We present observations of higher-frequency (~50–2500 Hz, ~0.1–0.7 fce) wave modes modulated at the frequency of colocated lower frequency (0.5–2 Hz, on the order of fci) waves. These observations come from the Van Allen Probes Electric Field and Waves instrument's burst mode data and represent the first observations of coupling between waves in these frequency ranges. The higher-frequency wave modes, typically whistler mode hiss and chorus or magnetosonic waves, last for a few to a few tens of seconds but are in some cases observed repeatedly over several hours. The higher-frequency waves are observed to be unmodulated before and after the presence of the electromagnetic ion cyclotron (EMIC) waves, but when the EMIC waves are present, the amplitude of the higher-frequency waves drops to the instrument noise level once every EMIC wave cycle. Such modulation could significantly impact wave-particle interactions such as acceleration and pitch angle scattering, which are crucial in the formation and depletion of the radiation belts. We present one case study with broadband, high-frequency waves observed to be modulated by EMIC waves repeatedly over a 2 h time span on both spacecraft. Finally, we show two additional case studies where other high-frequency wave modes exhibit similar modulation.
Bibliographical noteFunding Information:
This work was supported by RBSP-EFW funding provided by JHU/APL contract 922613, under NASA's prime contract NAS5-01072. Van Allen Probes data used in this paper can be found in the EFW (http://www.space.umn.edu/rbspefw-data/), EMFISIS (http://emfisis.physics.uiowa.edu/data/index), and ECT (http://www.rbsp-ect.lanl.gov/rbsp_ect.php) archives. Research at Augsburg College was supported by NSF grant PLR-1341493.
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