LANDAU DAMPING OF AURORAL HISS

Auroral hiss is observed to propagate over distances comparable to an Earth radius from its source in the auroral oval. The role of Landau d amping is investigated for upward propagating auroral hiss. By using a ray tracing code and a simplified model of the distribution function, the effect of Landau damping is calculated for auroral hiss propagati on through the environment around the auroral oval. Landau damping is found to be the likely mechanism for explaining some of the one-sided auroral hiss funnels observed by Dynamics Explorer 1. It is also found that Landau damping puts a lower limit on the wavelength of auroral h iss. Poleward of the auroral oval. Landau damping is found in a typica l case to limit omega/k(parallel to) to values of 3.4 x 10(4) km/s or greater, corresponding to resonance energies of 3.2 keV or greater and wavelengths of 2 km or greater For equatorward propagation, omega/k(p arallel to) is limited to values greater than 6.8 x 1O(4) km/s, corres ponding to resonance energies greater than 13 keV and wavelengths grea ter than 3 km. Independent estimates based on measured ratios of the m agnetic to electric field intensity also show that omega/k(parallel to ) corresponds to resonance energies greater than 1 keV and wavelengths greater than 1 km. These results lead to the difficulty that upgoing electron beams sufficiently energetic to directly generate auroral his s of the inferred wavelength are not usually observed. A partial trans mission mechanism utilizing density discontinuities oblique to the mag netic field is proposed for converting auroral hiss to wavelengths lon g enough to avoid damping of the wave over long distances. Numerous re flections of the wave in an upwardly flared density cavity could conve rt waves to significantly increased wavelengths and resonance velociti es.