Dynamics of Flare Processes and Variety of the Fine Structure of Solar Radio Emission over a Wide Frequency Range of 30 – 7000 MHz

Abstract

Radiobursts exibiting fine structure observed over two years during the rising phase of Cycle 24 by the Chinese Solar Broadband Radio Spectrometer (SBRS/Huairou) in the range 1-7.6 GHz and the spectrograph IZMIRAN in the meter range (25 -270 MHz) are analyzed. In five events zebra structure, various fiber bursts and fast pulsations were observed. These observations have great importance for testing different theoretical models of fine structure formation, as, for example, only for explaining the zebra-structure more than ten mechanisms have been proposed. Events on 15 and 24 February 2011 are of the greatest interest. In the course of the flare on 15 February (which occurred close to the center of disk) zebra structure was observed during three sequential flare brightenings. The polarization changed sign in the third. This behavior of polarization combined with images of the corresponding flare brightenings, obtained in extreme ultraviolet radiation by the Solar Dynamics Observatory (SDO/AIA, 171 Å) provides important clues. The polarization of radio emission in all three cases is related to the ordinary wave mode of radio emission. The zebra structure was present at frequencies 190-220 MHz in the Culgoora spectrum. The event on 24 February 2011 is remarkable, as the zebra structure at frequencies of 2.6-3.8 GHz was not polarized and it appeared during the magnetic reconnection observed by SDO/AIA 171 Å in this limb flare. In the event on 9 August 2011, for the first time, a superfine millisecond structure was registered simultaneously in the fast pulsations and the stripes of zebra structure. In the event on 1 August 2010 after the zebra structure two families of fibers bursts with an opposite frequency drift were observed. On 19 April 2012 the fibers against the background of type III bursts were observed by IZMIRAN and Nançay spectrographs. In the band of 42 -52 MHz a group of nine slowly drifting narrow-band (</~ 1 MHz) fibers formed a unique type II burst. Almost all events in the microwave range contain superfine structure in the form of the millisecond spikes, whose emission should be considered primary. It is possible that each type of fine structure is excited by the same mechanism, and the broad variety of events is related to the dynamics of flare processes.

Flare evolution and polarization changes in fine structures of solar radio emission in the 2013 April 11 event

“…Its maximum emission flux at 06:58:30 UT was very strong, it exceeded the background flaring continuum intensity by several times (Figure 3 and 4). Tan et al (2014) showed that this ZP was simultaneously observed with ORSC/Ondrejov spectrograph and all details (stripe separations, brightness, duration, the arched shapes of each stripe and the spiky super structure) are strictly identical to the spectrogram obtained by SBRS/Huairou. The middle panel in Figure 4 shows moderate left-handed polarization, of 35 ± 5%.…”

“…We do not have any convincing evidence that the mechanism, generated ZP, is the emission of Bernstein modes as this was proposed by Tan et al (2014) for the first strong ZP. First, an exact definition of the frequency separation between three (and sometimes four) zebra stripes is too problematic.…”

“…The first strong ZP burst occurred just at the beginning of the flare (Figure 4). Tan et al (2014) described it and correlated it with a sudden EUV flash observed by SDO/AIA images. However, in several subsequent radio maxima (see profiles in Figure 3) many Fig.…”

“…Therefore a corresponding shock wave could be driven by the CME front. The beginning of the event and related instrumentations were described by Tan et al (2014).…”

A Model of Zebra Patterns in Solar Radio Emission

“…It is impossible to consider the ten other ZP models in one publication; their importance requires additional analysis. A detailed description of the observational properties of the ZP and fiber bursts and their modern theoretical models are given in the literature (Chernov et al, 2015 and in two other reviews, Chernov, 2012 and 2016, which are freely accessible at http://www.izmiran.ru/~gchernov/).…”