Observation of broadband wave bursts with power law spectra above the plasma frequency in the underdense auroral plasma

Viking spacecraft spectral observations of high-frequency waves with frequencies f_e<f<f_ce between the electron plasma (f_e) and cyclotron (f_ce) frequencies in the strongly magnetized nightside auroral plasma occasionally exhibit impulsive emissions with featureless power law spectra extending sometimes up to frequencies close to f_ce. Most of these bursts are observed when the electric antenna is at large angles with respect to the ambient magnetic field. The index of the frequency electric spectra is in the range ∼3.5-4. Much flatter spectra are found for the transverse magnetic fluctuations. The electric to magnetic field ratio is large. Hence either the waves are of predominantly electrostatic nature, or the radiation propagates transverse to the ambient magnetic field, so that the Viking loop antenna underestimates their magnetic components. The bursts typically last several hundred milliseconds. Simultaneous electron observations exhibit the presence of upward suprathermal electrons associated with energetic downgoing electron fluxes having power law energy distribution with index close to 4 in an energy range extending from ∼2 keV up to 70-80 keV. Considering the wave bursts to be electromagnetic, propagating in the Z mode, and locally generated, we suggest that the observations might be explained in terms of a free electron maser mechanism whose free energy source lies in the observed power law energetic electron fluxes.