  Foreshock disturbances -- large-scale (~1000 km to >30,000 km), transient
(~5-10 per day - lasting ~10s of seconds to several minutes) structures [1,2] -
generated by suprathermal (>100 eV to 100s of keV) ions [3,4] arise upstream of
Earth's bow shock formed by the solar wind colliding with the Earth's
magnetosphere. They have recently been found to accelerate ions to energies of
several keV [5,6]. Although electrons in Saturn's high Mach number (M > 40) bow
shock can be accelerated to relativistic energies (nearly 1000 keV) [7], it has
hitherto been thought impossible to accelerate electrons at the much weaker (M
< 20) Earth's bow shock beyond a few 10s of keV [8]. 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
reported, their presence has been attributed to escaping magnetospheric
particles [9,10] or solar events [11]. These relativistic electrons are not
associated with any solar activity nor are they of magnetospheric origin.
Further, current theories of ion acceleration in foreshock disturbances cannot
account for electrons accelerated to the observed relativistic energies
[12-17]. These electrons are clearly coming from the disturbances, leaving us
with no explanation as to their origin.
