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Signatures of localised particle acceleration at a global coronal shock wave

C. Cuddy, D. M. Long, M. Nedal, S. Bhunia, P. T. Gallagher

Abstract

Extreme ultraviolet (EUV) waves are global waves in the solar corona which can accelerate particles. The efficiency of the acceleration depends on local plasma characteristics e.g. Alfvén speed and the geometry of the magnetic field. This shock-driven particle acceleration can produce radio signatures such as Type II radio bursts and herringbone emission. Here we investigate signatures of particle acceleration by a weak coronal shock on 10 March 2024. In particular, we combine EUV images with radio imaging and spectral observations to determine how and where this weak shock could accelerate energetic particles. A potential field source surface extrapolation was used to examine the pre-eruption ambient magnetic field while the evolution of the global wave was probed using running difference and base difference EUV images. The EUV images enabled the speed and Alfvén Mach number of the EUV wave to be characterised. The combination of radio images and dynamic spectra provide evidence of beams of shock-accelerated electrons localised to a dimming region at the time the EUV wave passes through it. The speeds and energies of these electrons were estimated from the drift rates of their herringbones. The EUV wave initially propagated West, channelled by loop systems, before changing direction northward. From the EUV intensity jump at the wavefront, the Alfvén Mach number was estimated to be approximately 1.005 at the time that the herringbones were produced. The herringbone drift rates revealed accelerated electron energies of 75-122 keV, using Newkirk density models with scaling factors of 1.3-2.6. These observations suggest that the weak lateral shock impacted quasi-perpendicular open field in a dimming region, enabling localised particle acceleration. This indicates that the geometry of the ambient magnetic field relative to the shock strongly governs where particles can be accelerated.

Signatures of localised particle acceleration at a global coronal shock wave

Abstract

Extreme ultraviolet (EUV) waves are global waves in the solar corona which can accelerate particles. The efficiency of the acceleration depends on local plasma characteristics e.g. Alfvén speed and the geometry of the magnetic field. This shock-driven particle acceleration can produce radio signatures such as Type II radio bursts and herringbone emission. Here we investigate signatures of particle acceleration by a weak coronal shock on 10 March 2024. In particular, we combine EUV images with radio imaging and spectral observations to determine how and where this weak shock could accelerate energetic particles. A potential field source surface extrapolation was used to examine the pre-eruption ambient magnetic field while the evolution of the global wave was probed using running difference and base difference EUV images. The EUV images enabled the speed and Alfvén Mach number of the EUV wave to be characterised. The combination of radio images and dynamic spectra provide evidence of beams of shock-accelerated electrons localised to a dimming region at the time the EUV wave passes through it. The speeds and energies of these electrons were estimated from the drift rates of their herringbones. The EUV wave initially propagated West, channelled by loop systems, before changing direction northward. From the EUV intensity jump at the wavefront, the Alfvén Mach number was estimated to be approximately 1.005 at the time that the herringbones were produced. The herringbone drift rates revealed accelerated electron energies of 75-122 keV, using Newkirk density models with scaling factors of 1.3-2.6. These observations suggest that the weak lateral shock impacted quasi-perpendicular open field in a dimming region, enabling localised particle acceleration. This indicates that the geometry of the ambient magnetic field relative to the shock strongly governs where particles can be accelerated.
Paper Structure (9 sections, 8 equations, 6 figures)

This paper contains 9 sections, 8 equations, 6 figures.

Table of Contents

  1. Introduction
  2. Observations
  3. EUV observations
  4. Radio observations
  5. Methods and results
  6. EUV analysis
  7. Radio analysis
  8. Magnetic field
  9. Discussion and conclusions

Figures (6)

  • Figure 1: AIA 211 Å running difference images from 12:11:09 UT on 10 March 2024. Left: Paths for tracking the EUV wave plotted on a 1 minute offset AIA 211 Å running difference image. Right: For each of the 27 running difference images made between 12:09:33 UT and 12:19:33 UT, there is a front shown here in a unique shade of red, which connects the manually selected points along the leading edge of the EUV wave. In the online version we present this information in a movie.
  • Figure 2: (a): The prolonged feature seen in the dynamic spectra is a type II radio burst captured between approx 240 and 40 MHz from approximately 12:11:30 UT to 12:20:45 UT by ORFEES and CALLISTO in Greenland and Algeria. The colour map used in this dynamic spectrum is not representative of the actual relative intensities. It was normalised differently across the two datasets with the aim of aiding the reader to best see the shape of the type II feature. (b): Zoom in on ORFEES herringbones between 12:13:00 UT and 12:13:20 UT. The points from identifying one of these herringbones are shown, at the frequencies corresponding to the NRH contours on the right (black crosses joined by a dashed black line). (c): Running difference image in 211 Å from 12:13:09 UT, with NRH contours over-plotted between 150.9 MHz and 270.6 MHz. We see that the EUV wave has just passed over the open field region and reverse herringbone emission has been triggered in that region. The plotted contours show the path of the herringbone identified. In the online version we present this information in a movie.
  • Figure 3: Stack-plots for each of the paths shown in the left hand side of Fig. \ref{['fig:paths_and_clicks']}. From top left to bottom right they are organised by decreasing angle. The stack-plots in the top row correspond to those along great arc paths, and the others correspond to those along straight line paths. The white horizontal line denotes the distance from AR13599 to the limb along the relevant path. For each frame where the wave is visible along the path in question, the distance from the AR of the nearest clicked point to the path is shown in red.
  • Figure 4: Left: 211 Å base difference ratio image of the area surrounding AR13602, at 12:13:09UT, using a quiet time base frame from 12:08:45UT. The red box indicates the subregion that the light-curve on the right corresponds to. This subregion was identified as consistently being within the dimming region during the time window that dimming occurred. Right: The base difference ratio light curve for the subregion, showing that it dims by up to 45.77%.
  • Figure 5: The Potential Field Source Surface (PFSS) magnetic field extrapolation just before the flare. The field lines are plotted on an AIA 193 Å image. The closed field lines are in white and open field lines are in cyan. The location of the herringbone contours in AR13602 is as indicated. The yellow arrows highlight the large loop systems to the North and South of the flaring active region, which is marked by the 'AR' label.
  • ...and 1 more figures