Yan Yihua

On the wave mode of subsecond pulses in the cm-range

In this paper we determined the wave mode of subsecond pulses (SSP). We used data on pulses with a degree of polarization over 30%, with the sources located at −60 to +60 deg from the central meridian, for the period 2000–2002. The superposition of SSRT radio maps and MDI magnetograms has shown that radio SSP sources are typically located near the polarity inversion line of the active region magnetic field. Such an arrangement indicates that SSP sources are located at the tops of magnetic loops. The ordinary mode of electromagnetic radiation is recorded in SSP sources located from the inversion line by no less than about 10 arc sec.

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.

A solar magnetic field model and its 3-D boundary element method solution

A force-free magnetic field model with constant α is established, and a boundary element method is proposed to solve the problem. The procedure ensures a unique solution as well as a finite magnetic energy content. The proposed formulation is effective in solving magnetic fields above the solar surface, and the validity of our procedure is demonstrated by satisfactory agreement between calculated and observed magnetograms.

On the origin of microwave type U-bursts

An analysis is made of the observations of U-type cm-bursts recorded simultaneously with high spatial (Siberian solar radio telescope) and spectral resolution (National Astronomical Observatories/Beijing spectropolarimeters). It is shown that the source positions of increasing and decreasing branches of the U-burst coincide within a few arcsec. The suggestion is made that the occurrence of the observed type U-bursts at microwave frequencies is associated with a pressure and density response to rapid heating pulses in flare loops.

Observational evidence for solar coronal decimetric radio pulsations with very short periods

Using the decimetric (700-1500MHz) radio spectrometer and the synchronous observational system with high temporal resolution at four frequencies (1420, 2130, 2840 and 4260 MHz) of Yunnan Observatory, two rare events were observed on 2001 June 24 and 1990 July 30. The former was a small radio burst exhibiting pulsations with short periods (about 29, 40 and 100 ms) in the impulsive phase. The latter was a large radio burst, which at 2840 MHz produced radio pulsations with period of about 30 ms. This paper focuses on pulsations with very short periods in the range of 29-40 ms. The mechanism of generation of such pulsations may be modulation of radio radiation by the periodic trains of whistler packets originating in unstable regions of the corona. Alternatively, these pulsations can be attributed to wave-wave non-linear interactions of electrostatic upper hybrid waves driven by beams of precipitating electrons in flaring loops.

Space Astronomy in China

We review the development of space astronomy in China. We mainly introduce the researches relevant to the developments of instruments for space astronomy in China. In particular, we describe in details the Chinese Space Solar Telescope. It should be emphasized that the data analysis for the observations taken from spacecrafts is an important part in the space astronomy and Chinese astronomers have made significant progresses in analyzing data taken from foreign spacecrafts in different branches in astronomy, which are not included in this review, however. The contents of this paper are as follows. 1. The un-completed project “Astronomy-1” satellite 2. Hard X-ray, Soft γ-ray and Infrared Astronomy by Balloon Flights 3. Cosmic Gamma-Ray Burst Spectrometers onboard Spacecrafts 4. Hard X-ray Modulation Telescope (HXMT) 5. Lunar Exploration Project of China 6. The Space Solar Telescope 7. The Long-term Plans 8. Ground-based Supporting Systems for Space Astronomy

Observational evidence of very long period pulsations of solar coronal microwave radiation

On 1998 September 23, a rare event was observed with the microwave (1.0-2.0 GHz and 2.6-3.8 GHz) radio spectrograph of National Astronomical Observatories. It was a complex type-IV burst accompanied by multi-periodic pulsations, type-III bursts and type-I like noise storms. This paper focuses on the pulsation components with multiple long periods (about 7.3, 4.9, 3.7, 1.2 and 0.4 minutes). The pulsations may be due to standing waves in closed loops. Magnetohydrodynamic standing Alfven waves driven by the photospheric velocity field penetrate across the magnetic field, and this leads to the modulation of the radio radiation. These pulsations have downward motions, so it is not excluded that disturbances propagating in a closed or an open field structure may have caused the pulsations. The resonance of Alfven waves is concerned with the photospheric 5-minute oscillations, hence pulsations of very long periods may be assumed to come from driving sources in the photosphere. This can imply a possible mutual coupling between coronal magnetic loops and the photosphere.

Preliminary analysis of a joint observation of the complex solar radio burst of 1998 September 23

We have carried out a preliminary comparative analysis of a large, complex, type III-IV solar burst observed on 1998-09-23 with the 1-2 GHz and 2.6-3.8 GHz spectrometers of Beijing Astronomical Observatory as well as with the Russian SSRT and the German decimetric-metric dynamical spectrometer. The contents for research on the acceleration of electrons in the solar corona and on the structure of coronal magnetic field have been extended. Some observed phenomena and the radiation mechanisms of moving type IV bursts and multiple pulsations axe briefly interpreted.

Non-Conforming Boundary Elements for 3-D Steady-State Electromagnetic Fields

In this paper, a family of conforming or non-conforming boundary elements are introduced to the 3-D BE analyses of electromagnetic fields. As in practical problems most volumes of interest are bounded by a combination of some regular surfaces such as planes, spheres, or cylinders etc., the use of planes to model the volume geometry is described, and the Gaussian quadrature scheme over each subelement for surface integrals is employed. Thus the boundary solution can be represented properly. Moreover, the possibility of simulating the fields including dis-continuous solutions more precisely is provided. The proposed method has been applied to some steady-state electromagnetic fields and the results obtained appear satisfactory.

An extension boundary element formulation for potential problems with nonlinear materials

Abstract In this paper, an extension BEM that uses adjacent nodes to improve the trial functions on each element for nonlinear material potential problems is proposed, so that better approximations can be obtained for the boundary-only formulation based on the Kirchhoff transformation. The proposed method can increase the accuracy of the solution without increasing the total number of nodes. Some potential problems are analysed by the above formulation and the validity of the proposed method is demonstrated by comparison with other numerical solutions.