Doppler radar observations of harmonics of electrostatic ion cyclotron waves in the auroral ionosphere

Observations of short-lived scattering events (0.5 s to 1 min) have been made with cross-beamed 50-MHz CW Doppler radars. Doppler spectra with high resolution in both frequency and time are obtained by detailed processing of time series digitized at high analogue to digital conversion rates. The spectra are highly structured, showing bursts of power at groups of discrete frequencies that are very short lived (usually approx.50 ms). Discrete spectral peaks can be found at Doppler velocities as high as 2500 m/s. The frequency separation between the successive discrete components of a group is often approximately the cyclotron frequency of one of the principal ions in the E region (NO/sup +/, O/sup +//sub 2/, N/sup +//sub 2/, and O/sup +/), so the components are postulated to be harmonics of electrostatic ion cyclotron (EIC) waves at elevated beam electron temperatures (more than 1000 K) in the upper E region. These transient events are closely correlated with intense auroral arc brightenings and/or the onset of auroral breakup, when high temperatures, strong field-aligned particle drifts, and field-aligned currents can be anticipated. Numerical solutions of the kinetic dispersion relation for a range of ionospheric parameters, especially the parallel electron drift velocity and beam temperature, revealmore » that both the higher and lower harmonics of EIC waves can become unstable. Thus some of the rapid time variation of the observed spectral peaks in the transient events can be explained by an appropriate accompanying change in ionospheric parameters. copyright American Geophysical Union 1988« less