Qiwu Song

STATISTICS OF FLARING LOOPS OBSERVED BY THE NOBEYAMA RADIOHELIOGRAPH. III. ASYMMETRY OF TWO FOOTPOINT EMISSIONS

Two footpoint (FP) emissions are compared in a total of 24 events with loop-like structures imaged by the Nobeyama Radioheliograph (NoRH), which are divided into two groups: when the optically thin radio spectrum in the looptop is harder than those in the two FPs (group 1) and when it is softer than those in at least one FP (group 2). There are always correlative variations of the brightness temperatures and polarization degrees, the spectral indices, the column densities of nonthermal electrons, and magnetic field strengths in the two FPs. The maximum differences of these parameters in the two FPs may reach one or two orders of magnitude (except the polarization degree). The logarithm of the ratio of the magnetic field strengths in the two FPs is always anti-correlated with the logarithms of the ratios of the brightness temperatures in the two FPs, but correlated with the differences of the spectral indices in the two FPs. Only two anti-correlations exist in group 1, between the difference of the absolute polarization degrees in the two FPs and the logarithm of the ratio of the brightness temperatures in the two FPs and between the difference of the spectral indices in the two FPs and the logarithm of the ratio of the column densities of nonthermal electrons in the two FPs. Only two positive correlations appear in group 1, between the difference of the absolute polarization degrees in the two FPs and the relative ratio of magnetic field strengths in the two FPs and between the logarithm of the ratio of the column densities of nonthermal electrons in the two FPs and the logarithm of the ratio of the brightness temperatures in the two FPs. These four statistics in group 2 are just opposite to those in group 1, which may be directly explained by gyrosynchrotron theory. Moreover, the asymmetry of the two FP emissions in group 2 is more evident than that in group 1, which may be explained by two kinds of flare models, respectively, in the two groups of events, i.e., the energy release above a single loop (group 1) and in the coalescence of two flaring loops (group 2). The asymmetry of the two FP emissions cannot simply be explained by the mirroring effect, but it may strongly depend on the asymmetry of the initial pitch angles of nonthermal electrons in the two FPs. The normal or abnormal events are defined as whether a stronger emission corresponds to a stronger or weaker polarization in the two FPs, with a comparable proportion of the normal and abnormal events in the two groups.

FREQUENCY DEPENDENCE OF THE POWER-LAW INDEX OF SOLAR RADIO BURSTS

We process solar flare observations of Nobeyama Radio Polarimeters with an improved maximum likelihood method developed recently by Clauset et al. The method accurately extracts power-law behaviors of the peak fluxes in 486 radio bursts at six frequencies (1-35 GHz) and shows an excellent performance in this study. The power-law indices on 1-35 GHz given by this study vary around 1.74-1.87, which is consistent with earlier statistics in different solar cycles and very close to the simulations of the avalanche model by Lu.

The calculation of coronal magnetic field and density of nonthermal electrons in the 2003 October 27 microwave burst

Based on Dulk and Marshs approximate theory about nonthermal gyrosynchrotron radiation, one simple impulsive microwave burst with a loop-like structure is selected for radio diagnostics of the coronal magnetic field and column density of nonthermal electrons, which are calculated from the brightness temperature, polarization degree, and spectral index, as well as the turnover frequency, observed by using the Nobeyama Radioheliograph and the Nobeyama Radio Polarimeters, respectively. Very strong variations (up to one or two orders of magnitude) of the calculated transverse and longitudinal magnetic fields with respect to the line-of-sight, as well as the calculated electron column density, appear in the looptop and footpoint sources during the burst. The absolute magnitude and varied range of the transverse magnetic field are evidently larger than those of the longitudinal magnetic field. The time evolution of the transverse magnetic field is always anti-correlated with that of the longitudinal magnetic field, but positively correlated with that of the electron column density. These results strongly support the idea that quantifying the energy released in a flare depends on a reconstruction of the coronal magnetic field, especially for the transverse magnetic field, and they are basically consistent with the recent theoretical and observational studies on the photospheric magnetic field in solar flares.

