomagnetically quiet periods the loss cone distribution of energetic outer zone ions can provide sufficient free energy to excite quasi‐electrostatic instability over a broad frequency range confined to bands centered between multiples of the ion gyrofrequency. Preliminary calculations of the wave growth near the lower cyclotron harmonics indicates that auroral zone (L∼10) wave amplitudes should readily approach the nonlinear range. The expected frequency range together with nonlinear saturation arguments suggest that this instability may be related to the more or less continual presence of the recently reported 1‐10 mV/m broad band electrostatic wave turbulence on auroral field lines. Such turbulence should cause strong diffusion scattering of the ambient ion population (1‐100 keV), thus accounting for the persistent nocturnal proton auroral precipitation from field lines just within the outer boundary of trapping. The instability, however, may be severely suppressed by thermal electron (1‐10 eV) Landau damping. It should therefore be less effective on the dayside (especially at lower L) and it is expected to be quenched entirely within the plasmasphere or detached plasma regions.

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