Quasi-Periodic Herringbone Structures

• Quasi-periodic herringbone structures are distinct features in type II solar radio bursts, marked by both forward- and reverse-drift stripes indicating episodic electron acceleration.
• Observations during major solar eruptions show mean drift rates of ~3.61 MHz/s and ~4.13 MHz/s, with electron speeds up to 0.23c, highlighting varying acceleration dynamics.
• The 17–21 s periodic modulation of these bursts supports models of shock rippling or MHD oscillatory modulation, providing direct diagnostics of coronal shock processes.

Quasi-periodic herringbone structures are distinctive, short-duration features embedded within type II solar radio bursts, exhibiting both forward- and reverse-drifting stripes in dynamic radio spectra. These structures trace the intermittent acceleration of electron beams by transient coronal shock fronts, typically driven by fast coronal mass ejections (CMEs) during major solar eruptive events. Their quasi-periodic nature, revealed by periodic modulation in radio flux, provides direct diagnostics of shock formation, electron acceleration, and coronal plasma dynamics above the solar photosphere (Zhang et al., 1 Feb 2026).

1. Observational Context and Instrumentation

The occurrence of quasi-periodic herringbone structures is closely associated with the impulsive phase of X-class flares and CME-driven shocks. During the X5.0 eruptive flare on 2023 December 31, multi-wavelength imaging and dynamic radio spectroscopic observations were coordinated using a comprehensive suite of instruments. Table 1 summarizes the key properties and roles of these instruments employed to capture both plasma emission signatures and CME evolution.

Instrument (Type)	Spectral/Imaging Range	Temporal/Spatial Resolution
SDO/AIA (EUV)	171, 211, 304 Å	12 s, 0.6″
GOES-16/SUVI (EUV)	195 Å, 304 Å	60–120 s, 2.5″
SOHO/LASCO-C2, C3 (wl)	Visible coronagraphy	12 min, 11.4″/56″
STEREO-A/SECCHI-COR2 (wl)	White light	15 min, 14.7″
e-CALLISTO (radio: multiple)	5–377 MHz	0.25 s
STEREO-A/S-WAVES (radio)	0.0025–16.025 MHz	60 s

Imaging (Figures 1–2 of (Zhang et al., 1 Feb 2026)) captures the EUV and white-light evolution of the prominence and CME, while dynamic spectra from several e-CALLISTO stations and STEREO-A/S-WAVES isolate the onset and nature of the herringbone structures (Figures 6–8).

2. Morphology and Identification of Herringbone Structures

Quasi-periodic herringbone structures manifest during the early stages of type II radio bursts, identifiable as simultaneous forward-drift and reverse-drift sigmoid features within dynamic frequency–time spectra. In the 2023-12-31 event, the type II burst began at ~21:45:40 UT in the 15–220 MHz range. Careful manual scrutiny isolated 40 forward-drift and 35 reverse-drift herringbone stripes (via linear fits in frequency–time space, Figure 9), each burst corresponding to accelerated electron beams traversing ambient coronal plasma.

The forward-drift herringbones map to electron beams escaping upstream (higher altitudes) of the shock, while reverse-drift features correspond to downstream-propagating beams. Both families exhibit short durations (mean 2.5 s and 3.1 s, respectively), emphasizing their impulsive and discrete production at the shock front.

3. Quantitative Properties: Drift Rates, Energetics, and Source Heights

Herringbone parameters are extracted by fitting straight lines to individual burst tracks in the frequency–time plane, yielding frequency drift rates ($D = df/dt$), initiating frequencies ($f_s$), and apparent bandwidths. These metrics directly inform models of electron energetics and coronal conditions.

