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Abstract Electron cyclotron harmonic waves (ECH) play a key role in scattering and precipitation of plasma sheet electrons. Previous analysis on the resonant interaction between ECH waves and electrons assumed that these waves are generated by a loss cone distribution and propagate nearly perpendicular to the background magnetic field. Recent spacecraft observations, however, have demonstrated that such waves can also be generated by low energy electron beams and propagate at moderately oblique angles . To quantify the effects of this newly observed ECH wave mode on electron dynamics in Earth's magnetosphere, we use quasi‐linear theory to calculate the associated electron pitch angle diffusion coefficient. Utilizing THEMIS spacecraft measurements, we analyze in detail a few representative events of beam‐driven ECH waves in the plasma sheet and the outer radiation belt. Based on the observed wave properties and the hot plasma dispersion relation of these waves, we calculate their bounce‐averaged pitch angle, momentum and mixed diffusion coefficients. We find that these waves most efficiently scatter low‐energy electrons (10–500 eV) toward larger pitch angles, on time scales of to seconds. In contrast, loss‐cone‐driven ECH waves most efficiently scatter higher‐energy electrons (500 eV–5 keV) toward lower pitch‐angles. Importantly, beam‐driven ECH waves can effectively scatter ionospheric electron outflows out of the loss cone near the magnetic equator. As a result, these outflows become trapped in the magnetosphere, forming a near‐field‐aligned anisotropic electron population. Our work highlights the importance of ECH waves, particularly beam‐driven modes, in regulating magnetosphere‐ionosphere particle and energy coupling.
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Beam‐Driven Electron Cyclotron Harmonic Waves in Earth's Magnetotail
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.
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- Award ID(s):
- 1902699
- PAR ID:
- 10374866
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Journal of Geophysical Research: Space Physics
- Volume:
- 126
- Issue:
- 3
- ISSN:
- 2169-9380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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