An official website of the United States government
Abstract Energetic electron precipitation into Earth's atmosphere is an important process for radiation belt dynamics and magnetosphere‐ionosphere coupling. The most intense form of such precipitation is microbursts—short‐lived bursts of precipitating fluxes detected on low‐altitude spacecraft. Due to the wide energy range of microbursts (from sub‐relativistic to relativistic energies) and their transient nature, they are thought to be predominantly associated with energetic electron scattering into the loss cone via cyclotron resonance with field‐aligned intense whistler‐mode chorus waves. In this study, we show that intense sub‐relativistic microbursts may be generated via electron nonlinear Landau resonance with very oblique whistler‐mode waves. We combine a theoretical model of nonlinear Landau resonance, equatorial observations of intense very oblique whistler‐mode waves, and conjugate low‐altitude observations of <200 keV electron precipitation. Based on model comparison with observed precipitation, we suggest that such sub‐relativistic microbursts occur by plasma sheet (0.1 − 10 keV) electron trapping in nonlinear Landau resonance, resulting in acceleration to ≲200 keV energies and simultaneous transport into the loss cone. The proposed scenario of intense sub‐relativistic (≲200 keV) microbursts demonstrates the importance of very oblique whistler‐mode waves for radiation belt dynamics.
more »
« less
This content will become publicly available on July 9, 2025
Oblique instability of quasi-parallel whistler waves in the presence of cold and warm electron populations
Whistler waves propagating nearly parallel to the ambient magnetic field experience a nonlinear instability due to transverse currents when the background plasma has a population of sufficiently low energy electrons. Intriguingly, this nonlinear process may generate oblique electrostatic waves, including whistlers near the resonance cone with properties resembling oblique chorus waves in the Earth’s magnetosphere. Focusing on the generation of oblique whistlers, earlier analysis of the instability is extended here to the case where low-energy background plasma consists of both a “cold” population with energy of a few eV and a “warm” electron component with energy of the order of 100 eV. This is motivated by spacecraft observations in the Earth’s magnetosphere where oblique chorus waves were shown to interact resonantly with the warm electrons. The main new results are: 1) the instability producing oblique electrostatic waves is sensitive to the shape of the electron distribution at low energies. In the whistler range of frequencies, two distinct peaks in the growth rate are typically present for the model considered: a peak associated with the warm electron population at relatively low wavenumbers and a peak associated with the cold electron population at relatively high wavenumbers; 2) overall, the instability producing oblique whistler waves near the resonance cone persists (with a reduced growth rate) even in the cases where the temperature of the cold population is relatively high, including cases where cold population is absent and only the warm population is included; 3) particle-in-cell simulations show that the instability leads to heating of the background plasma and formation of characteristic plateau and beam features in the parallel electron distribution function in the range of energies resonant with the instability. The plateau/beam features have been previously detected in spacecraft observations of oblique chorus waves. However, they have been attributed to external sources and have been proposed to be the mechanism generating oblique chorus. In the present scenario, the causality link is reversed and the instability generating oblique whistler waves is shown to be a possible mechanism for formation of the plateau and beam features.
more »
« less
- Award ID(s):
- 2031024
- PAR ID:
- 10539891
- Publisher / Repository:
- Frontiers Media S.A
- Date Published:
- Journal Name:
- Frontiers in Astronomy and Space Sciences
- Volume:
- 11
- ISSN:
- 2296-987X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Abstract Although electron cyclotron harmonic (ECH) waves are the primary contributor to plasma sheet electron scattering loss, experimental verification of their most widely accepted excitation mechanism, loss‐cone instability, has been lacking for decades. Using 10 years of time history of events and macroscale interactions during substorms satellite observations, we investigate ECH wave properties near dipolarization fronts, the predominant source of such waves. To our surprise we find that more than 30% of observed ECH waves have moderately oblique (∼70°) wave normal angles (WNA), much less than the ∼85° expected from classical loss‐cone instability. These moderately oblique WNA ECH waves carry a strong field‐aligned electric field that is used to identify them. They are often observed with cold, dense electrons that exhibit enhanced parallel flux at a few hundred eV energy, which suggests that low‐energy counterstreaming beams (likely of ionospheric origin) might be their free energy source. By solving the linear dispersion relation for parameters representative of such plasma sheet electron distributions, we confirm that ECH waves at WNA ∼ 70° can indeed be driven unstable by such beams. Our work reveals a previously unknown excitation mechanism for ECH waves and exposes the need for quantifying the conditions for and relative importance of beam‐driven waves compared to those excited by the loss‐cone instability in Earth's plasma sheet.more » « less
