G. P. Chernov

A superfine structure in solar microwave bursts

We have observed in the microwave range (with the radio spectrometer of the Huairu station (Beijing, NAOC) around 3 GHz) the ne structure of solar radio bursts called zebra patterns and b er bursts (seen drifting on the frequency stripes in emission and in absorption on the background burst continuum emission). In all seven observed bursts we discovered a new eect: zebra stripes have a superne structure, consisting of numerous fast spikes with duration at a limit of the time resolution of the spectrometer, 8 ms. Since for zebra patterns and microwave spikes dieren t emission mechanisms were proposed, these new observations require us to revise known theories. An alternative model of microwave millisecond spikes is based on the coupling of plasma waves (l) with ion- sound waves (s) : l + s ! t. Since the main features of zebra stripes and b er bursts are similar, we consider the zebra pattern of these bursts as whistler manifestations. Whistlers (w) yield a principal contribution in the ne structure radio emission (t) by coupling with Langmuir waves at sum as well as dierence frequencies: !l !w = !t. Allowance for the conversion of ion-sound waves into whistlers (and inversely in a pulsating regime) enables us to identify the zebra pattern consisting of spikes as a simultaneous manifestation of both those processes (l + s ! t and l + w ! t ) in radio sources, related to magnetic reconnection above are regions.

Recent data on zebra patterns

A comparative analysis of two recent solar radio outbursts around 3 GHz with zebra structures and fiber bursts in their dynamical radio spectra is carried out using all available ground-based and satellite data (SOHO, TRACE, RHESSI). The latest theoretical models of the zebra pattern are critically discussed . New data on microwave zebra structures and fiber bursts suggests that they are analogous to similar structures observed at meter wavelengths. It was discovered that in the 2,6-3,8 GHz frequency band more than 34 zebra stripes can appear simultaneously, and some isolated fiber bursts can continuously be transformed into zebra stripes. This fact indicates a single origin for both structures. The zebra pattern was observed when the signs of magnetic reconnections were revealed in images of 195 ˚ lines, and radio sources coincided with positions of some new sources in hard X-rays. All the main properties of the stripes in emission and absorption can be explained if they are associated with interactions between electrostatic plasma waves and whistlers, taking into account the quasi-linear diusion of fast particles with the loss-cone distribution on whistlers. In this model it is possible to obtain realistic values for the magnetic field strength of B … 160G at the plasma level of about 3 GHz. The double plasma resonance model for the zebra pattern based on the known realistic dependences of electron density and magnetic field yields a frequency dependence for the frequency separation between stripes that does not agree with the observations.

On the Zebra Structure in the Frequency Range near 3 GHz

We present 19 cases of zebra pattern structure (ZPS) and fiber bursts (FB) in radio bursts in frequency range around 3 GHz, and one such case in the range 5.2–7.6 GHz, using the new microwave spectrometer of NAOC between 2.6–3.8 and 5.2–7.6 GHz (China, Huairou station) with high resolution (10 MHz and 8 ms). The FB and ZPS have about the same spectral parameters: the frequency bandwidth of emission stripes Δf ~ 20 MHz, the frequency separation between the emission and the neighboring low frequency absorption -Δfea ~ 30 MHz and the frequency separation between emission stripes (when a periodic structure persists) Δfs ~ 60-70 MHz. Therefore we consider both these fine structures to be whistler manifestations, i.e., interactions of plasma electrostatic waves with whistler waves (generated by the same fast particles with loss-cone anisotropy) l + w → t. The duration of the fiber bursts of about 2s corresponds to whistler waves propagating undamped at about 2s, which requires a whistler increment < 0.5 s−1. In the frequency range 3–7 GHz the relation between the ratios of plasma to cyclotron frequencies and whistler to cyclotron frequencies is almost independent of the decrement of whistler electron damping. This finding is used to obtain the magnetic field strength in the region of generation. For a reasonable value of electron temperature (2–20 MK), we find B = 125–190 G when the electron density is (8-18) × 1010 cm-3 and B = 520–610 G when the electron density is (35–60) × 1010 cm-3. In two remarkable events, 1998-04-15 and 2000-10-29, the right-hand polarization is strong for all the fine structures and corresponds to ordinary wave.

