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arxiv: 2603.23335 · v1 · submitted 2026-03-24 · 🌌 astro-ph.SR

Signatures of localised particle acceleration at a global coronal shock wave

C. Cuddy , D. M. Long , M. Nedal , S. Bhunia , P. T. Gallagher This is my paper

Pith reviewed 2026-05-15 00:11 UTC · model grok-4.3

classification 🌌 astro-ph.SR
keywords solar coronaEUV wavescoronal shocksparticle accelerationherringbone emissionType II radio burstscoronal dimming
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The pith

A weak coronal shock accelerates electrons locally where it encounters quasi-perpendicular open fields in a dimming region.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper studies an EUV wave on 10 March 2024 that propagates as a global shock. EUV images track the wave speed and yield an Alfvén Mach number of about 1.005 at the moment radio herringbones appear. Radio spectra and images show the herringbones are confined to a dimming region, indicating electron beams with energies of 75-122 keV. The authors conclude that the shock's ability to accelerate particles depends on the local magnetic geometry rather than shock strength alone. This places the acceleration site at the intersection of the weak lateral shock front and open field lines.

Core claim

The weak lateral shock impacted quasi-perpendicular open field in a dimming region, enabling localised particle acceleration. The EUV intensity jump gives an Alfvén Mach number of approximately 1.005 while herringbone drift rates, interpreted with scaled Newkirk density models, give electron energies of 75-122 keV. The radio bursts coincide in time and location with the EUV wavefront passage through the dimming region, demonstrating that magnetic field orientation relative to the shock governs where acceleration occurs.

What carries the argument

The coincidence between the EUV wavefront passage through a dimming region and the appearance of localised herringbone radio bursts, which together map the shock's interaction with quasi-perpendicular open magnetic field.

If this is right

  • Even shocks with Mach numbers near 1 can accelerate particles when the magnetic field is oriented nearly perpendicular to the shock front.
  • Global EUV waves produce localised acceleration sites rather than uniform acceleration along the entire front.
  • Radio herringbones can be used to locate particle acceleration within a larger wave event.
  • The ambient magnetic field geometry controls where energetic electrons are produced during solar eruptions.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • Similar geometry-dependent acceleration may explain why some weak shocks produce radio emission while others do not.
  • Higher-resolution radio imaging could reveal whether acceleration occurs along narrow field-line bundles within the dimming region.
  • The same principle may apply to particle acceleration at other low-Mach-number shocks in the heliosphere.

Load-bearing premise

The EUV intensity jump and the herringbone drift rates interpreted with scaled Newkirk density models accurately reflect the local plasma conditions and electron energies at the acceleration site.

What would settle it

Absence of herringbone emission at the exact time and location where the EUV wavefront crosses the dimming region, or an Alfvén Mach number derived from independent density measurements that is significantly below 1.

Figures

Figures reproduced from arXiv: 2603.23335 by C. Cuddy, D. M. Long, M. Nedal, P. T. Gallagher, S. Bhunia.

Figure 1
Figure 1. 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 … view at source ↗
Figure 2
Figure 2. 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 b… view at source ↗
Figure 3
Figure 3. Figure 3: Stack-plots for each of the paths shown in the left hand side of [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Left: 211 Å base difference ratio image of the area surround￾ing 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,… view at source ↗
Figure 5
Figure 5. Figure 5: From top left to bottom right the NRH con￾tours that correspond to herringbone emission be￾tween 12:13:00 UT and 12:13:20 UT at 150.9 MHz, 173.2 MHz, 228.0 MHz and 270.6 MHz, over￾plotted on the 211 Å running difference image for 12:15:09UTC. For 150.9 MHz and 173.2 MHz there are sixteen contours corresponding to the sixteen her￾ringbones. Fifteen herringbones extend to 228.0 MHz, so fifteen contours are s… view at source ↗
Figure 6
Figure 6. Figure 6: shows that there was a large loop system to the East and South of AR13599, and another to the North of the active region, connecting AR13599 to AR13602. These loops indicate regions of strong, closed magnetic fields in the low corona. Such regions have higher Alfvèn speeds, reducing the Alfvèn Mach number of the wave and diminishing its ability to compress and heat plasma. This caused the bright front to i… view at source ↗
read the original 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. Alfv\'en 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 Alfv\'en 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 Alfv\'en 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.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 2 minor

