Zhang Hong-qi

Structures of chromospheric magnetic fields in the solar flare producing active region 5747

In this paper, the formation and the measurement of the Hβ line in chromospheric magnetic fields are discussed. The evolution of the chromospheric magnetic structures and the relation with the photospheric vector magnetic fields and chromospheric velocity fields in the flare producing active region AR 5747 are also demonstrated.The chromospheric magnetic gulfs and islands of opposite polarity relative to the photospheric field are found in the flare-producing region. This probably reflects the complication of the magnetic force lines above the photosphere in the active region. The evolution of the chromospheric magnetic structures in the active region is caused by the emergence of magnetic flux from the sub-atmosphere or the shear motion of photospheric magnetic fields. The filaments separate the opposite polarities of the chromospheric magnetic field, but only roughly those of the photospheric field. The filaments also mark the inversion lines of the chromospheric Doppler velocity field which are caused by the relative motion of the main magnetic poles of opposite polarities in the active region under discussion.

Diagnosis of solar chromospheric magnetic field

This paper discusses the measurements of the chromospheric magnetic field and the spatial configuration of the field at the lower solar atmosphere inferred by the distribution of the solar photospheric and chromospheric magnetic fields. Some questions in the study of the chromospheric magnetic field are also presented.

Discussion on the progress in research on helicity in solar physics in China

The Sun is the nearest star to the Earth. The Sun not only affects the environment of interplanetary space but, as a unique large-scale natural plasma laboratory, also helps us understand the basic process of electromagnetic interaction in the universe. Helicity is a basic parameter used in describing the properties of vector fields. It is important in understanding the origin of the solar magnetic field and the release mechanism of solar magnetic energy, which are key subjects in solar physics. In this paper, we discuss the basic concepts of helicity (including magnetic helicity, current helicity, and kinematic helicity) and major advances in helicity research conducted by Chinese solar physicists. We also inquire into the possibility of new open questions in the helicity research of solar physics.

Distribution of magnetic shear angle at the ascending phase of cycle 23

Using the vector magnetograms observed at Huairou Solar Observing Station of National Astronomical Observatories, the magnetic shear angles of solar active regions at the ascending phase of cycle 23 (1996–2000) were calculated. It is found that the statistical distribution of the magnetic shear angles can be fitted well by Gaussian curves. And the dominant sign of the magnetic shear angles is negative (positive) in the northern (southern) hemisphere. It is consistent with the N-S sign asymmetry of force-free field constant α and current helicity.

Observation of the chromospheric magnetic field of quiet-sun regions

Abstract Using the solar magnetic field telescope at Huairou Station of Beijing Astronomical Observatory, the solar photospheric and chromospheric magnetic fields in quiet-sun regions were observed for the first time. From a study of deep magnetograms of the solar photosphere, chromosphere and different level of the chromosphere, it is found that the network magnetic tubes do not diverge greatly from the photosphere to the chromosphere, contrary to the predictions of the popular canopy model. It is also found that some intranetwork magnetic tubes can reach the chromosphere, which is again contrary to the predication of the dynamo model.

A solar flare in the Fe I 5324 line on 24 June, 1993

A solar flare with both Hβ and Fe i 5324 emissions was observed in AR 7529 (S13, E65) on 24 June, 1993 at the Bejing Astronomical Observatory. Our calculations show that the Fe i 5324 emission region of the flare was located in the low photosphere at a height of about 180 km above τ5000 = 1, which is lower than many previous studies of white-light flares. To study a Fe i 5324 flare, which represents a kind of extreme case in solar flares, would be useful for clarifying some arguments in the researches of white-light flares as well as for understanding the mechanism of solar flares.The synthetic analyses from vairous features of the flare lead to the following possible exciting mechanism of the Fe i 5324 flare: owing to the flow of energetic electrons from the corona and probably also the thermal conduction downward into the lower atmosphere, a condensation with a temperature higher than that below it was formed near the transition region. Then the low photosphere was heated through backwarming. The Fe i 5324 flare occurred as an indicator of the excitation in the low photosphere.

Variation of Magnetic Fields and Electric Currents Associated with a Solar Flare

The high-resolution vector magnetograms obtained with the solar telescope magnetograph of the Beijing Astronomical Observatory of the active region AR 4862 on 7 October, 1987, close before and after a solar flare, were used to calculate the electric current densities in the region. Then the relations between the flare and the magnetic fields as well as the electric currents were studied. The results are: (i) the transverse magnetic fields, and hence the longitudinal electric currents in the region before and after the flare, are evidently different, while the longitudinal magnetic fields remain unchanged; (ii) this confirms the result obtained previously that the flare kernels coincide with the peaks of longitudinal electric density in active regions; (iii) the close relation between the flare kernels and the electric currents indicates that the variations of the transverse magnetic fields and the longitudinal electric currents arise not from the general global evolution of the active region, but from the flare. These results tend to the conclusion that the triggering of a solar flare might be related with the plasma instability caused by the surplus longitudinal electric currents at some local regions in the solar atmosphere.

Numerical solution of magnetic flux emergence in gravity-stratified solar atmosphere

Abstract Numerical MHD simulation in Lagrangian scheme is made of the dynamical evolution of the solar magnetic bipolar field in a gravity-stratified atmosphere when a parallel or an antiparallel dipole emerges from the subphotosphere. It is shown that the emergence will lead to converging and descending motion of the plasma, and that a pressure has to be added for the dipole to float. The floating results in the formation of a current sheet in the interface between the old and the new fields. The sheet may bring about explosive phenomena such as the Ellerman bombs. The computation reveals that the gas at the top of the arch is rising with a small velocity while the gas at the feet is falling with a large velocity. Similar to the Brunt-Vaisala oscillation, this peculiar property stems from the nonhomogeniety of the physical parameters in the gravity-stratified atmosphere. Also, we simulate usual solar active region by making a strong magnetic dipole of 1500 G float into a weak background field of 100 G. It is indicated that a diminished pressure should be added on the boundary in order to avoid strong shocks. The emerging process should last at least several hours. After emergence a strong current sheet is formed in the chromosphere which, on arriving in the lower corona, will become a potential source of solar flares, as envisaged by Heyvaerts and Priest. Finally, the results of computation will be used to inpterpret the magnetic emerging flux data obtained at Huairou Station, given in [1] and [2].

An analysis of the formation in magnetic field and the formation depth of seven solar photospheric FeI lines. (I)

Abstract To meet the calibration demands of the multi-channel solar magnetic telescope, we took the VAL quiet Sun atmosphere model and calculated the Stokes profiles of seven FeI photospheric lines, their formation depths, and contribution functions, thereby obtaining a more systematic picture of the features and properties of the abnormal and normal Zeeman lines and an explanation of the appearance of sharp peaks in the formation depth curves of the Stokes parameters Q and U near the line centre.

Observations of the magnetic and velocity fields of two solar flares

Abstract After we have installed a CCD imaging system on our solar magnetic telescope, we have obtained many two-dimensional, real time, fine-detailed magnetic and velocity field pictures. In this paper we report on the two sunspot groups assoicated with two flares. Based on our analysis we point out that mutual penetration of opposite polarity regions exists generally and bright cores of flares all occur at front of mutual squeezing between such regions. This provides a strong support for the squeezing force-free field model of flares. We also found that, in regions where flares occur, the direction of the flow field is downward.