Letter to the Editor

Relativistic electron beams accelerated by an interplanetary shock

¹ Department of Physics and Astronomy, University of Turku, 20500 Turku, Finland
e-mail: immanuel.c.jebaraj@gmail.com
² LPC2E/CNRS, UMR 7328, 3A Avenue de la Recherche Scientifique, Orléans, France
³ Space Sciences Laboratory, University of California, Berkeley, CA, USA
⁴ The Blackett Laboratory, Department of Physics, Imperial College London, London, UK
⁵ Center for mathematical Plasma Astrophysics, Department of Mathematics, KU Leuven, Celestijnenlaan 200B, 3001 Leuven, Belgium
⁶ Queen Mary University of London, School of Physics and Astronomy, London, UK
⁷ The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, 20723 USA
⁸ Swedish Institute of Space Physics, PO Box 537 751 21 Uppsala, Sweden
⁹ Institute of Experimental & applied Physics, Kiel University, 24118 Kiel, Germany
¹⁰ Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, 20771 USA

Received: 1 October 2023
Accepted: 10 November 2023

Abstract

Context. Collisionless shock waves have long been considered to be among 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 heliophysics and astrophysics is the acceleration of electrons to relativistic energies (> 300 keV) by traveling shocks.

Aims. The reason for an incomplete understanding of electron acceleration at IP shocks is due to scale-related challenges and a lack of instrumental capabilities. This Letter presents the 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 presents the characteristics of the electron beams close to the source and contributes to the understanding of their acceleration mechanism.

Methods. 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 min 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.

Results. By leveraging Solar Orbiter’s high temporal resolution Energetic Particle Detector (EPD), we successfully showcase 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.