Radio bursts in the 2017 September 6, X9.3 flare
by M. Karlicky and J. Rybak

Radio bursts and their fine structures are an integral part of solar flares. Although many of them are known as e.g. type II, III, V, J, U, and IV, still some unique bursts and fine structures, not observed so far, can be detected. This is the case of the X9.3 flare observed on September 6, 2017, where we found not only several unique bursts and fine structures, but also their interesting time association with the other flare phenomena observed in extreme ultraviolet (EUV), white-light, X-ray, and γ-ray emissions. Using our new method based on the wavelets we also detected quasi-periodic pulsations (QPPs) in the whole time–frequency domain of the analyzed radio spectrum (11:55–12:07 UT and 22–5000 MHz).

In the pre-impulsive phase of this flare we found a remarkable double drifting pulsation structure (DPS) at high frequencies (2200-4200 MHz) in association with the EUV brightenings caused by the interaction of magnetic ropes, as presented in the paper by Hou at al. (2018).

In the flare impulsive phase, at the time of the hard X-ray and γ-ray peaks and a sunquake start we found strong quasi-periodic pulsations (P in Figure 1). Just after these pulsations an exceptional radio burst drifting from 5000 to 800 MHz (burst B in Figure 1) was detected. Comparing this burst B with time associated EUV phenomena we found that this radio burst was probably generated by a rising magnetic rope. Moreover, in connection with this burst B, we recognized a U burst at about the onset time of an EUV writhed structure and a drifting chain of bursts as a signature of a shock wave at high frequencies (1050–1350 MHz).

We analyzed the quasi-periodic pulsations observed at the impulsive flare phase (P in Figure 1) by our new wavelet technique (Karlický, M. et al. (2017)) in detail. We found quasi-periodic pulsations in broad range of periods (1-30 s). Among these QPPs we detected pulsations bi-directionally drifting to higher and lower frequencies (Figure 2). Owing to their low frequency drift they indicate a presence of the magnetosonic waves generated at the primary flare energy-release site and propagating upwards and downwards in the solar atmosphere. The frequency, where the frequency drift changes from the positive to negative value enables us to determine the plasma density in the primary flare energy-release process (magnetic reconnection) as (1.1-1.5) x 1011 cm-3 assuming the emission on the fundamental frequency. Similar drifting phases of pulsations were found in further flare phase, but on lower frequencies. It indicates that the primary flare energy-release process during the flare moved to higher heights in the solar atmosphere.   


Figure 1 High-frequency part of the radio spectrum showing an unusual drifting burst B, pulsations P and type IV burst observed by the Ondřejov radiospectrographs on September 6, 2017.

Figure 2 Top panel: Phase map of pulsations P (see Figure 1) in the 2000–5000 MHz range at time of the first the γ-ray peak and at start of the sunquake at 11:55–11:57 UT for periods of 11–30 s. Arrows show the bidirectional drift of the pulsation phase. Bottom panel: The time profiles of the radio flux on 2050, 2500, and 3000 MHz.


The September 6, 2017 X9.3 flare was exceptional not only owing to its γ-ray emission,  sunquake, white light flare and CME, but also by the high-frequency drifting burst, interpreted as the radio emission from the rising filament and by the magnetoacoustic waves propagating upwards and downwards from the primary flare energy-release site in the flare impulsive phase.

Related article: This nugget is based on the paper by Karlický, M. and Rybák, J., 2020, ApJS, 250:31.


Hou, Y. J., Zhang, J., Li, T., Yang, S. H., Li, X. H., 2018, A&A, 619, A100

Karlický, M., Rybák, J., Monstein, C., 2017, SoPh, 292, 1

Karlický, M. and Rybák, J. 2020, ApJS, 250:31