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Bursts of whistler mode waves in the upstream of the bow shock: Geotail observations
Author(s) -
Zhang Y.,
Matsumoto H.,
Kojima H.
Publication year - 1998
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.67
H-Index - 298
eISSN - 2156-2202
pISSN - 0148-0227
DOI - 10.1029/98ja01371
Subject(s) - whistler , physics , foreshock , electron , bow shock (aerodynamics) , computational physics , geophysics , bow wave , solar wind , magnetic field , atomic physics , shock wave , geology , seismology , nuclear physics , quantum mechanics , aftershock , thermodynamics
Bursts of narrowband and short‐lived electromagnetic waves are frequently observed in the upstream (the electron foreshock and the pure solar wind) by the waveform capture instrument on board Geotail satellite. The electromagnetic waves with frequencies above 10 Hz are all right‐hand circularly polarized and identified as whistler mode waves. We call the waves narrowband and short‐lived whistlers (NSW). Nearly all the NSW in the electron foreshock propagate in a downstream direction parallel to the ambient magnetic field ( B o ) with an average θ kB of 16°, where θ kB is an angle between NSW wave vector and the B o . Their frequencies cover a range from 0.05 to 1.0Ω e with an average of 0.35Ω e , where Ω e is a local electron cyclotron frequency. Their amplitudes range from a few picoteslas to 100 pT with an average of 23 pT. Because of their parallel propagation, the NSW must be excited by electron cyclotron resonance. These features of the NSW suggest existence of electron beams which travel in an upstream direction parallel to the B o and which have a temperature anisotropy. Kinetic energies of the beams range from a few eV to about 200 eV (28 eV on average). All these characteristics of the electron beams revealed from the NSW are consistent with ISEE and WIND particle observations. The competition between electrostatic and whistler instabilities and the finite size of the beams are very likely the reasons why the NSW are short‐lived. These NSW can be well explained by a modeled electron beam with a losscone distribution in the electron foreshock. The NSW in solar wind are very similar to those in the electron foreshock. However, they have larger amplitudes (34 pT on average), lower frequencies (0.24Ω e on average), and higher cyclotron resonant energy (100 eV on average). They are very likely excited by halo electrons or nonlocal sources in the solar wind.

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