FAST Observations of Inertial Alfven Waves in the Dayside Aurora

Abstract

Bursts of large amplitude impulsive electric and magnetic field oscillations are a common feature observed from FAST when crossing the dayside auroral oval at altitudes from 1500–2500 km. The oscillations have transverse amplitudes of up to 1 V/m and 100nT and exhibit a parallel electric field component with amplitudes which may be as large as 100mV/m. Calculation of E1/B1 over 100 events yields an average value four times the local Alfven speed. The wave period is usually less than 0.25s with ‘perpendicular wavelengths’ which average to 7.1 electron skin depths (c/ωpe∼80m). Poynting flux calculations indicate predominately downward fluxes with magnitude up 10−2 Wm−2 usually accompanied by a smaller upwards component. Invariably these waves are accompanied by field‐aligned fluxes of down going and sometimes counterstreaming suprathermal electrons. Comparison with theoretical studies indicate that these observations are consistent with the characteristics of a shear Alfven wave with k⟂∼ωpe/c propagating in the inertial dispersive regime and interfering with a reflected component. However the observed large parallel electric field component, if real, has yet to be explained