Long‐duration whistler waves in the magnetosheath: Wave characteristics and the possible source region

A type of whistler waves termed “long-duration whistler waves” (LDWW) in ELF range (1 ∼ 64 Hz) in the magnetosheath is studied according to the wave characteristics and generation based on data observed by the search coil magnetometer onboard the Geotail satellite. LDWW are band-limited emissions near the lower hybrid frequency typically lasting several tens of minutes. Orientations of wave normal are determined from waveform data of vector magnetic field with an assumption of plane waves and are observed to be fairly well organized in a certain duration of LDWW. Ambiguity in propagation directions is removed by comparison of the phase relation between magnetic components and one component of electric field. The propagation vectors of LDWW are statistically aligned along the “Parker spiral” but are primarily reversed in the dusksides and in the dawnsides of the magnetosheath; that is, one is sunward, and the other is antisunward. This outstanding asymmetry strongly suggests that the bow shock region is the common source region with waves propagating away from the bow shock along the draped magnetic field in the magnetosheath. Electrons composing a type of distribution function in flat-topped shape are concurrently observed during a series of LDWW events and are likely to yield a favorite condition for LDWW propagating a long path free from attenuation caused by wave particle interactions.

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