K. Jiřička

Global statistics of 0.8-2.0 GHz radio bursts and fine structures observed during 1992-2000 by the Ondřejov radiospectrograph

681 solar radio events observed by the Ondřejov 0.8-2.0 GHz radiospectrograph during 1992-2000 are analyzed and corresponding bursts and fine structures classified into ten different classes. A new rare type of fine structure with rapid frequency variation we called lace pattern was included. Drifting pulsation structures, observed usually at the beginning of the impulsive flare phase, were recognized among pulsations. Furthermore, a new sub-class of zebra patterns with many zebra lines (~30) superimposed on fibers was identified. For all defined types of burst and fine structures basic characteristics of their parameters are presented. Distributions of various types of burst and fine structures in the years 1992-2000 in dependence on the changes of solar activity during the cycles 22 and 23, occurrences of studied types of burst in association with GOES class flares as well as their relationship to GOES flare maxima are shown. Finally, the association of the analyzed bursts with the metric type III bursts observed at Potsdam-Tremsdorf Observatory was studied.

Narrowband dm-spikes in the 2 GHz frequency range and MHD cascading waves in reconnection outflows

Two examples of clouds of narrowband dm-spikes, observed by the Ondřejov radiospectrograph in the 1–2 GHz frequency range, are analyzed. After transformation of the frequency scales to distances in the solar atmosphere, the power spectra analysis of size scales reveals a spectral index of −5/3, resembling that of Kolmogorov spectra of turbulent cascades. The narrowband dm-spikes are interpreted as radio emission from electrons accelerated in MHD cascading waves, probably generated in plasma outflows from magnetic field reconnection.

Tadpoles in Wavelet Spectra of a Solar Decimetric Radio Burst

In the solar decimetric type IV radio event observed on 2001 June 13, we have found wavelet tadpole patterns for the first time. They were detected simultaneously at all radio frequencies in the 1.1-4.5 GHz frequency range. The characteristic period of the wavelet tadpole patterns was found to be 70.9 s. The parameters of the tadpoles on different frequencies are very similar and the correlations between individual radio fluxes are high. These tadpoles are interpreted as a signature of the magnetoacoustic wave train moving along the flare loop through the radio source and modulating its gyrosynchrotron emission.

1–4 GHz type II-like radio bursts and pinch processes near the transition region

Analyzing 205 radio bursts observed by the Ondřejov radiospectrograph in the 1–4 GHz frequency range during 1992 and 1993, we found 6 examples of type II-like radio bursts coinciding with impulsive phases of solar flares. These bursts were interpreted as radio manifestations of MHD (shock) waves generated during impulsive phases of flares in the vicinity of the transition region. Assuming a magnetic-field perturbation origin of these waves, we studied pinch processes in the current sheet near the transition region. In the 2-D MHD numerical model of this current sheet we demonstrated that 2-D pinch processes induced by radiative losses can trigger the impulsive phase of some flares and so generate the observed high-frequency type II-like radio bursts.

Repeated activity of the 0.8–1.2 GHz radio source of March 20, 1993

During March 20, 1993, from 12:00 to 16:00 UT, repeated radio burst activity was observed in the 0.8–1.2 GHz frequency range. ‘Periods’ in intervals 0.1–0.5, 0.7–1.0, 2.8–3.9, 75–170 s, and 15–25 min were recognized. This long-lasting narrowband activity consisted mainly of pulsations and continua. In some intervals it was accompanied not only by spikes, broadband pulsations, and fibers in the 1–2 GHz frequency range, but also by type III and U burst activity at lower frequencies as well as by hard X-ray bursts. From several radio bursts, two characterized by different fine structures were selected and compared. The observed differences are explained by different distribution functions of superthermal electrons. The position of the 0.8–1.2 GHz radio source above the photosphere and the magnetic field in the fiber burst source were estimated to be 66 000–75 000 km and 120–135 G, respectively.

Separation of solar radio bursts in a complex spectrum

Radio spectra, observed during solar flares, are usually very complex (many bursts and fine structures). We have developed a new method to separate them into individual bursts and analyze them separately. The method is used in the analysis of the 0.8–2.0 GHz radio spectrum of the April 11, 2001 event, which was rich in drifting pulsating structures (DPSs). Using this method we showed that the complex radio spectrum consists of at least four DPSs separated with respect to their different frequency drifts (−115, −36, −23, and −11 MHz s −1 ). These DPSs indicate a presence of at least four plasmoids expected to be formed in a flaring current sheet. These plasmoids produce the radio emission on close frequencies giving thus a mixture of superimposed DPSs observed in the radio spectrum.