Fu Qi-jun

Nonlinear calibration and data processing of the solar radio burst

The processes of the sudden energy release and energy transfer, and particle accelerations are the most challenge fundamental problems in solar physics as well as in astrophysics. Nowadays, there has been no direct measurement of the plasma parameters and magnetic fields at the coronal energy release site. Under the certain hypothesis of radiation mechanism and transmission process, radio measurement is almost the only method to diagnose coronal magnetic field. The broadband dynamic solar radio spectrometer that has been finished recently in China has higher time and frequency resolutions. Thus it plays an important role during the research of the 23rd solar cycle in China. Sometimes when there were very large bursts, the spectrometer will be overflowed. It needs to take some special process to discriminate the instrument and interference effects from solar burst signals. According to the characteristic of the solar radio broadband dynamic spectrometer, we developed a nonlinear calibration method to deal with the overflow of instrument, and introduced channel-modification method to deal with images. Finally the interference is eliminated with the help of the wavelet method. Here we take the analysis of the well-known solar-terrestrial event on July 14th, 2000 as the example. It shows the feasibility and validity of the method mentioned above. These methods can also be applied to other issues.

The First Spectral Analysis of Solar Radio Spike Event

The first spectral analysis of a spike event has been performed. It was found that the peak frequency of spectrum of the spike event on Oct. 2, 1993 drifted from higher to lower frequency for the first group of spikes, but the frequency remained almost the same for the second group. Only if the mean energy of electron beam is a monotonic or random function of time and its pitch angle is a monotonically decreasing function of mean energy, then the different observed characteristics of evolution of the peak frequency with time may be interpreted by the location of resonant circle. The spike radio emissions with one and the same peak frequency probably originated from the same region of electron cyclotron maser instability.

An exploration of the origin of bidirectional electron beams in the solar corona with observations of pairs of type III radio bursts

Three paired type III radio bursts from metric to microwave regions observed with radio spectrometers at Yunnan and Bejing Observatories are investigated. The frequencies and heights of bidirectional electron beams are estimated. The normal and reverse drifts in the three events separate at three very different frequencies of 250, 1300 and 2900 MHz, but each within a narrow range between 4 and 100 MHz. Compact acceleration sources are indicated where originate both upward and downward moving electron beams. The magnetic structure topology, the size of the source of electron acceleration, the velocity of the electron beams and the mechanism of paired type III bursts are discussed.

Spike observations in flares in China

Variations on short time-scales have been found in solar flares at different wavelengths. Millisecond scale radio spikes are a quickly developing area of solar radio astronomy. The solar radio astronomy group of Beijing Astronomical Observatory (BAO) has found fine structures of microwave bursts with millisecond time-scale at 2840 MHz. In this paper, we briefly summarize the observations. A joint-observation network for observing solar radio bursts with high time resolution has also been established. The equipment in the network covers a frequency domain of more than 10:1, including 1.3, 2.0, 6, 10, 15, 20 cm, and meter wavelengths. In particular, a multi-channel polarimeter with super-fast sampling (10 μs) at 2600 MHz, an intensity interferometer with 1 ms sampling rate at 6 cm wavelength, and an auto-correlation radio spectrograph with 8 ms time constant at 21 cm wavelength are being established. We pay close attention to research on the spike emission features over wide bands, and their relationship to special characteristics in other spectral ranges.

Spectra of solar microwave burst events

Abstract The two solar radio spectrometers of National Astronomical Observatories for frequency bands 1.0–2.0 GHz and 2.6–3.8 GHz were put into operation, respectively, in January 1994 and September 1996. Since then up to April 10, 2000, they have recorded, respectively, 297 and 316 solar radio bursts and acquired high-quality dynamical materials with high temporal and frequency resolutions. This provides abundant information for the study of the temporal and spatial evolution processes of solar flares on various scales. The event of 1998-04-15, which was observed with both spectrometers, displayed diverse changes of amplitudes and structures in both time and frequency.

Solar microwave combined type-III bursts

Abstract We call a “microwave combined type III burst” when there is a microwave common type III burst at the low frequency end and a simultaneous microwave continuous type U burst at the high frequency end. The latter is the result of further evolution of single microwave type U bursts in one and the same magnetic ring, and, as such, is still a subtype of type III bursts. Hence the whole combined type III burst is also a subtype of type III bursts. The source of radiation of the microwave combined type III burst (i.e. high-energy electron beams) comes from one and the same region of acceleration. But in its interaction with magnetic rings in low coronal regions an electron beam is divided into two beams, one of captured electrons and one of escaping electrons. These beams have different trajectories of motion and give rise to simultaneous continuous type III burst at the high frequency end and common type III burst at the low frequency end. The formation of the combined burst is closely related to the structure of magnetic rings in the low coronal regions, so it is peculiar to the microwave range.

A statistical study of solar microwave fine structure events

Abstract This paper presents a statistical summary of microwave fine structure (FS) events recorded by Beijing Astronomical Observatory at 2840 MHz in 1991. The statistical analysis shows that the time scales are between tens and hundreds of ms for more than 67% of the events and the amplitudes are less than 200 sfu in more than 85% of the cases. We discuss the time scale and intensity of FS events and the difference in the occurrence rate between the 21st and 22nd solar cycles.

MHD oscillations in the solar spike radiation

Abstract In this paper the observed 1.4–1.6 s quasi-periodic oscillations in the spike radiation of the microwave outburst of 1981 May 16 are analysed in teras of MHD waves. We point out that the fast magnetoacoustic waves (“sausage” mode) propagating inside and outside a loop can modulate the magnetic field and the pitch angle distribution of the electron beams in the source region. The growth rate of electron-cyclotron-maser instability is then affected to give rise to the quasi-periodic oscillations. Quantitative estimates of relevant physical parameters are given.