Observations of ion cyclotron harmonic waves by the Viking satellite

During the first months of operation the Swedish magnetospheric satellite Viking has detected many examples of plasma waves with spectral features ordered by the proton gyro frequency. Even though the proton gyro frequency characterizes all such spectra, there are many variations from spectrum to spectrum. Different bands are excited and the emissions are located within different parts of the harmonic bands at different times; sometimes a little above the harmonics, sometimes slightly below, and sometimes in the middle of the bands. The amplitudes of the waves are small, on the order of 1 mV/m, often less. Some of these waves do not appear to have been previously reported and we present here a representative example that took place at an altitude of about 13000 km on geomagnetic field lines connected to morning side auroral oval. The generation mechanism is still uncertain but simultaneously measured particle data indicate that there is free energy available in the loss cone in the hot ion population. If the loss cone is the source of free energy, these waves are of importance for proton precipitation in diffuse aurora.