Attenuation of coronal magnetic fields in solar microwave bursts

Based on the observed data by the Nobeyama Radio Observatory and the nonthermal gyrosynchrotron theory, the calculated magnetic field in a loop-like radio source of the 2001 October 23 flare attenuates from hundreds to tens of Gauss,. except in the region with very weak magnetic fields. Meanwhile, the viewing angle between the magnetic field and line of sight has a similar attenuation from tens to around ten degrees, implying that the transverse magnetic component attenuates much faster than the longitudinal one. All of these results can be understood by the magnetic energy release process in solar flares. Moreover, the column density of nonthermal electrons decreases from 109-10 to 107-8 cm(-2) during the flare, except in the region with very weak magnetic fields, where its value is larger than that with strong magnetic fields due to the mirroring effect. The calculated error and harmonic number of gyrofrequency better suit the region with strong magnetic fields.

The contracting and unshearing motion of flare loops in the X 7.1 flare on 2005 January 20 during its rising phase

With the aim of studying the relationship between the relative motions of the loop-top (LT) source and footpoints (FPs) during the rising phase of solar flares, we give a detailed analysis of the X7.1 class flare that occurred on 2005 January 20. The flare was clearly observed by RHESSI, showing a distinct X-ray flaring loop with a bright LT source and two well-defined hard X-ray (HXR) FPs. In particular, we correct the projection effect for the positions of the FPs and magnetic polarity inversion line. We find that: (1) The LT source showed an obvious U-shaped trajectory. The source of the higher energy LT shows a faster downward/upward speed. (2) The evolution of FPs was temporally correlated with that of the LT source. The converging/separating motion of FPs corresponds to the downward/upward motion of the LT source. (3) The initial flare shear of this event is found to be nearly 50 degrees, and it has a fluctuating decrease throughout the contraction phase as well as the expansion phase. (4) Four peaks of the time profile of the unshearing rate are found to be temporally correlated with peaks in the HXR emission flux. This flare supports the overall contraction picture of flares: a descending motion of the LT source, in addition to converging and unshearing motion of FPs. All results indicate that the magnetic field was very highly sheared before the onset of the flare.

Progress of site survey for large solar telescopes in western China

Excellent sites are necessary for developing and installing ground-based large telescopes. For very-high-resolution solar observations, it had been unclear whether there exist good candidate sites in the west areas in China, including the Tibetan Plateau and the Pamirs Plateau. The project of solar site survey for the next-generation large solar telescopes, i.e., the Chinese Giant Solar Telescope (CGST) and the large coronagraph, has been launched since 2011. Based on the close collaboration among Chinese solar society and the scientists from NSO, HAO and other institutes, we have successfully developed the standard instruments for solar site survey and applied them to more than 50 different sites distributed in Xinjiang, Tibet, Qinghai, Sichuan, Yunnan and Ningxia provinces. We have built two long-term monitoring sites in Tibet and the large Shangri-La to take systematic site data. Clear evidence, including the key parameters of seeing factor, sky brightness and water vapor content, has indicated that a few potential sites in the large Tibetan areas should obtain the excellent astronomical conditions for our purpose to develop CGST and large coronagraph. We introduce the fresh site survey results in this report.

Statistical study of co-rotating interaction region properties with STEREO and ACE observations

We analyzed the data on co-rotating interaction regions (CIRs) measured by the A dvanced Composition Explorer (ACE) and Solar TErrestrial RElations Observatory (STEREO) from 2007 to 2010. The CIRs were observed by STEREO B (STB) , ACE and STEREO A (STA) one after another, and a total of 28 CIRs were identified in this work. Since the same characteristics of CIRs were detected by these three spacecraft at three different locations and times, these data can help us to study the evolutions of CIRs. For a single event, the properties of CIRs observed by the three spacecraft were quite different and could be explained by spatial or temporal variations. For all these 28 CIRs, STA and STB observed similar mean parameters, such as peak magnetic field strength (offset 11%), peak and change in solar wind speed (offset 3% and 10% respectively), peak proton temperature (offset 14%) and peak perpendicular pressure (offset 15%). Surprisingly, STA detected much higher (41%) peak density of protons than STB .