Electromagnetic ion cyclotron waves at proton cyclotron harmonics

[1] Waves with frequencies in the vicinity of the proton cyclotron frequency and its harmonics are commonly observed from the Fast Auroral Snapshot spacecraft when traversing regions of auroral particle acceleration. In areas of upward current, large-amplitude electromagnetic waves with frequencies within 5% of the local proton gyrofrequency Omega(p) and its harmonics are often observed where upstreaming ion beams exist. These waves have electric field (E-1) and magnetic field (B-1) amplitudes of up to 1 V m(-1) and 2 nT with the ratio E-1/B-1 as small as c. The waves occur in the low-altitude portion of the primary auroral acceleration potential, where plasma densities are less than or equal to1 cm(-3). It is shown how these waves grow through inverse Landau resonance with a cold field-aligned electron beam superimposed on an accelerated and magnetically mirrored plasma sheet electron component in the absence of any significant plasma densities at energies below similar to100 eV. Significantly, the drift velocity of the cold beam (nu(oeb)) is several times larger than its thermal velocity nu(eb), and it is this feature that allows the wave to become electromagnetic at cyclotron harmonics while simultaneously giving rise to broadband electrostatic emissions spanning the first few cyclotron harmonics as is observed.