Resonant generation of ion waves on auroral field lines by positive slopes in ion velocity space

Simultaneous observations of wave spectra and particle velocity distributions obtained by the S3‐3 satellite on auroral field lines are studied. Emissions below the lower hybrid frequency occur just below multiples of the proton gyrofrequency. At higher frequencies the observed spectra show peaks just above such multiples. The ion and electron distributions are modeled, and numerical methods are used to calculate the corresponding spatial wave growth rates. This theoretically obtained growth agrees rather well with the observed emissions. The major source of free energy is a positive slope perpendicular to the ambient magnetic field in the ion velocity distribution.