• Drift Rates:
  • Forward-drift: $D_f\in [1.33, 6.39]$ MHz s$^{-1}$, mean $\simeq 3.61$ MHz s$^{-1}$
  • Reverse-drift: $D_r\in [1.95, 9.44]$ MHz s$^{-1}$, mean $\simeq 4.13$ MHz s$^{-1}$
• Electron Beam Speeds: Using a plasma emission framework and the Newkirk coronal density model, speeds are computed as $v \simeq 2D/(\alpha f)$ (with $\alpha$ the density scale height):
  • $v_\mathrm{forward} \in [0.07, 0.41]\,c$ (mean $\simeq 0.23\,c$)
  • $v_\mathrm{reverse} \in [0.04, 0.26]\,c$ (mean $\simeq 0.11\,c$)
• Source Heights: Maximum $f_s$ of 21–52 MHz maps to heliocentric distances of $0.64$–$0.78\,R_\odot$ ($448$–$546$ Mm), consistent with the CME leading-edge at shock formation ($\simeq 0.75\,R_\odot$).

These values indicate production of herringbone structures close to the CME shock front and highlight substantial variability in both drift rates and electron speeds (see Table 2, Figure 10 in (Zhang et al., 1 Feb 2026)).

4. Quasi-Periodic Behavior and Temporal Modulation

A defining feature of these structures is their quasi-periodicity, evident as pulsations in the radio flux. Residual signals (after background subtraction) at fixed frequencies (30.3, 45.3, 47.4 MHz) reveal significant wavelet power at 17.5–21.3 s periods (Figure 11). A representative sinusoidal fit to the detrended flux $F'(t)$ is

$F'(t) \simeq A\,\sin\left[2\pi t/P +\phi\right],$

with $P$ in the above interval, uniform across frequencies and robust above 95% confidence.

This periodicity demonstrates that electron acceleration by the CME-driven shock occurs intermittently, rather than as a continuous process. The approximately four-minute duration of herringbone activity is modulated on these 17–21 s cycles, yielding a discrete ensemble of $\sim$75 bursts.

5. Physical Interpretation: Shock Acceleration and Periodicity Mechanisms

Herringbone formation is attributed to beams of electrons energized at or near a CME-driven quasi-perpendicular shock. Forward- and reverse-drifting herringbones trace respectively upstream and downstream escape of electrons from the shock front. The observed periodicity implicates time-dependent shock processes in modulating the acceleration efficiency.

Two leading physical mechanisms are discussed:

• Shock Rippling or Self-Reformation: The nonstationary structure of collisionless shocks (Holman & Pesses 1983; Burgess 2006; Yang et al. 2018) produces ripples or self-reforming surfaces, which stochastically inject and accelerate particles quasi-periodically.
• MHD Oscillatory Modulation: Magnetohydrodynamic oscillations (e.g., global kink, sausage modes) in the flaring region may temporally modulate the efficiency of shock or reconnection-powered acceleration (Takasao & Shibata 2016; Nakariakov & Melnikov 2009).

Numerical simulations support these interpretations, showing that electron reflection, drift, and acceleration occur quasi-periodically at perturbed, rippled shock fronts (Guo & Giacalone 2010; Hao et al. 2023). The strong temporal coincidence between quasi-periodic herringbones and CME-driven shock formation supports models invoking rapidly evolving plasma structures to episodically release electron beams (Zhang et al., 1 Feb 2026).

6. Significance and Implications

The detection and quantitative characterization of quasi-periodic herringbone structures establish critical diagnostics for high-energy particle acceleration astrophysics. Their properties—drift rates, electron speeds, periodicity, and spatial coincidence with CME shocks—demonstrate the sensitivity of type II burst phenomena to coronal shock dynamics and the structure of the ambient plasma.

These observations confirm that key acceleration regions reside $\sim$0.64–0.78 $R_\odot$ above the photosphere, coupled directly to CME evolution at speeds up to $\sim$2852 km s$^{-1}$, and that the quasi-periodicity of electron production imprints observable structure on solar radio emission. A plausible implication is that quasi-periodic modulation of shock-accelerated electron beams may also inform space weather modeling of energetic particle fluxes at Earth and elsewhere.

By linking herringbone phenomenology with coronal MHD processes, CME kinematics, and nonstationary shock physics, these results reinforce and extend the paradigm of intermittent “bursty” acceleration in eruptive solar activity, providing an empirical basis for advancing models of particle acceleration and coronal shock wave evolution (Zhang et al., 1 Feb 2026).