Spectral and spatial observations of microwave spikes and zebra structure in the short radio burst of May 29, 2003

Context. The unusual radio burst of May 29, 2003 connected with the M1.5 flare in AR 10368 has been analyzed. It was observed by the Solar Broadband Radio Spectrometer (SBRS/Huairou station, Beijing) in the 5.2‐7.6 GHz range. It proved to be only the third case of a neat zebra structure appearing among all observations at such high frequencies. Despite the short duration of the burst (25 s), it provided a wealth of data for studying the superfine structure with millisecond resolution (5 ms). Aims. We localize the site of emission sources in the flare region, estimate plasma parameters in the generation sites, and suggest applicable mechanisms for interpretating spikes and zebra-structure generation. Methods. We analyze of flare area structures and spectral parameters of millisecond spikes and their radio sources. We then interpret the superfine structure in the framework of known models. Results. Positions of radio bursts were obtained by the Siberian Solar Radio Telescope (SSRT) (5.7 GHz) and Nobeyama radioheliograph (NoRH) (17 GHz). The flare configuration includes two systems of loops with the common base near the N-spot. The loop bases coincide with polarized emission sources at 17 GHz. The sources in intensity gravitated to tops of short loops at 17 GHz, and to long loops at 5.7 GHz. Short pulses at 17 GHz (with a temporal resolution of 100 ms) are registered in the R-polarized source over the N-magnetic polarity (extraordinary mode). The positions of the subsecond pulse sources at 5.7 GHz change from pulse to pulse and are level with the tops of some loops over the magnetic field’s neutral line. Dynamic spectra show that all the emission comprised millisecond pulses (spikes) of 5‐10 ms duration in the instantaneous band of 70 to 100 MHz, forming the superfine structure of different bursts, essentially in the form of fast or slow-drift fibers and various zebra-structure stripes. Five scales of zebra structures have been singled out. The occurrence of the spikes is associated with the formation of two new radio sources with different polarities, which appeared simultaneously on SSRT and NoRH maps. This took place after new magnetic fluxes of opposite polarity had emerged in the leading spot and a new magnetic “delta” configuration had been formed. Conclusions. As the main mechanism for generating spikes (as the initial emission) we suggest the coalescence of plasma waves with whistlers in the pulse regime of interaction between whistlers and ion-sound waves. In this case one can explain the appearance of fibers and sporadic zebra-structure stripes exhibiting the frequency splitting.

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

The measurement of positions and sizes of radio sources in observations is important for understanding of the flare evolution. For the first time, solar radio spectral fine structures in an M6.5 flare that occurred on 2013 April 11 were observed simultaneously by several radio instruments at four different observatories: Chinese Solar Broadband Radio Spectrometer at Huairou (SBRS/Huairou), Ondrejov Radio Spectrograph in the Czech Republic (ORSC/Ondrejov), Badary Broadband Microwave Spectropolarimeter (BMS/Irkutsk), and spectrograph/IZMIRAN (Moscow, Troitsk). The fine structures included microwave zebra patterns (ZPs), fast pulsations and fiber bursts. They were observed during the flare brightening located at the tops of a loop arcade as shown in images taken by the extreme ultraviolet (EUV) telescope onboard NASAs satellite Solar Dynamics Observatory (SDO). The flare occurred at 06:58-07:26 UT in solar active region NOAA 11719 located close to the solar disk center. ZPs appeared near high frequency boundaries of the pulsations, and their spectra observed in Huairou and Ondrejov agreed with each other in terms of details. At the beginning of the flares impulsive phase, a strong narrowband ZP burst occurred with a moderate left-handed circular polarization. Then a series of pulsations and ZPs were observed in almost unpolarized emission. After 07:00 UT a ZP appeared with a moderate right-handed polarization. In the flare decay phase (at about 07:25 UT), ZPs and fiber bursts become strongly right-hand polarized. BMS/Irkutsk spectral observations indicated that the background emission showed a left-handed circular polarization (similar to SBRS/Huairou spectra around 3 GHz). However, the fine structure appeared in the right-handed polarization. The dynamics of the polarization was associated with the motion of the flare exciter, which was observed in EUV images at 171 angstrom and 131 angstrom by the SDO Atmospheric Imaging Assembly (AIA). Combining magnetograms observed by the SDO Helioseismic and Magnetic Imager (HMI) with the homologous assumption of EUV flare brightenings and ZP bursts, we deduced that the observed ZPs correspond to the ordinary radio emission mode. However, future analysis needs to verify the assumption that zebra radio sources are really related to a closed magnetic loop, and are located at lower heights in the solar atmosphere than the source of pulsations.

Concerning spikes in emission and absorption in the microwave range

In some events, weak fast solar bursts (near the level of the quiet Sun) were observed in the background of numerous spikes in emission and absorption. In such a case, the background contains the noise signals of the receiver. In events on 2005 September 16 and 2002 April 14, the solar origin of fast bursts was confirmed by simultaneous recording of the bursts at several remote observatories. The noisy background pixels in emission and absorption can be excluded by subtracting a higher level of continuum when constructing the spectra. The wavelet spectrum, noisy profiles in different polarization channels and a spectrum with continuum level greater than zero demonstrates the noisy character of pixels with the lowest levels of emission and absorption. Thus, in each case, in order to judge the solar origin of all spikes, it is necessary to determine the level of continuum against the background of which the solar bursts are observed. Several models of microwave spikes are discussed. The electron cyclotron maser emission mechanism runs into serious problems with the interpretation of microwave millisecond spikes: the main obstacles are too high values of the magnetic field strength in the source (omega(Pe) t) in a source related to shock fronts in the reconnection region.