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Observations of the Source Region of Whistler Mode Waves in Magnetosheath Mirror Structures
Author(s) -
Kitamura N.,
Omura Y.,
Nakamura S.,
Amano T.,
Boardsen S. A.,
Ahmadi N.,
Le Contel O.,
Lindqvist P.A.,
Ergun R. E.,
Saito Y.,
Yokota S.,
Gershman D. J.,
Paterson W. R.,
Pollock C. J.,
Giles B. L.,
Russell C. T.,
Strangeway R. J.,
Burch J. L.
Publication year - 2020
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1029/2019ja027488
Subject(s) - physics , whistler , computational physics , poynting vector , magnetosheath , amplitude , magnetic field , geophysics , earth's magnetic field , field line , magnetosphere , optics , magnetopause , quantum mechanics
In the magnetosheath, intense whistler mode waves, called “Lion roars,” are often detected in troughs of magnetic field intensity in mirror mode structures. Using data obtained by the four Magnetospheric Multiscale (MMS) spacecraft, we show that reversals of gradient of magnetic field intensity along the magnetic field correspond to reversals of the field‐aligned component of Poynting flux of whistler mode waves in the troughs. Such a characteristic is consistent with the idea that the whistler mode waves are effectively generated near the local minima of magnetic field intensity because of the smallest cyclotron resonance velocity and propagate toward regions of larger magnetic field intensity along the magnetic field lines on both sides. We use the reversal of the Poynting flux as an indicator of wave source regions. In these regions, we find that pancake or an outer edge of butterfly electron distributions above ~100 eV are good candidates for wave generation. Unclear correlations of phase difference and amplitude variations of whistler mode waves in cases of ~40 km spacecraft separation indicate that a simple plane wave approximation with a constant amplitude is not valid at this spatial scale that is much smaller than the ion gyroradius. The whistler mode waves consist of small coherent wave packets from multiple sources with spatial scales smaller than tens of electron gyroradii transverse to the background magnetic field in a mirror mode structure.