title: Relativistic Electrons Produced by Foreshock Disturbances Observed Upstream of Earth's Bow Shock.
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 (∼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≤M<20) bow shock. Here we report observations of electrons energized by foreshock disturbances to energies up to at least ∼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.
authors.affiliation: NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors.forename: L B
authors.initials: LB
authors.lastname: Wilson
authors.affiliation: NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
authors.forename: D G
authors.initials: DG
authors.lastname: Sibeck
authors.affiliation: The Aerospace Corporation, El Segundo, California 90245, USA.
authors.forename: D L
authors.initials: DL
authors.lastname: Turner
authors.affiliation: Department of Radio Science, Aalto University, Espoo 02150, Finland.
authors.forename: A
authors.initials: A
authors.lastname: Osmane
authors.affiliation: Department of Astrophysical Sciences, Princeton University, Princeton, New Jersey 08544, USA and University of Chicago, Department of Astronomy and Astrophysics, Chicago, Illinois 60637, USA.
authors.forename: D
authors.initials: D
authors.lastname: Caprioli
authors.affiliation: Department of Earth, Planetary, and Space Sciences, and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, California 90095, USA.
authors.forename: V
authors.initials: V
authors.lastname: Angelopoulos