Qi-Jun Fu

The Magnetic Rope Structure and Associated Energetic Processes in the 2000 July 14 Solar Flare

In the reconstructed nonlinear force-free magnetic field of NOAA Active Region 9077 before the X5.7/3B (10:24 UT) flare on 2000 July 14, we reveal for the first time the presence of a magnetic rope from the extrapolation of the three-dimensional magnetic field structure. This magnetic rope is located in a space above the magnetic neutral lines of the filament. The calculated field lines of the rope rotate around its axis for more than three turns. Overlying the rope are multilayer magnetic arcades with different orientations. These arcades are in agreement with the Transition Region and Coronal Explorer observations. The estimated free magnetic energy in this rope system is about 1.6 × 1032 ergs. Such magnetic field structure provides a favorable model for the interpretation of the energetic flare processes as revealed by Hα, EUV, and radio observations. In particular, the intermittent cospatial brightening of the rope in EUV 1600 A image leading to the onset of the flare suggests that the rope instability may have triggered the flare event, and the drifting pulsation structure in the decimetric frequency range is considered to manifest the initial phase of the coronal mass ejection.

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.

Dual pulsations in solar radio bursts at short centimeter wavelengths

It is well known that the oscillating MHD waves drive periodic variations in the magnetic field. But how the MHD waves can be triggered in the flaring loops is not yet well known. It seems to us that this problem should be connected with the physical processes occurring in the flare loop during a solar flare. A peculiar solar flare event at 04:00–04:51 UT on May 23, 1990 was observed simultaneously with time resolutions 1 s and 10 ms by Nanjing University Observatory and Beijing Normal University Observatory, which are about 1000 km apart, at 3.2 cm and 2 cm wavelengths, respectively. Two kinds of pulsations with quasi-periods 1.5 s and 40 s were found in radio bursts at the two short centimeter waves; however, the shorter quasi-periodic pulsations were superimposed upon the longer ones. From the data analysis of the above-mentioned quasi-periodic pulsations and associated phenomena in radio and soft X-ray emissions during this flare event published in Solar Geophysical Data (SGD), the authors suggest that the sudden increase in plasma pressure inside (or underlying) the flare kernel due to the upward moving chromospheric evaporated gas, which is caused by the explosive collision heating of strong non-thermal electrons injected downwards from the microwave burst source, plays the important role of triggering agents for MHD oscillations (fast magneto-acoustic mode and Alfvén mode) of the flare loop. These physical processes occurring in the flare loop during the impulsive phase of the solar flare may be used to account for the origin and observational characteristics of quasi-periodic pulsations in solar radio bursts at the two short centimeter wavelengths during the flare event of 1990 May 23. In addition, the average physical parameters N, T, B inside or underlying the flare kernel can be also evaluated.

Fine structures in solar radio bursts at a 21 cm wavelength and pulsation modulation

The solar burst of 13 July, 1986 at 21 cm wavelength was recorded with a time constant of 8 ms. In the course of the burst lasting for about 40 min there appeared distinct stages of the bursts evolution. They consisted of the pattern of energy release in flares which was proposed by Sturrock et al. (1984). There were pulsations with periodicities of 0.178 and 4 s superimposed on the flux density. The pulsations were quasiperiodical with features of almost unchanged mean periods. The relative amplitude of the pulsation modulation changed with the phase of the burst; in general, it reached 10–20% in the rising phase. The possible mechanisms of pulsations are discussed and some plasma parameters of oscillation sources are deduced.

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.

SPIKE EMISSION AT 21 CM WAVELENGTH IN POST-FLARE LOOPS

Electron-cyclotron maser instability produced by accelerated energetic electrons is todays most favoured process for spike emission on ultra-short time scales at dm wavelengths. The process of electron acceleration occurs primarily in pre-impulsive and impulsive phases. We present observations of spike bursts at 21 cm in a limb event that occurred in AR 5629 on 1989 August 17. Optical observations of the event from Beijing Astronomical Observatory show coronal loops that interact just before the spike groups appeared. This implies that additional particle acceleration might be produced by the interaction between emerging loops and original large loops, and become the source of the energetic electrons responsible for the spike emission appearing in the descending phase. Some important parameters are deduced.

A pair of solar spike emissions

Using the 2.6–3.8 GHz solar radio spectrometer of the National Astronomical Observatories of China (NAOC), a pair of microwave millisecond spike (MMS) emissions were observed, and their frequency drift rate was measured. The separatrix frequency of the MMS pair was at 2900 MHz. Its emission layer was about 2×104km above the photosphere. The polarization degree was wave-like variation with an average value of about 25% in LCP. An MMS pair differs greatly from the type III bursts pair. For the latter, in a certain frequency range, there is no emission around separatrix frequency. This phenomenon may help better understand the mechanism of MMS.

Statistics and analysis of microwave type III bursts at low frequencies

Abstract68 groups of type III bursts recorded with the dynamic spectrograph (from1 to 2 GHz) at the Beijing Astronomical Observatory are statistically analyzed in this present article, with the frequency drift, duration, bandwidth and polarization of each of these events being analyzed in detail. These observational features are different from those of metric, decimetric and microwave high frequency type III bursts, showing that there possibly exist transition phenomena in the microwave low frequency band and the microwave type III bursts at low frequencies are possibly produced by the harmonic plasma radiation caused by nonthermal electronic beams and by the electron gyro-maser radiation.

A group of solar microwave type III bursts with reversed drift rates on Nov 4, 1997

The most important feature of the microwave type III bursts on Nov. 4, 1997 is the periodically reversed drift rates, which may be contributed to a group of electron beams trapped by a huge magnetic tube (10(4) km). It is suggested that these electron beams are accelerated by the same mechanism, because there is a power law distribution with index 3.2 in the energetic spectrum of the beam. On the other hand, the energy release in each pulse is quasi-quantized, which is confirmed by the statistical correlation between the rising time and the burst flux. Both of these two results are based on the model of plasma instability responsible for the burst.The most important feature of the microwave type III bursts on Nov. 4, 1997 is the periodically reversed drift rates, which may be contributed to a group of electron beams trapped by a huge magnetic tube (104 km). It is suggested that these electron beams are accelerated by the same mechanism, because there is a power law distribution with index 3.2 in the energetic spectrum of the beam. On the other hand, the energy release in each pulse is quasi-quantized, which is confirmed by the statistical correlation between the rising time and the burst flux. Both of these two results are based on the model of plasma instability responsible for the burst.

Broadband radio spectral observations of the solar eclipse on 2008-08-01 and its implications on the quiet Sun atmospheric model

Based on the joint-observations of the radio broadband spectral emissions of the solar eclipse on August 1, 2008 at Jiuquan (total eclipse) and Huairou (partial eclipse) at the frequencies of 2.00–5.60 GHz (Jiuquan), 2.60–3.80 GHZ (Chinese solar broadband radiospectrometer, SBRS/Huairou), and 5.20–7.60 GHz (SBRS/Huairou), the authors assemble a successive series of broadband spectra with a frequency of 2.60–7.60 GHz to observe the solar eclipse synchronously. This is the first attempt to analyze the solar eclipse radio emission under the two telescopes located at different places with broadband frequencies in the periods of total and partial eclipses. With these analyses, the authorsmade a semiempirical model of the coronal plasma density of the quiet Sun, which can be expressed as ne ≃ 1.42×109(r−2+1.93r−5) (cm−3), in the space range of r=1.039–1.212 R⊙, and made a comparison with the classic model.