Herringbone structures during an X-class eruptive flare

In this paper, we report quasi-periodic herringbone structures during the impulsive phase of an X-class flare, coinciding with the distinct acceleration phase of eruptive prominence ejection on 2023 December 31. The prominence propagates non-radially in the southeast direction with an inclination angle of $\sim$35$\fdg$4. The fast coronal mass ejection (CME) at a speed of $\sim$2852 km s$^{-1}$ drives a shock wave and a coronal EUV wave. The herringbone structures lasting for $\sim$4 minutes take place at the initial stage of a group of type II radio burst. The herringbones in the frequency range 20$-$70 MHz are characterized by simultaneous forward-drift and reverse-drift bursts with average durations of $\sim$2.5 s and $\sim$3.1 s. The frequency drift rates of these bursts fall in a range of 1.3$-$9.4 MHz s$^{-1}$ with average values of $\sim$3.6 and $\sim$4.1 MHz s$^{-1}$, respectively. The speeds of electron beams producing the herringbones are estimated to be 0.04$-$0.41 $c$, with average values of $\sim$0.23 $c$ and $\sim$0.11 $c$ for forward-drifting and reverse-drift bursts, respectively. The heights of particle acceleration regions are estimated to be 0.64$-$0.78 $R_{\sun}$ above the photosphere, which are consistent with the height of CME front ($\sim$0.75 $R_{\sun}$) when the shock forms. Quasi-periodic pulsations with periods of 17.5$-$21.3 s are found in the radio fluxes of herringbones, suggesting that electrons are accelerated by the CME-driven shock intermittently.

💡 Research Summary

On 31 December 2023 an X5.0 solar flare erupted from active region 13536 near the eastern limb (N04E79). The flare’s impulsive phase (≈21:36 UT to 21:55 UT) was accompanied by a non‑radial, loop‑like prominence that rose toward the southeast with an inclination of about 35.4°. The prominence accelerated rapidly from ~48 Mm to ~516 Mm in roughly 500 s, reaching speeds of ~1500 km s⁻¹. Simultaneously, a fast coronal mass ejection (CME) was observed by SOHO/LASCO and STEREO‑Ahead COR2. The CME’s leading edge first appeared at 21:53 UT with an initial speed of ~1025 km s⁻¹, then accelerated to >2800 km s⁻¹ by 22:00 UT, attaining a halo configuration with a central position angle of ~112°. At a heliocentric distance of ~0.75 R☉ the CME drove a shock front, which in turn generated a coronal EUV wave propagating southward at ~600 km s⁻¹ (a faster component of ~950 km s⁻¹ was identified as the shock‑driven component).

Radio observations from e‑Callisto (Australia, Alaska, Mexico‑Lance‑B) and S/WAVES covered 15–220 MHz. A type II burst began around 21:45 UT, drifting from ~160 MHz downwards. At the onset of the type II burst, a group of herringbone structures appeared in the 20–70 MHz band and persisted for about four minutes. These herringbones consist of simultaneous forward‑drift (FD) and reverse‑drift (RD) bursts. The average duration of FD bursts is 2.5 s, RD bursts 3.1 s. Their frequency drift rates average 3.6 MHz s⁻¹ (FD) and 4.1 MHz s⁻¹ (RD), spanning a range of 1.3–9.4 MHz s⁻¹. Assuming plasma emission, the corresponding electron beam speeds are 0.04–0.41 c, with mean values of ~0.23 c for FD and ~0.11 c for RD bursts. Using a density model, the inferred heights of the acceleration region are 0.64–0.78 R☉, essentially coincident with the CME front height (~0.75 R☉) at shock formation.

A power‑spectral analysis of the radio fluxes reveals quasi‑periodic pulsations (QPP) with periods of 17.5–21.3 s embedded in the herringbone emission. These QPPs suggest that the CME‑driven shock accelerates electrons intermittently, possibly due to shock reformation, rippling, or interaction with upstream turbulence.

The study draws several key conclusions: (1) The coexistence of an ultra‑fast CME and a strong shock front provides the optimal conditions for herringbone generation; (2) The close match between electron beam speeds, acceleration heights, and CME front location confirms that the shock front itself is the primary accelerator; (3) The detected QPPs indicate that electron acceleration is not steady but occurs in bursts, offering a diagnostic of shock dynamics; (4) Although herringbones appear in only ~20 % of type II events, their occurrence probability rises dramatically when a CME exceeds ~2500 km s⁻¹ and drives a high‑Mach (M_A≈2–3) shock.

By integrating high‑cadence EUV imaging, white‑light coronagraphy, and broadband radio spectroscopy, the paper provides a comprehensive observational case that links CME kinematics, shock formation, and fine‑scale radio signatures. The results support the shock‑drift acceleration scenario with a quasi‑perpendicular geometry (θ_BN≈90°) and highlight the utility of herringbone structures as probes of coronal shock physics. Future work combining higher‑frequency radio imaging, in‑situ particle measurements, and kinetic simulations could further elucidate the spatial extent and temporal evolution of shock‑accelerated electron beams.