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Electron Scattering by Low-frequency Whistler Waves at Earth?s Bow Shock
Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 USA..
Kyushu Univ, Fukuoka, Fukuoka, Japan..
Kyushu Univ, Fukuoka, Fukuoka, Japan..
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2019 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 886, no 1, article id 53Article in journal (Refereed) Published
Abstract [en]

Electrons are accelerated to nonthermal energies at shocks in space and astrophysical environments. While shock drift acceleration (SDA) has been considered a key process of electron acceleration at Earth?s bow shock, it has also been recognized that SDA needs to be combined with an additional stochastic process to explain the observed power-law energy spectra. Here, we show mildly energetic (?0.5 keV) electrons are locally scattered (and accelerated while being confined) by magnetosonic-whistler waves within the shock transition layer, especially when the shock angle is large (<CDATA<i). When measured by the Magnetospheric Multiscale mission at a high cadence, ?0.5 keV electron flux increased exponentially in the shock transition layer. However, the flux profile was not entirely smooth and the fluctuation showed temporal/spectral association with large-amplitude (<CDATA<i), low-frequency (<CDATA<i where <CDATA<i is the cyclotron frequency), obliquely propagating (<CDATA<i, where <CDATA<i is the angle between the wave vector and background magnetic field) whistler waves, indicating that the particles were interacting with the waves. Particle simulations demonstrate that, although linear cyclotron resonances with ?0.5 keV electrons are unlikely due to the obliquity and low frequencies of the waves, the electrons are still scattered beyond 90; pitch angle by (1) resonant mirroring (transit-time damping), (2) non-resonant mirroring, and (3) subharmonic cyclotron resonances. Such coupled nonlinear scattering processes are likely to provide the stochasticity needed to explain the power-law formation.

Place, publisher, year, edition, pages
Institute of Physics (IOP), 2019. Vol. 886, no 1, article id 53
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Astronomy, Astrophysics and Cosmology
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URN: urn:nbn:se:kth:diva-265500DOI: 10.3847/1538-4357/ab4a81ISI: 000499366900001OAI: oai:DiVA.org:kth-265500DiVA, id: diva2:1379593
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QC 20191217

Available from: 2019-12-17 Created: 2019-12-17 Last updated: 2019-12-17Bibliographically approved

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Lindqvist, Per-Arne

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