Perpendicular ion acceleration in field-aligned density striations

A basic process similar to transit-time absorption is evaluated as a possible source of fast perpendicular ions under conditions typically encountered during sounding rocket nights in the auroral ionosphere. Electric fields localized within density striations aligned with the geomagnetic field have a nonuniform perpendicular amplitude that results in irreversible energy absorption. The efficiency of the process depends on the ratio of the ion Larmor radius to the perpendicular scale length of the amplitude gradient and on the scaled frequency v = omega/Omega(i). This absorption mechanism does not depend on a Landau resonance and does not require the electric field to be larger than a threshold value, because it is active in the linear regime; Theory, numerics, and particle-in-cell simulations demonstrate the process for electrostatic fields with frequencies a few times the ion gyrofrequency, but the process does not depend explicitly on the fluctuations being lower hybrid waves. Calculations of the evolving ion distribution function due to coherent electric fields suggest that the effect of spatial localization should be considered in the analysis of fast ions of geophysical origin.