Resolving spatial and temporal shock structures using LOFAR observations of type II radio bursts

¹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
² Turku Collegium for Science, Medicine and Technology, University of Turku, 20014 Turku, Finland
³ Center for Solar-Terrestrial Research, New Jersey Institute of Technology, Newark, New Jersey, USA
⁴ ASTRON – the Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁵ Space Radio-Diagnostics Research Centre, University of Warmia and Mazury, R. Prawochenskiego 9, 10-719 Olsztyn, Poland
⁶ Astronomy & Astrophysics Section, Dublin Institute for Advanced Studies, DIAS Dunsink Observatory, Dublin D15 XR2R, Ireland
⁷ Leibniz Institute for Astrophysics Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany

^⋆ Corresponding author; diana.morosan@utu.fi

Received: 28 October 2024
Accepted: 14 February 2025

Abstract

Context. Collisionless shocks are one of the most powerful particle accelerators in the Universe. In the heliosphere, type II solar radio bursts are signatures of electrons accelerated by collisionless shocks launched at the Sun. Spectral observations of these bursts show a variety of fine structures often composing multiple type II lanes. The origin of these lanes and structures is not well understood and has been attributed to the inhomogeneous environment around the propagating shock.

Aims. Here, we aim to determine the large-scale local structures near a coronal shock wave using high-resolution radio imaging observations of a complex type II radio burst observed on 3 October 2023.

Methods. By using inteferometric imaging from the Low Frequency Array (LOFAR), combined with extreme ultraviolet observations, we investigate the origin of multiple type II lanes at low frequencies (30−80 MHz) relative to the propagating shock wave.

Results. We identify at least three radio sources at metric wavelengths corresponding to a multi-lane type II burst. The type II burst sources propagate outwards with a shock driven by a coronal mass ejection. We find a double radio source that exhibits increasing separation over time, consistent with the expansion rate of the global coronal shock. This suggests that the overall shock expansion is nearly self-similar, with acceleration hotspots forming at various times and splitting at a rate proportional to the shock’s expansion.

Conclusions. Our results show the importance of increased spatial resolution in determining either the small-scale spatial properties in coronal shocks or the structuring of the ambient medium. Possible shock corrugations or structuring of the upstream plasma at the scale of 10⁵ km can act as hotspots for the acceleration of suprathermal electrons. This can be observed as radiation that exhibits double sources with increasing separation at the same expansion rate as the global shock wave.