Huirong Ji

A broadband spectrometer for decimeter and microwave radio bursts

Observations of solar microwave bursts with high temporal and spectral resolution have shown interesting fine structures (FSs) of short duration and small bandwidth which are usually superimposed on the smooth continuum. These FSs are very intense (up to 1015 K) and show sometimes a high degree of circular polarization (up to 100%). They are believed to be generated by electron cyclotron maser emission (ECME) in magnetic loops. Another type are the microwave type III bursts, which are drifting microwave FSs, and are probably the signatures of travelling electron beams in the solar atmosphere. The exact emission mechanisms for these phenomena, in particular the source configuration, the plasma parameters and the distribution of radiating electrons are not clear. For a detailed study of these problems new observations of intensity and polarization with high resolution in time and in frequency in decimeter and microwave wavebands are essential. In order to investigate these features in greater detail, spectrometers with high temporal and spectral resolution are being developed by the solar radio astronomy community of China (Beijing Astronomical Observatory (BAO), Purple Mountain Observatory (PMO), Yunnan Astronomical Observatory (YAO), and Nanjing University (NJU)). The frequency range from 0.7 to about 12 GHz is covered by about five spectrometers in frequency ranges of 0.7–1.4 GHz, 1–2 GHz, 2.4–3.6 GHz, 4.9–7.3 GHz, and 8–12 GHz, respectively. The radiospectrometers will form a combined type of swept-frequency and multi-channel receivers. The main characteristics of the solar radio spectrometers are: frequency resolution: 1–10 MHz; temporal resolution: 1–10 ms; sensitivity: better than 2% of the quiet-Sun level. We pay special attention to the sensitivity and the accuracy of polarization. Now, the 1–2 GHz radiospectrometer is being set up. The full system will be set up in 3–4 years.

A New Solar Radio Spectrometer at 1.10–2.06GHz and First Observational Results

An improved Solar Radio Spectrometer working at 1.10–2.06 GHz with much improved spectral and temporal resolution, has been accomplished by the National Astronomical Observatories and Hebei Semiconductor Research Institute, based on an old spectrometer at 1–2 GHz. The new spectrometer has a spectral resolution of 4 MHz and a temporal resolution of 5 ms, with an instantaneous detectable range from 0.02 to 10 times of the quiet Sun flux. It can measure both left and right circular polarization with an accuracy of 10% in degree of polarization. Some results of preliminary observations that could not be recorded by the old spectrometer at 1–2 GHz are presented.

A model of magnetic fields in a large-scale solar active region☆☆☆

The magnetic field configuration in a super-active region (i.e., a large, island-like delta -type spot) is simulated with a non-linear force-free field that varies slowly with time. Such a complex magnetic field exhibits the main observational characteristics of vector magnetic fields. These are the extreme inequality of positive and negative magnetic fluxes (the ratio of positive and negative fluxes is 1:6), the U-shaped magnetic inversion line, as well as the difference between dipoles and quadrupoles. The results of simulation may be used to interpret the following observed phenomena: (1) Large flares often occur in regions of mixed polarities or quadrupoles near a U-shaped inversion line. (2) In the quasi-bipolar regions near U-shaped inversion line, almost no large flares appear or there are merely small flares. (3) Large-scale rotational motion and flux emergence within active regions may give rise to quadrupolar magnetic topological separatrices and produce large current sheets, and hence intense magnetic reconnections and major flares are induced. The method used in the computation of non-linear force-free fields is a natural extension of Woodburys relaxation method in polar coordinates. The common ill-posed solutions are thereby avoided. This makes the linear and non-linear force-free solutions to be mutually consistent, so there is a continuous and smooth transition between the two.

A radio spectrometer at 2.6-3.8 GHz

The properties, structure and performance of a Solar Radio Dynamic Spectrometer working at 2.6-3.8 GHz developed by Beijing Astronomical Observatory, Purple Mountain Observatory and Nanjing University is described, Some results of initial observation are presented.