Low-frequency oscillations associated with a polar cap auroral arc: Local spectral density estimation

Enhanced low-frequency electrostatic oscillations measured from a rocket in the high-latitude upper F region during the crossing of a polar cap auroral arc are analyzed using local spectral analysis techniques. The turbulent spectra were reduced from measurements made by Langmuir probes whose orientation remained approximately perpendicular to B throughout the flight. The increased low-frequency turbulence appears at an altitude of 400 km, coincident with electrostatic analyzer indications of increased energetic particle flux. Langmuir probe measurements simultaneously show an increasing temperature and generally decreasing, although irregular, electron density profiles. Electrostatic wave data are analyzed using cross-correlation analysis to determine phase shift; local wave number approximations provide an estimate of the plasma wave spectral density, S(k, ω). Theoretical expectations for threshold excitation and growth rates of possible instabilities in a two-component high-latitude F region ionosphere are investigated. The most likely source for the measured oscillations appears to be excitation of a collisionless electrostatic ion cyclotron mode