Auroral beam/plasma interaction observed directly

Abstract

On 9 December 1981 rocket borne energetic electron spectrometers measured energy spectra over a stable auroral arc. An associated microprocessor accurately timed the electron detection pulses to calculate auto-correlation functions for each of 16 energy levels between 300 eV and 19 keV. Energy spectra measured up to 230 km altitude contained a secondary peak around 5 keV, corresponding to the auroral beam. Derived velocity distribution functions contain a plateau or table extending round from 0 to 90° pitch angle with a weak positive gradient (+ ve dƒ(ν)/dν) near zero pitch angle. Autocorrelation functions made at energy levels corresponding to the location of the positive gradient showed the electrons of this region of phase space to be strongly modulated (∼ 30%) at a frequency of 2.65 MHz or approximately at twice the electron gyrofrequency. This observation provides the most direct measurement of the auroral beam/ionospheric plasma interaction to date. It provides hard experimental evidence to support the theories which have previously predicted that a major wave-particle interaction responsible for the evolution of the auroral distribution function occurs at heights where the upper hybrid frequency equals twice the local electron gyrofrequency.