Modeling of field-aligned electron bursts by dispersive Alfven waves in the dayside auroral region

[ 1] A linear, one-dimensional gyrofluid code including electron inertia, electron pressure gradient, and finite ion gyroradius effects is applied to an inhomogeneous dayside auroral field line to determine the characteristics of propagating Alfven waves with small perpendicular wavelengths. Test particles are then used to study the behavior of both magnetosheath ( 100 eV) and background ionospheric electrons ( 2 eV) under the influence of dispersive Alfven waves. Although the test particle approach is not self-consistent, the gyrofluid/test particle simulation is able to reproduce many of the features observed by low-altitude satellites. The test particle simulations verify results from previous studies, such as reproducing electron energy and pitch-angle dispersions, and in doing so, validate the approach. The test particle simulations also show how resonant particles can lead to low-energy field-aligned electron bursts that are commonly observed in the dayside auroral region. The new results in this study reveal the plasma conditions necessary for electron resonance. We show that an increased mass density ( significant O+ density) in the acceleration region is an essential prerequisite to generate an electron burst. The primary effect of the O+ is to decrease the phase speed of the Alfven wave. Furthermore, the full gyrokinetic effects of the O+ act to produce a region in which the Alfven speed profile is gradually slowing, which allows electrons remaining within the wave to lower altitudes. In these electron bursts, the energy gain experienced by the majority of electrons ranges from tens to hundreds of eV. The trapping occurs if the parallel electric field is substantial enough (-0.2 mV/m) in the acceleration region to accelerate the background electrons. An integrated energy flux of accelerated electrons is estimated to be 3 erg s(-1) cm-(2,) about 20% of the Alfven wave Poynting flux.