Collisionless shock waves have long been considered amongst the most prolific
particle accelerators in the universe. Shocks alter the plasma they propagate
through and often exhibit complex evolution across multiple scales.
Interplanetary (IP) traveling shocks have been recorded in-situ for over half a
century and act as a natural laboratory for experimentally verifying various
aspects of large-scale collisionless shocks. A fundamentally interesting
problem in both helio and astrophysics is the acceleration of electrons to
relativistic energies (more than 300 keV) by traveling shocks. This letter
presents first observations of field-aligned beams of relativistic electrons
upstream of an IP shock observed thanks to the instrumental capabilities of
Solar Orbiter. This study aims to present the characteristics of the electron
beams close to the source and contribute towards understanding their
acceleration mechanism. On 25 July 2022, Solar Orbiter encountered an IP shock
at 0.98 AU. The shock was associated with an energetic storm particle event
which also featured upstream field-aligned relativistic electron beams observed
14 minutes prior to the actual shock crossing. The distance of the beam's
origin was investigated using a velocity dispersion analysis (VDA).
Peak-intensity energy spectra were anaylzed and compared with those obtained
from a semi-analytical fast-Fermi acceleration model. By leveraging Solar
Orbiter's high-time resolution Energetic Particle Detector (EPD), we have
successfully showcased an IP shock's ability to accelerate relativistic
electron beams. Our proposed acceleration mechanism offers an explanation for
the observed electron beam and its characteristics, while we also explore the
potential contributions of more complex mechanisms.Comment: Main text: 6 pages, 2 figures. Supplementary material: 6 pages, 7
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