Relativistic Electrons Produced by Foreshock Disturbances Observed Upstream of Earth's Bow Shock

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Relativistic Electrons Produced by Foreshock Disturbances Observed Upstream of Earth’s Bow Shock

L. B. Wilson, III, D. G. Sibeck, D. L. Turner, A. Osmane, D. Caprioli, and V. Angelopoulos

Phys. Rev. Lett. 117, 215101 – Published 14 November 2016

Abstract

Charged particles can be reflected and accelerated by strong (i.e., high Mach number) astrophysical collisionless shock waves, streaming away to form a foreshock region in communication with the shock. Foreshocks are primarily populated by suprathermal ions that can generate foreshock disturbances—large-scale (i.e., tens to thousands of thermal ion Larmor radii), transient ( $\sim 5 - 10 per day$ ) structures. They have recently been found to accelerate ions to energies of several keV. Although electrons in Saturn’s high Mach number ( $M > 40$ ) bow shock can be accelerated to relativistic energies (nearly 1000 keV), it has hitherto been thought impossible to accelerate electrons beyond a few tens of keV at Earth’s low Mach number ( $1 \leq M < 20$ ) bow shock. Here we report observations of electrons energized by foreshock disturbances to energies up to at least $\sim 300 keV$ . Although such energetic electrons have been previously observed, their presence has been attributed to escaping magnetospheric particles or solar events. These relativistic electrons are not associated with any solar or magnetospheric activity. Further, due to their relatively small Larmor radii (compared to magnetic gradient scale lengths) and large thermal speeds (compared to shock speeds), no known shock acceleration mechanism can energize thermal electrons up to relativistic energies. The discovery of relativistic electrons associated with foreshock structures commonly generated in astrophysical shocks could provide a new paradigm for electron injections and acceleration in collisionless plasmas.

Received 16 July 2016

DOI:https://doi.org/10.1103/PhysRevLett.117.215101

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

Physics Subject Headings (PhySH)

Collisionless shock in plasma Particle acceleration in plasmas Plasma instabilities Solitons Space & astrophysical plasma Whistler plasma waves

Planetary bow shock

Plasma Physics

Authors & Affiliations

L. B. Wilson, III^* and D. G. Sibeck

NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA

D. L. Turner

The Aerospace Corporation, El Segundo, California 90245, USA

A. Osmane

Department of Radio Science, Aalto University, Espoo 02150, Finland

D. Caprioli

Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA and University of Chicago, Department of Astronomy and Astrophysics, Chicago, Illinois 60637, USA

V. Angelopoulos

Department of Earth, Planetary, and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90095, USA

^*lynn.b.wilsoniii@gmail.com

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Vol. 117, Iss. 21 — 18 November 2016

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Images

Figure 1
Three example foreshock disturbances with energetic electron enhancements. All data were observed by THEMIS-C, with disturbance centers indicated by the vertical magenta lines. Magnetic fields are shown in units of nanotesla (nT) in the geocentric solar ecliptic coordinate basis. Particle data are shown in units of intensity ( ${cm}^{- 2} s^{- 1} {sr}^{- 1} {eV}^{- 1}$ ) (or flux) as omnidirectional averages in the bulk flow rest frame with uniform color schemes (legends at far right) and vertical axis ranges by row. The low and high energy electron and high energy ion data are all shown as stacked line plots of intensity versus time, where each line corresponds to a different energy. The low energy ion data are shown as a dynamic energy spectrogram of energy versus time with a color scale [right-hand side of (l)] for intensity. (a)–(c) $| B_{0} |$ (nT) at 4 samples per second. (d)–(f) $B_{0}$ (nT) at 4 samples per second. (g)–(i) low energy electron (i.e., $\sim 50 eV - 12 keV$ ) intensity. (j)–(l) Low energy ion (i.e., $\sim 10 eV - 25 keV$ ) intensity. (m)–(o) High energy electron (i.e., $\sim 30 - 300 keV$ ) intensity. (p)–(r) High energy ion (i.e., $\sim 30 - 430 keV$ ) intensity.
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Figure 2
Example pitch-angle spectra inside and outside electron enhancement periods for each of the example disturbances in Fig. 1. One-dimensional directional cuts (with respect to $B_{0}$ ) of the merged particle velocity distributions from the low and high energy electron data, in units of phase space density ( $s^{+ 3} {cm}^{- 3} {km}^{- 3}$ ), on log-log plots with uniform horizontal and vertical axis ranges of $\sim 10^{- 26} - 10^{- 12} s^{+ 3} {cm}^{- 3} {km}^{- 3}$ and $\sim 0.050 - 400 keV$ , respectively. The color-coded boxes above each plot correspond to the color-coded arrows in Fig. 1. Each panel shows cuts parallel (red), perpendicular (green), and antiparallel (blue) to $B_{0}$ , where missing data points indicate an absence of significant flux. The orange dashed line shows a power law [defined in (a)] for perspective. (a)–(c) Distributions during the peak energetic electron enhancements in Fig. 1. (d)–(f) Distributions during periods of inactivity in Fig. 1.
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Figure 3
Example time evolution of particle spectra in field-aligned coordinates during an enhancement within the foreshock bubble shown in Fig. 1 on 2008-07-14. Magnetic field data are in units of nanotesla (nT) in the geocentric solar ecliptic coordinate basis. Particle data are shown as one-dimensional cuts (similar to Fig. 2) in units of phase space density ( $s^{+ 3} {cm}^{- 3} {km}^{- 3}$ ), on log-log plots with the same uniform horizontal and vertical axis ranges as in Fig. 2. The color-coded boxes above all particle distribution plots [(c)–(e)] correspond to the color-coded time ranges in the magnetic field plots below [(a) and (b)].
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