Lion roars in the magnetosheath: The Geotail observations

Details Lion roars in the magnetosheath: The Geotail observations Lion roars in the magnetosheath: The Geotail observations

Mar 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998jgr...103.4615z&link_type=abstract

Journal of Geophysical Research, Volume 103, Issue A3, p. 4615-4626

Physics

Plasma Physics

20

Magnetospheric Physics: Magnetosheath, Magnetospheric Physics: Plasma Waves And Instabilities, Space Plasma Physics: Wave/Particle Interactions, Space Plasma Physics: Waves And Instabilities

Scientific paper

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 (B0). About 70% (type B) of the lion roars are not associated with the decrease of the B0. Both types of the lion roars cover a frequency range from 0.02Ωe to 0.75Ωe. However, the dominant frequencies are around 0.12Ωe, where Ωe 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 (θkB~+/-10°) with respect to the B0. 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 B0. 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, θkB up to 87°) 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.

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