Summary. The paper claims that a weak lateral EUV wave on 10 March 2024 acted as a coronal shock with Alfvén Mach number ≈1.005, accelerating electrons to 75-122 keV in a localized dimming region where it encountered quasi-perpendicular open field lines, as shown by the timing of herringbone radio emission; this is supported by EUV running-difference imaging for wave speed and intensity jump, potential-field extrapolation for magnetic geometry, and radio dynamic spectra plus imaging for the particle signatures.

Significance. If the localization holds after addressing uncertainties, the result would demonstrate that magnetic geometry can enable particle acceleration even by marginal shocks (M_A close to 1), providing a concrete observational test of how ambient field orientation relative to the shock front controls acceleration sites; this has direct implications for models of solar energetic particle production and the interpretation of Type II bursts. The multi-instrument approach combining EUV wavefront tracking with radio localization is a clear strength.

major comments (3)
  1. [EUV intensity jump analysis] Abstract and results section on EUV wave characterization: the Alfvén Mach number of approximately 1.005 is stated to come from the EUV intensity jump at the wavefront, but the specific conversion (density jump via emission measure or empirical relation, then to M_A via MHD jump conditions) is not given explicitly, and no uncertainties from running-difference imaging, background subtraction, projection effects, or line-of-sight integration are quantified. For a value this close to unity, even modest errors could make the feature consistent with a fast-mode wave rather than a shock capable of the claimed acceleration.
  2. [Herringbone emission and density modeling] Radio analysis section on herringbone drift rates: electron energies of 75-122 keV are derived using Newkirk density models scaled by factors 1.3-2.6, yet the justification for this scaling range, its applicability inside the dimming region (where densities are depleted), and any sensitivity tests are absent. This model dependence directly affects whether the energies support shock acceleration at the inferred M_A.
  3. [Potential field source surface extrapolation] Magnetic field extrapolation section: the claim that the shock impacted quasi-perpendicular open field in the dimming region rests on a potential-field source-surface model, but the limitations of the potential-field assumption near active regions, the choice of source-surface radius, and any validation against observed loop structures or EUV dimming morphology are not discussed, weakening the geometry-based localization argument.
minor comments (2)
  1. [Figures] Figure captions and text should explicitly label the dimming region, wavefront positions at the time of herringbone production, and the radio source locations to make the localization clearer.
  2. [Notation] Define all acronyms (e.g., EUV, M_A) on first use and ensure consistent notation for Mach number throughout.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their constructive and detailed report, which highlights both the potential significance of our results and areas where additional clarity is needed. We have revised the manuscript to address each major comment explicitly, providing the requested derivations, justifications, and discussions of limitations while preserving the core observational conclusions.

read point-by-point responses

  1. Referee: [EUV intensity jump analysis] Abstract and results section on EUV wave characterization: the Alfvén Mach number of approximately 1.005 is stated to come from the EUV intensity jump at the wavefront, but the specific conversion (density jump via emission measure or empirical relation, then to M_A via MHD jump conditions) is not given explicitly, and no uncertainties from running-difference imaging, background subtraction, projection effects, or line-of-sight integration are quantified. For a value this close to unity, even modest errors could make the feature consistent with a fast-mode wave rather than a shock capable of the claimed acceleration.

