Measurability of the Nonlinear Response of Electron Distribution Function to Chorus Emissions in the Earth's Radiation Belt

We conduct test particle simulations to study the perturbations in a hot electron velocity distribution caused by a rising chorus element propagating parallel to the ambient magnetic field in the Earth's outer radiation belt. The wavefield is constructed from the nonlinear growth theory of chor...

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Bibliographic Details
Published in:Journal of geophysical research. Space physics 2021-09, Vol.126 (9), p.n/a
Main Authors: Hanzelka, M., Santolík, O., Omura, Y., Kolmašová, I.
Format: Article
Language:English
Subjects:
Analyzers
Backward waves
Chorus waves
Distribution functions
Earth magnetosphere
electromagnetic hole
Electromagnetic radiation
Electron distribution
Electron velocity distribution
Emissions
Energy resolution
Equator
Hot electrons
Liouville theorem
Magnetic equator
Magnetic fields
Mathematical models
nonlinear interaction
Nonlinear response
Outer radiation belt
particle measurements
Perturbation
Pitch (inclination)
Radiation
Radiation belt electrons
Radiation belts
Radiation counters
Resonant interactions
Space density
Spacecraft
Spacecraft instruments
Step functions
test particle simulation
Velocity
Velocity curve
Velocity distribution
whistler‐mode chorus
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Summary:We conduct test particle simulations to study the perturbations in a hot electron velocity distribution caused by a rising chorus element propagating parallel to the ambient magnetic field in the Earth's outer radiation belt. The wavefield is constructed from the nonlinear growth theory of chorus emissions of Omura (2021, https://doi.org/10.1186/s40623-021-01380-w), with additional considerations about saturation and propagation of the transverse resonant current being applied to model the subpacket structure. Using Liouville's theorem, we trace electrons back in time to reconstruct the evolution of electron velocity distribution at the magnetic equator. The electromagnetic hole created by nonlinear trapping and transport effects appears as a depression in the velocity distribution, aligned with the resonance velocity curve. We analyze the decrease of particle flux in this depression and estimate the energy resolution, pitch angle resolution, time resolution and geometric factor of particle analyzers needed to observe the perturbation. We conclude that particle detectors on current or recently operating spacecraft are always lacking in at least one of these parameters, which explains the missing direct observations of sharp phase space density depressions during chorus‐electron nonlinear resonant interaction. However, with a dedicated experiment and appropriate measurement strategy, such observations are within the possibilities of the current technology. Similarity of the simulated density perturbation and a step function mathematical model is used to draw an analogy between the backward wave oscillator regime of chorus generation and the nonlinear growth theory. Plain Language Summary The plasma environment in the Earth's magnetosphere supports natural growth of various electromagnetic waves, including the whistler‐mode chorus emissions, which consist of nonlinear chirping tones. These emissions can reach large amplitudes and play a major role in energization of radiation belt electrons. Nonlinear theories of chorus generation imply microscopic perturbations in resonant electron populations. A long‐standing problem is that these predictions were never directly confirmed by experimental observations. Here, we analyze perturbations of electron distribution functions numerically, taking into account spacecraft measurements of short subpackets within each chirping element. We reveal distinct perturbations, which are just below the measurability limits of exi
ISSN:2169-9380
2169-9402
DOI:10.1029/2021JA029624