Q. J. Fu

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.

An Estimation of the Coronal Magnetic Field Strength From Spectrographic Observations in the Microwave Range

Solar radio emission observations in the microwave frequency range show fine structures consisting of a number of almost parallel narrow-frequency bands. We interpret these bands as plasma emission at cyclotron harmonics. This emission is generated by the anisotropic electron beam, which excites longitudinal waves at a normal Doppler effect resonance. Subsequently, the longitudinal waves convert to radio emission at the second harmonic of the longitudinal wave frequency, and sometimes to the fundamental harmonic. The magnetic field strength is estimated on the basis of such a model in the microwave burst sources at ∼ 100–200xa0G. Estimates of the density variations are also made.

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.

Drifting radio bursts and fine structures in the 0.8-7.6 GHz frequency range observed in the NOAA 9077 AR (July 10-14, 2000) solar flares

The 0.8{7.6 GHz global and detailed radio spectra of the four most intense flares observed in the NOAA 9077 active region (July 10{14, 2000) are presented. The radio bursts of these flares and their sequence reveal features indicative of topological similarities among the flares under study. The drifting pulsation structures were found to be the typical signatures of these flares. Furthermore, many other ne structures such as narrowband drifting lines, drifting harmonic structure with zebra patterns, drifting branches of narrowband dm-spikes, and structures with fast positively and negatively drifting bursts are shown in the context of the whole radio flares. Some of them were observed for the rst time. The relationships among them and the resulting interpretations are summarized. The characteristic periods of the drifting pulsation structures and the magnetic eld in the zebra radio source are determined.

Discovery of unusual large group delay in microwave millisecond oscillating events

Two events demonstrating periodic narrowband millisecond pulsations of radio emission both in in- tensity and polarization degree are analyzed. Large time delays between L -a ndR-polarization components are found. The radio emission is shown to be generated as unpolarized by a plasma mechanism at the second harmonic of the upper-hybrid frequency. Observed oscillations of the radiation polarization degree arise due to group delay between extraordinary and ordinary modes along the line of sight. The theoretically predicted dependence of the group delay on frequency (f 3 ) agrees excellently with the observed delay frequency dependence. Physical parameters of the emission source and delay site are determined within the proposed model.

Anisotropic Beam Model for the Spectral Observations of Radio Burst Fine Structures on 1998 April 15

A fine structure consisting of three almost equidistant frequency bands was observed in the high frequency part of a solar burst on 1998 April 15 by the spectrometer of Beijing Astronomical Observatory in the range 2.6–3.8 GHz. A model for this event based on beam-anisotropic instability in the solar corona is presented. Longitudinal plasma waves are excited at cyclotron resonance and then transformed into radio emission at their second harmonic. The model is in accordance with the observations if we suppose a magnetic field strength in the region of emission generation of about 200 G.

Ion-sound model of microwave spikes with fast shocks in the reconnection region

A new model for solar spike bursts is considered based on the interaction of Langmuir waves with ion-sound waves: l+s→t. Such a mechanism can operate in shock fronts, propagating from a magnetic reconnection region. New observations of microwave millisecond spikes are discussed. They have been observed in two events: 4 November 1997 between 05:52–06:10xa0UT and 28 November 1997 between 05:00–05:10xa0UT using the multichannel spectrograph in the range 2.6–3.8xa0GHz of Beijing AO. Yohkoh/SXT images in the AR and SOHO EIT images testify to a reconstruction of bright loops after the escape of a CME. A fast shock front might be manifested as a very bright line in Te SXT maps (up to 20xa0MK) above dense structures in emission measure (EM) maps. Moreover one can see at the moment of spike emission (for the 28 November 1997 event) an additional maximum at the loop top on the HXR map in the AR as principal evidence of fast shock propagation. The model gives the ordinary mode of spike emission. Sometimes we observed a different polarization of microwave spikes that might be connected with the depolarization of the emission in the transverse magnetic field and rather in the vanishing magnetic field in the middle of the QT region. Duration and frequency band of isolated spikes are connected with parameters of fast particle beams and shock front. Millisecond microwave spikes are probably a unique manifestation of flare fast shocks in the radio emission.

The Generating Region of Bidirectional Electron Beams in the Corona

Metric and decimetric type III bursts and microwave spike emissions with negative and positive frequency drift rates which were observed with radio spectrometers at Yunnan and Beijing Observatories are presented. The frequencies and heights at which the bidirectional electron beams originated are estimated. Three events reveal a separatrix frequency (at 250, 1300, and 2900xa0MHz) between normal- and reverse-drifting radio bursts, indicating a compact acceleration source where electron beams are injected in both upward and downward directions. These cases may indicate that the changeover frequencies of bidirectional electron beams are within a large band from 250 to 2900xa0MHz and the frequency bands of separatrices are in very small (4 to 100xa0MHz) and different bands. These type III bursts appear to be a plasma emission phenomenon from a beam of electrons which seem to have widely separated acceleration regions from the high to the low corona. These cases suggest that current sheets that separate open and closed magnetic fluxes in the low corona, and oppositely directed open field lines in the high corona are possible sites for bidirectional electron acceleration. The regions of magnetic topology from closed to open magnetic field structures should be very large (from about 20u2009000 to 107u2009000xa0km above the photosphere).

Observations of microwave type-U bursts

Two microwave type-U bursts observed with the 2.6–3.8xa0GHz spectrometer of Beijing Astronomical Observatory (BAO) are described and analysed in this paper. The microwave type-U bursts have very short durations (about 200xa0ms), narrow bandwidth, high-frequency drift rates of the ascending and descending branches, and a high degree of circular polarization (80%). The sense of polarization remains the same from the ascending to the descending branch.

Group of type III bursts and its associated spikes

A group of type III bursts in solar radio bursts, observed with the 2.6–3.8 GHz spectrometer of the Beijing Astronomical Observatory on April 15 1998, is selected and analyzed. The type III bursts are associated with a large number of spikes, which have been recorded by the spectrometer at high temporal resolution (8 ms). We report the observational characteristics of the type III bursts and the corresponding spikes including duration, frequency bandwidth, drift rate and quasi-periodicity. Afterwards we discuss the space scale, the brightness temperature and the magnetic field strength at the emission source region. Finally we investigate the mechanism which would cause the quasi-periodicity of the type III bursts and the short time scale of the spikes theoretically. It is suggested that the type III bursts and the spikes differ clearly between their source regions in the space scale, the brightness temperature, the magnetic field strength and the magnetic structure.