First observation of rising-tone magnetosonic waves
Santolík, O.; Chen, L.; Cao, J. B.; Huang, S. Y.; Fu, H. S.; Khotyaintsev, Y. V.; Angelopoulos, V.; Omura, Y.; Taubenschuss, U.; Zhima, Z.
China, Sweden, United States, Czech Republic, Japan
Abstract
Magnetosonic (MS) waves are linearly polarized emissions confined near the magnetic equator with wave normal angle near 90° and frequency below the lower hybrid frequency. Such waves, also termed equatorial noise, were traditionally known to be "temporally continuous" 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.