Electron cyclotron maser (ECM) instability is today’s most favoured process for microwave spike emission. Although ECM looks attractive, the emission mechanism is still unclear due to the imperfection of present theories, inadequacy of observational data, and uncertainty as to conditions in the source region. To find solar active phenomena, both statistically and individually coincident with radio spike events, is essential for understanding the ambient conditions needed for generating spike emission, locating sites where spike emission is produced, and providing clues for particle acceleration and energy release in flares.

In this paper, evidence for an association between spike emission at 21 cm and fast variation of the magnetic configuration in post-flare loops is presented. Such associations may be helpful for solving the questions mentioned above.

AR 5395 was the super-active region in Solar cycle 22. In this active region, there was rapid relative motion of magnetic elements, growth and decay of umbrae, frequent newly emerging magnetic flux and various mass motions.

Data include daily vector magnetograms (Huairou Observing Station, Beijing Observatory, Hα filtergrams and photographs of sunspot group with fine structures (Yunnan Obs. and Purple Mountain Obs.) on March 9 - 15, 1989. In this period 42 flares have been observed by Yunnan and Purple Mountain Obs. (table 1). Observational data showed that all the 42 flares occurred in four segments (marked as A,B,C,D) of U-shaped neutral line in this active region (Fig. 1).

This paper presents a study of 9 “spotless” flares of H-alpha importance 1 or more which occurred in the years 1981-1990. The flares occurred in positions where no active region has been reported. All the flares were found to occur in evident and well-localized plages, large and diffusive plages as well as in regions where no plage was observed. Only one of all the flares studied in the paper was reported to be accompanied by the filament activity.

During the period of high solar activity (1988-1990) the Sayan and Baikal Observatories of the ISTP carried out observations of the chromosphere in the H-alpha line, the photosphere and of sunspot magnetic fields; at the Siberian Solar Radio Telescope, solar radio emission observations at 5.2 cm wavelength were made. Using those observations a study is made of the evolution of “centers of magnetic activity” (Bumba, 1986) in nine active regions with different flare productivity, for which the most complete observations were available. In three cases (NOAA Nos. 5229, 5643 and 5669) they were the targets of the cooperative international Max’91 program.

We observed a quiescent prominence on the east limb of the Sun, on June 7, 1988 (Mein P. et al 1990) and a quiescent filament (N 5, W 5) on June 17, 1986 (Schmieder et al 1991). These two observations are made with the Multichannel Subtractive Double Pass spectrograph of the turret dome refractor of the Pic du Midi Observatory, providing simultaneous 2D pictures in 9 channels for Hα.

We examine the solar activity indexes (relative intensity of sunspots and faculae) of an active region #83137 during 1983 June 21–26. The results are compared with daily variation of magnetic fields of the region to find a characteristic of solar activity variation in the descending phase of solar cycle.

A pre-eruption disturbance of an active filament has been observed. The disturbance is characterized by untwisting rotation and vigorous upward motion. It starts at the site where microflares took place, and appears in the late phase of magnetic flux cancellation beneath the central part of the filament, when more than 1020 Mx flux had disappeared.

In this paper, we propose a mechanism of solar flare based on the 3-D nonlinear mode coupling of resistive tearing instability. The results show that the nonlinear coupling of tearing modes leads the rapid destabilization of some high modes. Furthermore, tearing mode turbulence is formed and anomalous resistivity is produced, which in turn, quickens the development of tearing instability and accelerate the magnetic reconnection process. It is suggested that the fast magnetic reconnection as a mechanism of solar flare may be associated with this self-excited process caused by the nonlinear mode coupling of tearing instability in the solar corona. Using our model, we successfully explain all the main typical characters of the flares of February 4 and 6, 1986.

In this paper, the evolution history of longitudinal magnetic field of a decaying active region (NOAA AR6703) was studied. We also described the X1.9/4B flare occurring in AR6703, as well as the accompaning events, and emphasized in analysizing the features of longitudinal magnetic fields before the flare.

High resolution spectra are being taken in ionized calcium K-line at different latitudes and integrated over the visible 180 degree longitude. These have been analysed to study the variability of the line parameters as a function of latitude. These spectra are being obtained on a regular basis and large data base will be used to study chromospheric rotation, differential rotation in chromosphere, activity and variability of K-line parameters as function of latitude and solar cycle phase. Further, an optical arrangement has been worked out to obtain the spectra along the latitudes of the solar image in the light integrated over longitudes. This involves the unidirectionally focusing of the image in N-S direction.

To study the evolution and decay of large convective cells known as supergranules, a solar telescope was set up at Maitri, Indian permanent station in Antarctica region during the local summer months (December 1989 through March 1990). A continuous sequence of calcium-K-line filtergrams for 106 hours with an interval of about 10 minute was obtained. The analysis of this data indicates that most probable life time of the calcium K network is about 20 hours. The life time depends upon the size of the cell and is larger for bigger cells. The data also show that cells (of given size) associated with remanent magnetic field regions live longer than those in the region free from magnetic field regions.

