This content will become publicly available on April 1, 2025
 Award ID(s):
 1914670
 NSFPAR ID:
 10530206
 Publisher / Repository:
 A&A
 Date Published:
 Journal Name:
 Astronomy & Astrophysics
 Volume:
 684
 ISSN:
 00046361
 Page Range / eLocation ID:
 A143
 Format(s):
 Medium: X
 Sponsoring Org:
 National Science Foundation
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Abstract The electron VDF in the solar wind consists of a Maxwellian core, a suprathermal halo, a fieldaligned component strahl, and an energetic superhalo that deviates from the equilibrium. Whistler wave turbulence is thought to resonantly scatter the observed electron velocity distribution. Wave–particle interactions that contribute to Whistler wave turbulence are introduced into a Fokker–Planck kinetic transport equation that describes the interaction between the suprathermal electrons and the Whistler waves. A recent numerical approach for solving the Fokker–Planck kinetic transport equation has been extended to include a full diffusion tensor. Application of the extended numerical approach to the transport of solar wind suprathermal electrons influenced by Whistler wave turbulence is presented. Comparison and analysis of the numerical results with observations and diagonalonly model results are made. The offdiagonal terms in the diffusion tensor act to depress effects caused by the diagonal terms. The role of the diffusion coefficient on the electron heat flux is discussed.more » « less

Abstract Resonant interactions of energetic electrons with electromagnetic whistler‐mode waves (
whistlers ) contribute significantly to the dynamics of electron fluxes in Earth's outer radiation belt. At low geomagnetic latitudes, these waves are very effective in pitch angle scattering and precipitation into the ionosphere of low equatorial pitch angle, tens of keV electrons and acceleration of high equatorial pitch angle electrons to relativistic energies. Relativistic (hundreds of keV), electrons may also be precipitated by resonant interaction with whistlers, but this requires waves propagating quasi‐parallel without significant intensity decrease to high latitudes where they can resonate with higher energy low equatorial pitch angle electrons than at the equator. Wave propagation away from the equatorial source region in a non‐uniform magnetic field leads to ray divergence from the originally field‐aligned direction and efficient wave damping by Landau resonance with suprathermal electrons, reducing the wave ability to scatter electrons at high latitudes. However, wave propagation can become ducted along field‐aligned density peaks (ducts), preventing ray divergence and wave damping. Such ducting may therefore result in significant relativistic electron precipitation. We present evidence that ducted whistlers efficiently precipitate relativistic electrons. We employ simultaneous near‐equatorial and ground‐based measurements of whistlers and low‐altitude electron precipitation measurements by ELFIN CubeSat. We show that ducted waves (appearing on the ground) efficiently scatter relativistic electrons into the loss cone, contrary to non‐ducted waves (absent on the ground) precipitating onlykeV electrons. Our results indicate that ducted whistlers may be quite significant for relativistic electron losses; they should be further studied statistically and possibly incorporated in radiation belt models. 
Abstract The electron resonant interaction with whistler‐mode waves is characterized by transport in pitch angle–energy space. We calculate electron diffusion and advection coefficients (a simplified characterization of transport) for a large range of electron pitch angle and energy using test particle simulations. Nonlinear effects are analyzed by comparing the diffusion coefficients using test particle simulations and quasilinear theory, and by evaluating the advection rates. Dependence of nonlinear effects on the wave amplitude and bandwidth of whistler‐mode waves is evaluated by running test particle simulations with a broad range of wave amplitude and bandwidth. The maximum amplitudes where the quasilinear approach is valid are found to increase with increasing bandwidth, from 50 pT for narrowband waves to 300 pT for broadband waves at
L ‐shell of 6. Moreover, interactions between intense whistler‐mode waves and small pitch angle electrons lead to large positive advection, which limits the applicability of diffusion‐based models. This study demonstrates the parameter range of the applicability of quasilinear theory and diffusion model for different wave amplitudes and frequency bandwidths of whistler‐mode waves, which is critical for evaluating the effects of whistler‐mode waves on energetic electrons in the Earth’s magnetosphere. 
Abstract Suprathermal electrons (~0.1–10 keV) in the inner magnetosphere are usually observed in a 90°‐peaked pitch angle distribution, formed due to the conservation of the first and second adiabatic invariants as they are transported from the plasma sheet. We report a peculiar field‐aligned suprathermal electron (FASE) distribution measured by Van Allen Probes, where parallel fluxes are 1 order of magnitude higher than perpendicular fluxes. Those FASEs are found to be closely correlated with large‐amplitude hiss waves and are observed around the Landau resonant energies. We demonstrate, using quasilinear diffusion simulations, that hiss waves can rapidly accelerate suprathermal electrons through Landau resonance and create the observed FASE population. The proposed mechanism potentially has broad implications for suprathermal electron dynamics as well as whistler mode waves in the Earth's magnetosphere and has been demonstrated in the Jovian magnetosphere.

Context. Whistler waves are electromagnetic waves produced by electrondriven instabilities, which in turn can reshape the electron distributions via wave–particle interactions. In the solar wind they are one of the main candidates for explaining the scattering of the strahl electron population into the halo at increasing radial distances from the Sun and for subsequently regulating the solar wind heat flux. However, it is unclear what type of instability dominates to drive whistler waves in the solar wind.Aims. Our goal is to study whistler wave parameters in the young solar wind sampled by Parker Solar Probe (PSP). The wave normal angle (WNA) in particular is a key parameter to discriminate between the generation mechanisms of these waves.Methods. We analyzed the crossspectral matrices of magnetic field fluctuations measured by the searchcoil magnetometer (SCM) and processed by the Digital Fields Board (DFB) from the FIELDS suite during PSP’s first perihelion.Results. Among the 2701 wave packets detected in the crossspectra, namely individual bins in time and frequency, most were quasiparallel to the background magnetic field; however, a significant part (3%) of the observed waves had oblique (> 45°) WNA. The validation analysis conducted with the time series waveforms reveal that this percentage is a lower limit. Moreover, we find that about 64% of the whistler waves detected in the spectra are associated with at least one magnetic dip.Conclusions. We conclude that magnetic dips provide favorable conditions for the generation of whistler waves. We hypothesize that the whistlers detected in magnetic dips are locally generated by the thermal anisotropy as quasiparallel and can gain obliqueness during their propagation. We finally discuss the implications of our results for the scattering of the strahl in the solar wind.