A statistical study of auroral electromagnetic ion cyclotron waves

This paper discusses results from a statistical study of narrowband auroral electromagnetic ion cyclotron (EMIC) waves recorded by the Freja magnetic field experiment from December 1, 1993, to November 30, 1994. The study involves data collected at all magnetic local times (MLT), invariant latitudes between 40 degrees and 75 degrees, and altitudes between 1200 and 1750 km. Wave events have been identified using a two-step process. First, magnetic field spectra in the frequency range between 5 and 256 Hz were surveyed using an automated procedure which detects spectral peaks. Second, the results of this automated procedure were confirmed by visually inspecting wave spectrograms which contained these spectral peaks. A total of 316 auroral EMIC wave events were identified. The occurrence of these waves peak at auroral latitudes in the premidnight sector (1800-0100 MLT). The events have a strong seasonal dependence with peak occurrence in the winter. These properties are the same as those of auroral electron precipitation, which is suggested to be the free energy source of these waves. The spectral properties of the waves were also analyzed. The wave usually contained either one or two spectral peaks. In the cases where two spectral peaks were observed one peak was below the local O+ gyrofrequency (f(o)+) and one was above the He+ gyrofrequency (f(He)+). The observations suggest that auroral EMIC waves can be generated below f(o)+. The wave spectra were consistent with wave generation at altitudes near or above Freja's altitude. Analysis of the spectra suggest that the auroral EMIC waves originated from a source altitude between 1500 and 5500 km.