-
In this study, we present simultaneous multi-point observations of magnetospheric oscillations on a time scale of tens of minutes (forced-breathing mode) and modulated whistler-mode chorus waves, associated with concurrent energetic electron precipitation observed through enhanced BARREL X-rays. Similar fluctuations are observed in X-ray signatures and the compressional component of magnetic oscillations, spanning from ∼9 to 12 h in MLT and 5 to 11 inLshell. Such magnetospheric oscillations covering an extensive region in the pre-noon sector have been suggested to play a potential role in precipitating energetic electrons by either wave scattering or loss cone modulation, showing a high correlation with the enhancement in X-rays. In this event, the correlation coefficients between chorus waves (smoothed over 8 min), ambient magnetic field oscillations and X-rays are high. We perform an in-depth quasi-linear modeling analysis to evaluate the role of magnetic field oscillations in modulating energetic electron precipitation in the Earth’s magnetosphere through modulating whistler-mode chorus wave amplitude, resonance condition between chorus waves and electrons, as well as loss cone size. Model results further show that the modulation of chorus wave amplitude plays a dominant role in modulating the electron precipitation. However, the effect of the modulation in the resonant energy between chorus waves and energetic electrons due to the background magnetic field oscillations cannot be neglected. The bounce loss cone modulation, affected by the magnetic oscillations, has little influence on the electron precipitation modulation. Our results show that the low frequency magnetospheric oscillations could play a significant role in modulating the electron precipitation through modulating chorus wave intensity and the resonant energy between chorus waves and electron.more » « less
-
Abstract Electron cyclotron harmonic (ECH) waves, potential drivers for diffuse aurora precipitation, have been extensively investigated for decades. The generation mechanism of ECH waves, however, remains an open question. Theoretical work in 1970s has demonstrated that ECH waves can be excited by loss cone distributions of hot plasma sheet electrons. Recent THEMIS spacecraft observations, however, indicate that the waves can also be excited by low energy electron beams. Utilizing interferometry techniques to analyze the phase difference between electric potentials measured by individual probes on Electric Field Instrument antenna pairs on THEMIS spacecraft, we compute the wavenumber of both beam‐driven ECH waves and loss‐cone‐driven ECH waves. These wavenumber measurements as well as other wave properties obtained from spacecraft measurements prove to be consistent with expectation from linear instability analysis. This provides us with independent verification of the generation mechanism and linear dispersion relation of beam‐driven and loss‐cone‐driven ECH waves. Our statistical results demonstrate that the median value of the wave vectors of beam‐driven ECH waves, characterized by wave normal angles () less than 80°, is 0.011 m−1; and that of loss‐cone‐driven ECH waves, characterized by wave normal angles larger than 85°, is 0.00765 m−1. Direct wavenumber measurements of ECH waves allow us to better understand the interaction between ECH waves and electrons in Earth's magnetosphere.more » « less
-
Abstract Energetic electron losses in the Earth's inner magnetosphere are dominated by outward radial diffusion and scattering into the atmosphere by various electromagnetic waves. The two most important wave modes responsible for electron scattering are electromagnetic ion cyclotron (EMIC) waves and whistler‐mode waves (whistler waves) that, acting together, can provide rapid electron losses over a wide energy range from few keV to few MeV. Wave‐particle resonant interaction resulting in electron scattering is well described by quasi‐linear diffusion theory using the cold plasma dispersion, whereas the effects of nonlinear resonances and hot plasma dispersion are less well understood. This study aims to examine these effects and estimate their significance for a particular event during which both wave modes are quasi‐periodically modulated by ultra‐low‐frequency (ULF) compressional waves. Such modulation of EMIC and whistler wave amplitudes provides a unique opportunity to compare nonlinear resonant scattering (important for the most intense waves) with quasi‐linear diffusion (dominant for low‐intensity waves). The same modulation of plasma properties allows better characterization of hot plasma effects on the EMIC wave dispersion. Although hot plasma effects significantly increase the minimum resonant energy,Emin, for the most intense EMIC waves, such effects become negligible for the higher frequency part of the hydrogen‐band EMIC wave spectrum. Nonlinear phase trapping of 300–500 keV electrons through resonances with whistler waves may accelerate and make them resonant with EMIC waves that, in turn, quickly scatter those electrons into the loss‐cone. Our results highlight the importance of nonlinear effects for simulations of energetic electron fluxes in the inner magnetosphere.more » « less
