We describe some new aspects of the observed relationship between the longitudinal magnetic field (Blong) and the Doppler velocity (VD) and propose an explanation of the effect in terms of a two-component penumbra model.

In regards to the spatial associations of flares to velocity fields, Martres and Soru-Escaut (1977) found that flares occurred in the vicinity of the site where the magnetic neutral lines B∥=0 and the velocity neutral lines V∥=0 lines were crossing in the photosphere; Harvey and Harvey (1976) revealed that flares were related to velocity patterns different from Evershed flows. Recently Ai et al. (1991) discovered that flares occurred on the red-shift side of the velocity inversion lines (V∥=0) of the Hβ Dopplergrams observed half to two hours before flares. In this paper we further examined the associations with high resolution data obtained by Solar Magnetic Field Telescope at Huairou. A reconnection model applied to the magnetosphere is adopted here to tentatively interpret some of the observational results.

The active region Boulder # 5395 (N34, L257), which appeared on the disk in March 1989, is one of the biggest active regions in fifty years. Study on the structure and dynamical characteristics of the region can help to understand the physics of solar flares. Many authors have studied morphology and sunspot motion of the region (e.g. Wang et al. 1991, and Zhao 1990); the magnetic emergence and shear; the relationship between the extrusion and the flares and the characteristics of the magnetic and velocity fields in the flare sites (Chen 1990; Li et al. 1990; Zhang et al. 1990). It comes into question that most of the regions display the features mentioned above but only a few of them produce such a high activity as AR5395 does. In other word, AR5395 must possess some particular features that are probably related to its high activity. Our attempt is to find what the special features are.

Modeling techniques for solar magnetic fields are discussed. As an extension of the source surface model for large scale solar magnetic fields, a method based on the Green’s function approach to the Laplace equation is proposed. In the force-free field modeling, a proper definition of the boundary condition is an important but still unresolved problem. A condition based on the continuity of the Maxwell stress is newly proposed.

It has been recognized that the magnetic flux observed on the solar surface appears first in low latitudes, and then this flux is gradually dispersed by super granular convective motions and meridional circulation. Theoretically, the magnetic flux transport could be explained by the interactions between magnetic fields and plasma flows on the solar surface through the theory of magnetohydrodynamics.

To understand this physical scenario, a quasi-three-dimensional, time-dependent, MHD model with differential rotation, meridional flow and effective diffusion as well as cyclonic turbulence effects is developed. Numerical experiments are presented for the study of Bipolar Magnetic Regions (BMRs). When the MHD effects are ignored, our model produced the classical results (Leighton, Astrophys. J., 146, 1547, 1964). The full model’s numerical results demonstrate that the interaction between magnetic fields and plasma flow (i.e., MHD effects), observed together with differential rotation and meridional flow, gives rise to the observed complexity of the evolution of BMRs.

The sizes of solar active regions were quantitatively studied using Debrecen Heliographic Results 1977 data. The size-total area dependence was also examined. The size of a region was defined as the distance between the area-weighted mean positions of p- and f-polarity subgroups at the time when the total (umbral + penumbral) area of the spot group is at its maximum. Excluding the groups for which this occurred on the invisible hemisphere and other dubious cases, 68 active regions were left in the present one-year sample. Despite the smallness of this sample, the average size of the regions was found to be 58 400 km with a relatively low error of 3000km (though the individual regions show a considerable scatter in size). It is proposed that the toroidal magnetic flux tubes lie in a sufficiently subadiabatic layer to be linearly stable and they are only destabilized by finite-amplitude convective disturbances that lift parts of them into the unstable layers. In such circumstances the typical size will be determined by the horizontal correlation length of the finite disturbances, thereby explaining the observed size.

Active region coronal magnetic fields are expected to be in a twisted tangled state due to photospheric convective motions. These motions can drive the magnetic field to a statistically steady state where energy is released impulsively (Lu and Hamilton 1991). These relaxation events in the magnetic field can be interpreted as avalanches of many small reconnection events. We argue that the frequency distribution of these magnetic reconnection avalanches must be a power law. Furthermore, we calculate the expected distributions in a simple model of magnetic energy release events in a 3-dimensional complex magnetized plasma, and compare these to the distributions of solar flares. These distributions are found to match the observed power law distributions of solar flare energies, peak fluxes, and durations. This model implies that the energy-release process is fundamentally the same for flares of all sizes. Observational predictions of this model are discussed.

The monochromatic images of unipolar sunspots in the Stokes parameters Q and U of magneto-sensitive lines display a complicated structure. This is caused by the magneto-optical effect and also connected with the 3-D structure of spot magnetic fields. In the process of numerical simulation it is possible to check the regularities of the change of the angle of inclination of magnetic lines of force with distance from spot center.

Thermal stability of a current sheet is investigated when the magnetic field is not perfectly anti-parallel at the sheet. If the effect of Hall electric current is taken into account, a new instability (the Hall instability) appears. The result is applied to the activation of dark filaments often seen prior to solar flares. The growth time of the instability is evaluated and found to be consistent with the observed time scale of the filament activation.

