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Quasiperiodic Emissions: Fine Structure Corresponding to a Bouncing Wave
Journal article   Open access   Peer reviewed

Quasiperiodic Emissions: Fine Structure Corresponding to a Bouncing Wave

F. Němec, O. Santolík, G. B. Hospodarsky and W. S. Kurth
Geophysical research letters, Vol.51(2), e2023GL106459
01/28/2024
DOI: 10.1029/2023GL106459
url
https://doi.org/10.1029/2023GL106459View
Published (Version of record) Open Access

Abstract

Abstract Quasiperiodic (QP) emissions are whistler‐mode electromagnetic waves observed in the Earth's inner magnetosphere whose intensity has a nearly periodic time modulation with typical modulation periods on the order of minutes. Some events exhibit, on top of the main modulation period, an additional fine inner modulation with modulation periods on the order of seconds. We use high‐resolution multi‐component electromagnetic wave data obtained by the Van Allen Probes spacecraft to investigate one such event. Detailed wave analysis demonstrates that the fine inner modulation is due to a wave packet bouncing back and forth between the hemispheres. The presence of a density duct is important for the formation of the event, as demonstrated by the increased ratio of wave power propagating away from the equator (a tentative source region) within the duct. The main QP modulation period corresponds to the plasma number density modulation observed just outside the plasmasphere. Plain Language Summary The intensity of electromagnetic waves in the near‐Earth space, the magnetosphere, sometimes has a nearly periodic temporal modulation on the order of minutes. The origin of such waves, so‐called quasiperiodic emissions, is not yet fully understood. On top of the main modulation period, some events exhibit an additional fine inner modulation with periods on the order of seconds. We use wave propagation directions determined from the Van Allen Probes measurements to demonstrate that this shorter modulation corresponds to a wave packet bouncing in between the hemispheres. By examining the ratio of wave power propagating away from and toward the geomagnetic equator (a tentative source region), we further demonstrate that the presence of a region with enhanced density, guiding waves along a given magnetic field line, is important for the formation of the event. Additionally, the main modulation period of the event corresponds to the plasma number density modulation observed just outside the plasmasphere, possibly linked to a plasmapause surface wave. Our results, revealing the presence and origin of the fine inner structure of the waves, provide important observational constraints for models trying to explain the generation of quasiperiodic emissions. Key Points Whistler‐mode quasiperiodic event has a fine inner structure that is related to the wave bouncing between hemispheres Wave generation is related to the presence of a density duct, but the event can also be observed outside the duct Plasma number density just outside the plasmasphere is modulated with a period that corresponds to the period of the quasiperiodic event

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