OBSERVATION OF BROAD-BAND WAVE BURSTS WITH POWER-LAW SPECTRA ABOVE THE PLASMA FREQUENCY IN THE UNDERDENSE AURORAL PLASMA

Viking spacecraft spectral observations of high-frequency waves with f requencies f(e)<f<f(ce) between the electron plasma (f(e)) and cyclotr on (f(ce)) frequencies in the strongly magnetized nightside auroral pl asma occasionally exhibit impulsive emissions with featureless power l aw spectra extending sometimes up to frequencies close to f(ce). Most of these bursts are observed when the electric antenna is at large ang les with respect to the ambient magnetic field. The index of the frequ ency electric spectra is in the range similar to 3.5-4. Much flatter s pectra are found for the transverse magnetic fluctuations. The electri c to magnetic field ratio is large. Hence either the waves;are of pred ominantly electrostatic nature, or the radiation propagates transverse to the ambient magnetic field, so that the Viking loop antenna undere stimates their magnetic components. The bursts typically last several hundred milliseconds. Simultaneous electron observations exhibit the p resence of upward suprathermal electrons associated with energetic dow ngoing electron fluxes having power law energy distribution with index close to 4 in an energy range extending from similar to 2 keV up to 7 0-80 keV. Considering the wave bursts to be electromagnetic, propagati ng in the Z mode, and locally generated, we suggest that the observati ons might be explained in terms of a free electron maser mechanism who se free energy source lies in the observed power law energetic electro n fluxes.