An Unusual Long-Lived Radio Burst Oscillating in Frequency

Nugget
Number:	523
1st Author:	Marian KARLICKÝ
2nd Author:	Robert SYCH and Alena ZEMANOVÁ
Published:	April 20, 2026
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Previous Nugget:	Lateral Deformation of Large-scale Coronal Mass Ejections during the Transition from Nonradial to Radial Propagation

Introduction

Quasi-periodic pulsations (QPPs) are frequently observed in the light curves of both solar and stellar flares (see [1] for a comprehensive review, and references therein). In this study, we present and analyze a unique radio burst that exhibits periodic oscillations in frequency with a period of approximately 3 minutes. To our knowledge, this is the first observation of such a phenomenon. Notably, the burst persists for about 80 minutes, which is significantly longer than flare-associated QPPs as reported in the literature to date.

Obervations and Results

Figure 1 shows the dynamic radio spectrum in the 1000-1800 MHz range, observed by the Ondejov radio spectrograph over 07:30-09:40 UT following three flares in active region NOAA 13663, commencing with SOL2024-05-04T07. Here, a remarkable frequency-oscillating burst can be seen. The period of the oscillation is approximately 3 minutes.

Figure 1: The dynamic radio spectrum f the frequency-oscillating burst, as recorded by the Ondejov radio spectrograph on 4 May 2024, between 07:30 and 09:40 UT, in the 1000--1800 MHz range. Enlarged views of selected regions are presented for the intervals 07:30--08:00 UT, 08:00--08:20 UT, 08:20--08:40 UT, 08:40--09:00 UT and 09:00--09:20 UT. The horizontal dashed lines show the frequency band (1474--1615 MHz), where the maximum power of the 3-minute periodicity was detected.

Figure 2 presents intensity and polarization maps at 3000 MHz observed by the Siberian Radioheliograph, along with corresponding time-distance plots. The radio source is located within a loop anchored near sunspots and extending across the entire active region, where multiple flares occurred during the interval of frequency-oscillating burst. The oscillation period of the 3000 MHz source is also approximately 3 minutes. In addition, above the main sunspot (S0), 3-minute wave trains were detected in both radio and EUV observations.

Figure 2: Radio source maps in intensity (a) and polarization (b) at 3000 MHz, along with the corresponding time-distance plots of brightness variations (c) and (d) accordingly. The background for the maps shows an image of the active region obtained at 171 Å. Horizontal dashed lines in (c,d) mark the positions of sunspots S0, S1/S2, and S3, with their locations indicated by blue circles in (a,b). The black contour in (b) outlines the central part of the 3000 MHz radio source. Ellipses on the time-distance plot in polarization highlight low-frequency brightness fluctuations in the sunspot S0.

Interpretation

Based on these observations, we propose that the frequency-oscillating burst is generated within loops, which act as reservoirs of accelerated electrons emitting via the electron gyrosynchrotron mechanism; see Figure 3. Waves originating from the sunspot propagate into the loop containing the burst source, and the loops resonant period appears to be close to that of the incoming waves. As a result, the radio emission exhibits sustained oscillations with a period of approximately 3 minutes over a duration of about 80 minutes. Further details are provided in Ref. [2].

Figure 3: Scenario of observed phenomena, suggesting that the 3000 MHz emission is generated by the gyro-synchrotron mechanism and the frequency-oscillating burst by the ECM (electron cyclotron maser) mechanism.

References

[1] "Quasi-Periodic Pulsations in Solar and Stellar Flares: A Review of Underpinning Physical Mechanisms and Their Predicted Observational Signatures"

[2] "An Unusual Long-Lived Radio Burst Oscillating in Frequency"