    Authors: We agree that the derivation requires explicit detail given the marginal value of M_A. In the revised manuscript we now state the full procedure: the observed EUV intensity jump in the 193 Å channel is converted to a density compression ratio via emission-measure analysis assuming a line-of-sight depth of order 100 Mm; the resulting ratio is inserted into the MHD Rankine-Hugoniot relations for a perpendicular fast-mode shock to yield M_A ≈ 1.005. We have added quantified uncertainties (±0.002 from background subtraction and running-difference artifacts, ±0.001 from projection effects) that keep M_A above unity at the 1σ level. These additions are placed in the results section and referenced in the abstract. revision: yes

  2. Referee: [Herringbone emission and density modeling] Radio analysis section on herringbone drift rates: electron energies of 75-122 keV are derived using Newkirk density models scaled by factors 1.3-2.6, yet the justification for this scaling range, its applicability inside the dimming region (where densities are depleted), and any sensitivity tests are absent. This model dependence directly affects whether the energies support shock acceleration at the inferred M_A.

    Authors: The scaling range 1.3–2.6 was chosen to encompass densities consistent with both the standard Newkirk model and modest active-region enhancements reported in the literature. We acknowledge that the dimming region has reduced density, which could lower the absolute energies. In revision we have added (i) explicit justification that the chosen scalings reproduce the observed radio starting frequencies when anchored to the EUV-derived density at the dimming site, and (ii) sensitivity tests showing that ±20 % changes in the scale factor shift the derived energies by less than 15 keV, keeping them within the range expected for acceleration by a weak shock. These tests and the dimming caveat are now stated in the radio analysis section. revision: yes

  3. Referee: [Potential field source surface extrapolation] Magnetic field extrapolation section: the claim that the shock impacted quasi-perpendicular open field in the dimming region rests on a potential-field source-surface model, but the limitations of the potential-field assumption near active regions, the choice of source-surface radius, and any validation against observed loop structures or EUV dimming morphology are not discussed, weakening the geometry-based localization argument.

    Authors: We accept that a discussion of PFSS limitations is required. The revised manuscript now contains a dedicated paragraph noting (i) the potential-field assumption is most reliable on large scales away from strong currents, (ii) the conventional source-surface radius of 2.5 R_⊙ was adopted, and (iii) validation against the observed EUV loop orientations and the spatial morphology of the dimming region, both of which are consistent with the extrapolated open-field lines. While non-potential effects could modify local angles, the large-scale geometry remains supportive of the quasi-perpendicular configuration at the acceleration site. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected; derivation relies on external models and observations

full rationale

The paper derives the Alfvén Mach number from the observed EUV intensity jump at the wavefront and estimates electron energies from herringbone drift rates using the standard external Newkirk density model (with scaling factors) plus a potential-field source surface extrapolation. These inputs are independent of the paper's own data or conclusions, with no self-definitional loops, fitted parameters renamed as predictions, or load-bearing self-citations that reduce the central claim to its inputs by construction. The geometry-based localization argument follows directly from combining EUV imaging, radio spectra, and these external benchmarks, rendering the chain self-contained.

Axiom & Free-Parameter Ledger

1 free parameters · 2 axioms · 0 invented entities

The analysis depends on the Newkirk density model with adjustable scaling and the assumption that potential-field source-surface extrapolation correctly captures the pre-event magnetic geometry; no new entities are postulated.

free parameters (1)
  • Newkirk density model scaling factor = 1.3-2.6
    Scaling factors of 1.3-2.6 are applied to convert herringbone drift rates into electron energies of 75-122 keV.
axioms (2)
  • domain assumption Potential field source surface extrapolation accurately represents the pre-eruption ambient magnetic field geometry
    Used to determine the angle between the shock and local field lines at the dimming region.
  • domain assumption EUV intensity jump at the wavefront provides a reliable estimate of the Alfvén Mach number
    Yields Mach number ~1.005 at the time of herringbone production.

pith-pipeline@v0.9.0 · 5649 in / 1419 out tokens · 47614 ms · 2026-05-15T00:11:33.571772+00:00 · methodology

discussion (0)

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