Lion roars in the magnetosheath: The Geotail observations

Lion roars in the magnetosheath are studied based on the waveform data recorded by the Geotail plasma wave instrument (PWI). It is found that about 30% (type A) of the lion roars are associated with the mirror waves or the decrease of the ambient magnetic field (B-o). About 70% (type B) of the lion roars are not associated with the decrease of the B-o. Both types of the lion roars cover a frequency range from 0.02 Omega(e), to 0.75 Omega(e). However, the dominant frequencies are around 0.12 Omega(e), where Omega(e) is the electron cyclotron frequency. The amplitudes of the lion roars range from a few picoteslas to a few hundreds of picoteslas. Their average amplitude is about 110 pT. Most of these lion roars propagate in a slightly oblique direction (theta(kB) similar to +/-10 degrees) with respect to the B-o. One important feature of these lion roars is that they propagate mostly in a single direction, either in the direction of or in the opposite direction to the B-o. This suggests that the sources of the lion roars are very likely away from the location where they are observed. Five percent of the lion roars (only the type B lion roars) propagate in two opposite directions simultaneously. This indicates a possibility that the lion roars are generated locally in the vicinity of the spacecraft for these cases. Those highly oblique lion roars (the type B, theta(kB) up to 87 degrees) are usually observed near the bow shock and they are very likely the downstream propagating whistlers excited in the bow shock region. The nearly monotonic decrease of the wave normal distributions of the lion roars suggests that the Landau damping may play an important role. The relatively narrow wave normal distribution of the types A lion roars than that of the type B lion roars suggests that the type A lion roars may be closer to their sources than the type B lion roars.