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

Abstract

Foreshock disturbances -large-scale (∼1000 km to >30,000 km), solitary (∼5-10 per day, transient (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 . One type was found to have a distinct suprathermal electron population with energies >70 keV, which was attributed to a magnetospheric origin 7 . Although electrons in Saturn's high Mach number (M > 40) bow shock can be accelerated to relativistic energies (nearly 1000 keV) 8 , 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 9 . Here we report observations of electrons energized by foreshock disturbances from 10s of eV up to at least ∼300 keV. We observe a single isotropic power-law from 100s of eV to 100s of keV, unlike previous studies 7 . All previous observations of energetic foreshock electrons have been attributed to escaping magnetospheric particles 7,10,11 or solar events 12 . We observe no solar activity and the single isotropic power-law cannot be explained by any magnetospheric source. Further, current theories of ion acceleration in foreshock disturbances cannot account for electrons accelerated to the observed relativistic energies [13][14][15][16][17][18] . These electrons are clearly coming from the disturbances, leaving us with no explanation for the acceleration mechanism.We examine in detail particle velocity distribution functions measured by the low energy electron/total ion electrostatic analyzers 19 The disturbances occur in the ion foreshock 3,4 region upstream of the quasi-parallel bow shock, where the shock normal angle (θ Bn ) between the upstream quasi-static magnetic field (Bo) and the shock normal vector satisfies θ Bn < 45 • . We focused on observations near short large-amplitude magnetic structures 4 , hot flow anomalies 1 , and foreshock bubbles 2 . During the four orbital passes examined in detail, we identified 30 foreshock disturbances, 10 of which had clear energetic electron enhancements (five at short large-amplitude magnetic structures, two at hot flow anomalies, and three at foreshock bubbles). See Extended Data Fig. 1 for spacecraft orbits and Methods for further details of foreshock disturbances.We observe energetic (≥30 keV) electron enhancements as short duration (∼10s of seconds to few minutes) enhancements in the electron fluxes above background by factors of ∼10-200 (Fig. 1). They are localized to the large fluctuations in Bo (Figs 1a-1c) within the foreshock disturbances. The electron flux-time profiles for energies from ∼0.25 to >200 keV show no energy dispersion, i.e., fluxes increase at all energies simultaneously (Figs 1g-1i and 1m-1o). The low energy electron and ion (Figs 1j-1l) data look qualitatively similar for disturbances with and without (Extended Data Fig. 2) energetic ...

Statistical study of particle acceleration in the core of foreshock transients

“…In ~30% of our events, the maximum energy is larger than 25 keV (minimum SST energy range) in Figure g. In some of the events, energies are hundreds of keV to 700 keV, consistent with the 100–300 keV events reported by Wilson et al [] (their events are shown in green in Figure g).…”

“…For example, diffusive shock acceleration at shocks requires particles that have large initial energy, but where such particles obtain this energy is still unknown (the so‐called “injection” problem) [ Jokipii , ]. We have shown that foreshock transients often preaccelerate particles before they interact with the shock and bring them toward the bow shock supporting previous results [e.g., Liu et al ., ; Wilson et al ., ]. Foreshock transients could in principle form everywhere throughout the universe upstream of shocks, and thus, they may be quite important for addressing the injection problem.…”

Ion Acceleration Inside Foreshock Transients

“…This study, along with other recent observations (e.g., Liu, Hietala, et al, ; Liu, Angelopoulos, & Hietala, ; Liu, Angelopoulos, Hietala, & Wilson, ; Liu, Lu, et al, ; Wilson et al, ), demonstrates foreshock transients' ability to accelerate particles, implying that they have an important role in shock acceleration like providing a source for diffusive shock acceleration and repopulating foreshock ions, and potentially contributing to enhanced magnetospheric fluxes. These topics deserve more attention and multipoint studies in the future.…”

Energetic ion leakage from foreshock transient cores

“…Wilson et al . [] also reported that electrons in the foreshock transients can be accelerated to 100–300 keV. A recent statistical study shows that although the electron energy estimates of the suprathermal component in foreshock transient cores is almost always higher than in the background foreshock, an ion mean energy enhancement in the core is only occasionally observed [ Liu et al ., ].…”