Utilizing simultaneous twin Van Allen Probes observations of whistler mode waves at variable separations, we are able to distinguish the temporal variations from spatial variations, determine the coherence spatial scale, and suggest the possible mechanism of wave modulation. The two probes observed coherently modulated whistler mode waves simultaneously at an unexpectedly large distance up to ~4.3 RE over 3 h during a relatively quiet period. The modulation of 150–500 Hz plasmaspheric hiss was correlated with whistler mode waves measured outside the plasmasphere across 3 h in magnetic local time and 3 L shells, revealing that the modulation was temporal in nature. We suggest that the coherent modulation of whistler mode waves was associated with the coherent ULF waves measured over a large scale, which modulate the plasmaspheric density and result in the modulation of hiss waves via local amplification. In a later period, the 500–1500 Hz periodic rising-tone whistler mode waves were strongly correlated when the two probes traversed large spatial regions and even across the plasmapause. These periodic rising-tone emissions recurred with roughly the same period as the ULF wave, but there was no one-to-one correspondence, and a cross-correlation analysis suggests that they possibly originated from large L shells although the actual cause needs further investigation.
Bibliographical noteFunding Information:
J.L. and J.B. would like to acknowledge the NASA grant NNX13AI61G. We also thank EMFISIS subaward 1001057397:01; the ECT subaward 13-041; NSF Geospace Environment Modeling grant AGS-1723342, and NASA grants NNX14AN85G, NNX11AR64G, NNX17AD15G, and NNX15AI96G. This work was also supported by JHU/APL contracts 967399 and 921647 under NASA's prime contract NAS5-01072. We acknowledge the Van Allen Probes data from the EMFISIS instrument obtained from https://emfisis.physics.uiowa.edu/data/index, the EFW instrument data obtained from http://www.space.umn.edu/missions/rbspefw-home-university-of-minnesota/, and the MagEIS and HOPE data were downloaded from Van Allen Probe ECT website at http://www.rbsp-ect.lanl.gov/. We also thank the World Data Center for Geomagnetism, Kyoto, for providing SYM-H and AE indexes (http://wdc.kugi.kyoto-u.ac.jp/aeasy/index.html), and the Space Physics Data Facility at the NASA Goddard Space Flight Center for providing the OMNI2 data (ftp://spdf.gsfc.nasa.gov/pub/data/omni/omni_cdaweb/).
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- coherent waves
- periodic rising tone
- whistler mode