The solar atmosphere is a turbulent and magnetized environment, with the release of magnetic energy readily manifesting as emission across the electromagnetic spectrum. Solar radio emission dominates the radio sky, with the brightest solar radio bursts generated via the plasma emission process. The emission has a complex frequency-time structure with many features that are yet to be understood.
Observations
Using the LOw Frequency ARray (LOFAR) radio telescope (van Haarlem et al. 2013), Ma et al. (Nature Comm 2026) have detected “repeating spike-like burst pairs’’- brief flashes of radio energy that occur in pairs, separated by a characteristic delay of about four seconds. The findings reveal a new diagnostic of turbulent plasma processes high above the Sun’s surface, providing a powerful tool for probing the Sun’s magnetic environment and particle acceleration.
Solar radio bursts are known to exhibit complex fine structures, but the newly discovered signals stand out. Each event consists of two nearly identical, narrowband radio spikes occurring at the same frequency: a short-lived “earlier’’ (E) burst followed by a weaker, delayed (D) “echo-like’’ burst approximately 4 seconds later, as shown in Figure 1.
Figure 1. Dynamic spectrum showing repeating bursts. (a) Wide-field dynamic spectrum. (b) Zoomed-in spectrum. (c) Time flux profile of repeating burst pair I, fitted with asymmetric Gaussian functions. Figure adopted from Suli Ma et al 2026
In total, over 600 such pairs were analyzed, revealing consistent patterns in their timing, intensity, and spatial origin. By combining high-resolution spectroscopy with radio imaging, the team traced these bursts to an active region on the Sun (Figure 2). The key breakthrough came from observing that the second burst in each pair originates from a different location in the corona—often displaced by hundreds of arcseconds.
Figure 2. Centroids of each burst component overlaid on the surrounding magnetic environment. The background of panel a shows the HMI magnetogram, while panels b-f show the AIA 171 Å image. The light pink and green curves represent magnetic field lines from a PFSS extrapolation, corresponding to open and closed field lines, respectively. The red pluses and blue diamonds in panels a-d mark the centroid positions of the E and D components, respectively, with the color gradients corresponding to different frequencies as indicated by the colorbar within each panel.
Interpretation
This spatial separation, along with the reduced intensity and slower frequency drift, indicates that the delayed burst is not independently generated. Instead, it is likely a scattered echo of the first burst, formed as radio waves reflect and propagate through turbulent plasma in the corona, as shown in Figure 3. Using the method introduced in Kontar et al. (2019), we performed computer simulations that support this interpretation, showing that radio waves can be redirected and delayed by anisotropic turbulence—density fluctuations aligned with the Sun’s magnetic field. These conditions can produce echoes with precisely the observed timing and spatial offsets as shown in Figure 4.
Figure 3. Mechanism of the repeating burst pair. The actual source (gold star) emits at the harmonic frequency $2f_{\mathrm{pe2}}$. Direct rays (red) produce the earlier (E) component; downward rays reflect at the $f_{\mathrm{pe1}}$ layer ($f_{\mathrm{pe1}} = 2f_{\mathrm{pe2}}$) and then scatter upward (blue), creating the delayed (D) component.
The study suggests that these bursts originate at heights of around one solar radius above the surface, much higher than typical flare emission sites. This implies that magnetic reconnection and electron acceleration—key drivers of solar activity—may occur in previously underappreciated regions of the corona. The bursts are likely produced when small-scale reconnection events accelerate electrons, generating plasma waves that emit radio signals. These signals then follow multiple paths through turbulent plasma, producing the delayed echo signature.
Figure 4. Simulated time profiles and images of radio emission at 40 MHz. a, Comparison of time profiles for fundamental and harmonic sources (anisotropy ($\alpha = 0.1$), b, Harmonic sources with different anisotropy factors ($\alpha = 0.1\text{–}0.3$ show similar delays. c–h, Simulated images (2D histograms) of the direct (earlier) and echo (delayed) components. For harmonic emission (e–h), the delayed component is spatially displaced from the direct one, consistent with the observed source offsets. For fundamental emission (c,d), the two components remain co-spatial. The crosses mark the fitted centroids.
The discovery has several important consequences:
- New diagnostic of coronal turbulence: The timing and structure of the burst pairs provide a way to measure plasma density variations and turbulence.
- Insights into magnetic geometry: The directional scattering reveals how radio waves are guided along magnetic field lines.
- Clues to long-standing mysteries: The same scattering processes may explain why radar signals sent from Earth are weakly reflected by the Sun.
Conclusions
Repeating spike-like burst pairs constitute a newly identified and abundant class of solar radio fine structure. Their defining characteristics—paired emission at identical frequency, separated by a ~4 s delay and spatial offset—are best explained as turbulent echoes of harmonic plasma emission. The location of the burst sources high in the corona suggests ongoing magnetic reconnection and electron acceleration well above typical flare heights. These findings offer new insights into coronal turbulence effects while advancing diagnostics of coronal plasma and the elusive nature of solar radio echoes from ground-based transmitters.
Based on the recent paper: Suli Ma, Eduard P. Kontar, Daniel L. Clarkson, Huadong Chen & Yihua Yan, Imaging spectroscopy reveals spike-like repeating radio burst pairs in the solar corona, Nature Communications, 17, 5131 (2026). https://doi.org/10.1038/s41467-026-74137-2 [ArXiv:2605.23484]
References
van Haarlem, M.P., Wise, M.W., Gunst, A.W., et al. 2013, Astron. Astrophys., 556, A2
Kontar, E.P., Chen, X., Chrysaphi, N., et al. 2019, Astrophys. J., 884, 122
Ma, S., Kontar, E.P., Clarkson, D.L., et al. 2026, Nat Commun 17, 5131 (2026)
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