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Found 2 entries in the Bibliography.


Showing entries from 1 through 2


2014

Wave normal angles of whistler-mode chorus rising and falling tones

We present a study of wave normal angles (θk) of whistler mode chorus emission as observed by Time History of Events and Macroscale Interactions during Substorms (THEMIS) during the year 2008. The three inner THEMIS satellites THA, THD, and THE usually orbit Earth close to the dipole magnetic equator (\textpm20\textdegree), covering a large range of L shells from the plasmasphere out to the magnetopause. Waveform measurements of electric and magnetic fields enable a detailed polarization analysis of chorus below 4 kHz. When displayed in a frequency-θk histogram, four characteristic regions of occurrence are evident. They are separated by gaps at f/fc,e≈0.5 (f is the chorus frequency, fc,e is the local electron cyclotron frequency) and at θk\~40\textdegree. Below θk\~40\textdegree, the average value for θk is predominantly field aligned, but slightly increasing with frequency toward half of fc,e (θk up to 20\textdegree). Above half of fc,e, the average θk is again decreasing with frequency. Above θk\~40\textdegree, wave normal angles are usually close to the resonance cone angle. Furthermore, we present a detailed comparison of electric and magnetic fields of chorus rising and falling tones. Falling tones exhibit peaks in occurrence solely for θk>40\textdegree and are propagating close to the resonance cone angle. Nevertheless, when comparing rising tones to falling tones at θk>40\textdegree, the ratio of magnetic to electric field shows no significant differences. Thus, we conclude that falling tones are generated under similar conditions as rising tones, with common source regions close to the magnetic equatorial plane.

Taubenschuss, Ulrich; Khotyaintsev, Yuri; ik, Ondrej; Vaivads, Andris; Cully, Christopher; Le Contel, Olivier; Angelopoulos, Vassilis;

Published by: Journal of Geophysical Research: Space Physics      Published on: 12/2014

YEAR: 2014     DOI: 10.1002/2014JA020575

Chorus; wave normal

First observation of rising-tone magnetosonic waves

Magnetosonic (MS) waves are linearly polarized emissions confined near the magnetic equator with wave normal angle near 90\textdegree and frequency below the lower hybrid frequency. Such waves, also termed equatorial noise, were traditionally known to be \textquotedbllefttemporally continuous\textquotedblright in their time-frequency spectrogram. Here we show for the first time that MS waves actually have discrete wave elements with rising-tone features in their spectrogram. The frequency sweep rate of MS waves, ~1 Hz/s, is between that of chorus and electromagnetic ion cyclotron (EMIC) waves. For the two events we analyzed, MS waves occur outside the plasmapause and cannot penetrate into the plasmasphere; their power is smaller than that of chorus. We suggest that the rising-tone feature of MS waves is a consequence of nonlinear wave-particle interaction, as is the case with chorus and EMIC waves.

Fu, H.; Cao, J.; Zhima, Z.; Khotyaintsev, Y.; Angelopoulos, V.; ik, O.; Omura, Y.; Taubenschuss, U.; Chen, L.; Huang, S;

Published by: Geophysical Research Letters      Published on: 11/2014

YEAR: 2014     DOI: 10.1002/grl.v41.2110.1002/2014GL061867

discrete; frequency sweep rate; magnetosonic wave; nonlinear wave-particle interaction; Plasmapause; rising tone



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