The narrowband decimetric spikes belong to the most interesting solar radio bursts, because they are connected to the primary flare process.  Their brightness temperature is up to 10¹⁵ K. In the radio spectrum, they occur in clouds of many narrowband bursts with a relative bandwidth of about 1-3 % and duration less than 100 ms (Benz, 1986). Many models of these spikes were proposed, such as those based on the electron-cyclotron maser mechanism or based on the plasma emission mechanism.  Karlický et al. (1996) studied the frequency bandwidths of spikes by the Fourier method. They found the power-law spectra with power-law indices close to -5/3, the Kolmogorov turbulence index. Therefore, it was suggested that the spikes are generated by suprathermal electrons in the magnetohydrodynamic turbulence.

Figure 1. Upper: Radio spectrum observed at 13:41:30 – 13:42:30 UT during the 2022 May 10 flare. Bottom: Its detail at  13:42:00 – 13:42:03 UT. The white line marks an example of the frequency drift of these spikes. The black horizontal lines in the both spectra show the frequency at which the periods were analyzed.

Figure 2. SDO/AIA 171 A observation of reconnecting threads (1 red and 2 blue) at the time of dm-spikes at 13:42:09 UT, see also the movies in our paper.

Observations and results

We studied the radio emission occurring as narrowband decimetric spikes observed during the 2022 May 10 and 2022 August 26 flares. Using the wavelet analysis technique we found that the spikes in their radio spectra were distributed in groups that occurred quasi-periodically with the periods 5.1 s in the 2022 May 10 flare and 9.1 s in the 2022 August 26 flare. An example of these spikes observed during the 2022 May 10 flare is shown in Figure 1, upper part.

In some parts of the groups of spikes, subgroups of spikes distributed with the quasi-periods of 0.19 s (2022 May 10 flare), and 0.17 s and 0.21 s (2022 August 26 flare) were found. Some of these subgroups of spikes had positive frequency drift (930, 1100, and 1710 MHz s^-1), while others had negative drift  (−1000 and −1330 MHz s-1), which was observed for the first time. An example of subgroups with the quasi-period of 0.19 s observed during the 2022 May 10 flare is shown in Figure 1 bottom. There, the spikes drift from lower to higher frequencies, as indicated by the white line.

The spikes at 13:42 UT on 2022 May 10 occurred during the precursor phase of the X1.5-class X-ray flare as recorded by GOES-16. At the time of the spikes observation, a pair of short, mutually interacting loops were identified in the SDO/AIA EUV observations. One of these loops was interpreted as a bright thread belonging to the small erupting filament, while the other loop was an oppositely-oriented  overlaying loop, see Figure 2.

Conclusions

We propose that the mentioned loops reconnected in the dynamic quasi-periodic regime with the period 0.19 s, and that this reconnection is modulated by an oscillation of one of the interacting loops (the period 5.1 s). We note that similar periods of spikes group (0.1–0.9 s) were recently also found by Huang et al. (2022).

The drift of groups of spikes were observed for the first time. In agreement with Karlický et al. (1996), we propose that the accelerated electrons from the reconnection process are trapped in the reconnecting plasma outflows and move with them. Owing to this motion, the sources of spikes moved to higher or lower plasma densities, and thus the drifting groups of spikes were generated.

During the 2022 August 26 flare similar periods and drifts of groups of spikes were found. However, the magnetic structure of flare in SDO/AIA images was quite complex, making it difficult to uniquely identified reconnecting structures as in the 2022 May 10 flare. Nevertheless, we found indications that the groups of spikes and their drifts were generated by similar processes as in the 2022 May 10 flare.

Based on the paper published in Solar Physics 299: 113 (2024) by Marian Karlický, Jaroslav Dudík and Ján Rybák

References

Benz, A.O., 1986, Solar Phys. 104, 9

Huang, J., Tan, C., Chen, X. et al., 2022, Universe 8, 348

Karlický, M., Sobotka, M., Jičička, K., 1996, Solar Phys. 168, 375