Magnetic shear is sometimes defined observationally as the rotation of the photospheric vector magnetic field away from the field direction of a corresponding potential state. It is often assumed that the observational shear is a direct measure of the energy stored in coronal currents. However, this is not true in general as is illustrated here by considering the changes in observational shear in a variety of flare models. Some of these models predict rotations in the photospheric transverse field which are too small to be detected by present day instruments, but other models predict relatively large rotations in highly localized regions. In principle, observations of the transverse field might confirm or eliminate models in the latter group.

(1) Current techniques may now permit an acceptable resolution of the 180 degree ambiguity in determining the direction of transverse fields. (2) The non-potentiality of active regions can be described by parameters of angular shear, source field, line-of-sight current, force-free factor and force-free current. A 2-dimensional description of angular shear is necessary, and an alternative, FFT method of calculating the line-of-sight current seems recommendable. (3) For a sampling region, the degree of non-potentiality quantified by all the parameters shows great enhancement right before a 1M flare, and reduces somehow during the flare. (4) A highly intermittent distribution of force-free factor with fully mixed signs is obtained, consistent with a theoretical approach.

In solar active regions, over extended periods of time the plasma-magnetic field configuration evolves quasistatically through a sequence of nearly force-free equilibrium states. This evolution may be understood as the continual distortion of an existing equilibrium by wavelike disturbances propagating upward from the photosphere and subsequent fast relaxation to a new, neighbouring equilibrium. In the present paper the build-up of magnetic energy, which is presumably necessary for flares and other explosive events, during a quasistatic evolution is considered. If during the slow evolution the magnetic energy is increased, then the relaxation processes represent inverse cascades of energy. We study the conditions under which such cascades are possible within the framework of mean-field MHD. In contrast to the convection zone, where the dynamo for the global magnetic field of the Sun works, the solar atmosphere is convectively stable and the first order smoothing approximation justified. It turns out then that current helicity (B.∇ × B) is an important quantity decisive for whether magnetic energy can be built up.

In order to determine the sense of the photospheric transverse magnetic field measured with a magnetograph, it is shown that: (1) If the magnetic lines of force through the photospheric plane are both divergent and convex upward, Krall’s criterion

is true for the transverse field B⊥ much more strongly than for the longitudinal field Bz and the inequality in general becomes

(2) If the magnetic lines of force are only divergent upward, we have instead the inequality

for a force-free field.

The nonlinear evolution of tearing modes in the presence of sheared mass flow is studied in the cylindrical geometry. It is demonstrated that a sufficient large sheared mass flow can destabilize the development of resistive tearing instability. It is suggested that the coupling of sheared mass flow with shear magnetic field may be a triggering of solar flares.

It is well accepted that the source of energy for flares arises in the non-potential magnetic field from the dynamic evolution of active regions above the photosphere. So far, however, contradictory results have been suggested for the question of whether the occurrence of flares results in detectable changes of the magnetic fields and electric currents in the regions (Svestka, 1976)

The early researches of Severny’s group showed that the longitudinal and transverse magnetic fields in active regions change evidently after flares, which are characterized by the simplification of magnetic configuration as well as the decrease of magnetic strength and gradient (Severny, 1962, 1969). Then, after studying the magnetograms of two active regions measured by the Kitt Peak’s magnetograph, Harvey suggested that there were changes in longitudinal magnetic fields with a time scale of hours, but they could be attributed to the global evolution of active regions and not directly related to the occurrence of flares (Harvey et al., 1970). Moreover, from the analyses of the Kitt Peak’s magnetograms for the big flare of 3B importance on 4 August 1972, Livingston indicated that the longitudinal magnetic fields before and after the flare remained unchanged (Livingston, 1973). On the other hand, from the study of the magnetograms obtained by the Big Bear Observatory’s video magnetograph before and after a 2B flare in the region McMath 13225 on 10 September 1970, Tanaka reached the conclusion that in weak field areas of less than 100G, the longitudinal field changed about 30–100% at the onset of the flare, and this change was associated with the flare (Tanaka, 1978). In recent years, some authors have investigated in detail the magnetograms of the well known region AR2372 on 6 April 1980 obtained with the video magnetograph of the Marshall Space Flight Center (Krall et al., 1982; Hagyard, 1984; Ding et al., 1985; Lin and Gaizauskas, 1987). Their results showed that the change of the magnetic fields occurred before a 1B/X2 flare. Therefore we have a rather confused picture about the variation of magnetic fields associated with flares.

Two dimensional, three components compressible MHD equations are solved with multistep implicit scheme. The spontaneous reconnection of current sheets with force free field background is simulated. In comparison with the case where the background is potential field, a great difference is found. Our conclusion is that the force free magnetic field can accelerate reconnection. It may have important scientific significance to solar physics, for force free field is widely present in solar atmosphere.

The two-ribbon flare of May 13, 1981 is one of the most notable flares during solar cycle 21. There are rich observational data of this flare. We study carefully the characteristics about the evolution and motion of the filament related with the flare as well as the relationship between these characteristics and the development of the flare. The results indicate that the increase of the filament current plays an important role in the occurrence and development of the flare.

In this paper, based on observational information we proposed an opening bipolar magnetic field model and studied numerically the dynamic evolutionary processes of the magnetic fields by solving a complete set of MHD equations. The aim is to discuss one of the mechanisms of energy build up of solar flares produced in the active region Boulder 5395.