Two simple examples are presented to show that concepts about the physical nature of sunspot groups may significantly influence the statistical data analysis process. In particular, the second example shows that the well-known difference in the decay rates of preceding (p-) and following (f-) polarity parts of sunspot groups may lead to a fake proper motion effect when area-weighted group positions are used. This effect may be responsible for some recent contradictory findings concerning the motions of sunspot groups. It is therefore argued that while area-weighting is adequate when calculating the mean positions of p- and f-parts of a sunspot group separately, defining the position of the group as a whole by the unweighted average of the mean positions of the p- and f-parts is more satisfactory from the theoretical point of view (whenever it is possible to distinguish between spots of different polarities). Similarly, it is best not to “correct” sunspot proper motions for internal differential rotation within groups.

In this paper a 3-d formulation is applied to extrapolate the solar magnetic field from the measured magnetogram directly, and no additional data treatment is needed. The validity of the proposed formulation is demonstrated by comparison between calculated and analytical or observed results of some magnetic force-free field problems.

A generalization of Jefferies et al’s weak-field approximation for the inference of the strength and polar angle of magnetic field vectors from Stokes profiles is obtained while the inference of azimuthal angle follows a relationship derived by Landi Degl’Innocenti’s perturbative solutions to the transfer equations in which the magneto-optical effect is taken account. It is found that the weak-field condition (ΔλH / ΔλD ≤1) is not necessary for the new method when the fitting is done in the line wings.

Explosive events are the earliest indicators of flare activity and potentially predict the imminent occurrence of a flare at a specific location. They are highly energetic small-scale phenomena which are frequently detected throughout the quiet and active sun. The observations show that explosive events are related to emerging magnetic flux and tend to occur on the edges of high photospheric magnentic field regions. The cancellation of photospheric magnetic flux are the manifestation of explosive events, so that they are identified as the magnetic reconnection of flux elements. We assume that emerging flux are convected to the network boundaries with the typical velocity of intranetwork elements. Two-dimension (2D) compressible MHD simulations are performed to explore the reconnection process between emerging intranework flux and network field. The numerical results clearly show the cancellation of magnetic flux and the acceleration of the plasma flow.

Numerical simulations of solar prominence formation are presented employing photospheric horizontal motions as boundary conditions. Three different combinations of magnetic field configurations and footpoint motions are considered: (1) a bipolar arcade with a footpoint shear, (2) an arcade exposed to a shearing and converging motion, (3) two adjacent arcades undergoing a shearing and converging motion. In each case, it is found that the dynamic evolution of magnetic fields can force the plasma into a thermal instability leading to the formation of a prominence.

The presently widely accepted view that the solar dynamo operates near the base of the convective zone makes it difficult to relate the magnetic fields observed in the solar atmosphere to the fields in the dynamo layer. The large amount of observational data concerning photospheric magnetic fields could in principle be used to impose constraints on dynamo theory, but in order to infer these constraints the above mentioned “missing link” between the dynamo and surface fields should be found. This paper proposes such a link by modeling the passive vertical transport of thin magnetic flux tubes through the convective zone.

The historical development of optical instruments for solar physics is outlined, from white light to unpolarized and polarized monochromatic light, to Stokes profiles and simultaneous fields of view, from points to lines, plane to cube. An evolutionary series and classificaton of instruments for the solar magnetic field is described. As a next step the 2-D real time polarizing spectrograph has been proposed. The planned instruments in China for measurements of solar magnetic and velocity fields are briefly introduced.

The techniques and instrumentation for inferring magnetic fields using a spectrometer or spectrograph rather than a narrowband filter are reviewed. With array detectors and/or Fourier transform spectroscopy, the polarimetric data is acquired over the relevant spectral range strictly simultaneously with no possibility of spatial misregistration. Several recent examples of spectrometer-based magnetographs are discussed and compared, and new observations from the NASA/NSO Spectromagnetograph of magnetic flux, velocity field, continuum intensity, equivalent width, and line depth in active regions and the whole sun are shown.

The Solar Flare Telescope was constructed at Mitaka in 1989. This instrument comprises four telescopes which respectively observe (a) Hα images, (b) continuum images, (c) vector magnetic fields, and (d) velocity fields in the photosphere. The instrument aims at the study of energy build-up and energy release in solar flares, in cooperation with the Solar-A satellite. The whole system has been in regular operation since 1992 July. The methods of measuring the magnetic and velocity fields are described.

The THEMIS telescope will provide high resolution observations of the solar vector magnetic field. We present some capabilities of this instrument, concerning the polarization analysis, the correlation tracker and the two spectrographs. THEMIS is expected to come into operation in 1995–1996.

The Advanced Stokes Polarimeter is a new instrument dedicated to quantitative measurement of vector magnetic fields in lines formed at several heights in the solar atmosphere at high angular resolution. We present results for a small, symmetric sunspot observed under good seeing conditions near disk center on 25 March 1992. The narrow penumbral “spines” of more vertical field reported by Degenhardt and Wiehr (1991) and Title et al. (1992) are clearly seen in these observations. These spines are characterized by higher field strength than the surrounding penumbra. The inferred field azimuth indicates that these spines flare out with height, as would be expected of stronger field intrusions within the background penumbral field. The observations indicate the penumbral field “canopy” extends well beyond the outer edge of the penumbra. Outside of the sunspot, the small flux elements of mixed polarity are nearly vertical in orientation.