We have tried to find empirical evidence for the role of photospheric motions in the building up of the flare productive magnetic patterns in Active Regions.

The bright Hα faculae are associated with V∥ structures different from a classical Evershed flow and particularly ‘anomalous’ in the regions and periods of high flare occurrence. The flares observed occurred at ‘crossings’ of the lines V∥ = 0(V ≠ 0) and H∥ = 0 and at places where V∥ = 0 showed abrupt changes of direction. It is suggested that these anomalous V∥ structures are evidence of vortex motions.

On the basis of Hα pictures of two proton flares of medium size, the volume of the separate parts of magnetic-channel flares, are given. At the time of the main release of energy the volume of the whole system is determined at ∼ 2.5 × 1027 cm3 and the size of areas III after Petschek model at ∼ 7.0 × 1020 cm3.

The phenomenon of dashes in the eruptive loop prominences is found to represent the part of the solar flare process which includes all the levels of the Sun's atmosphere. For 75% of observations a coincidence of dashes and flares in space and time was discovered. The rest of the dash observations may be interpreted as the excitation of dashes by the flares arising outside the limb.

Following the solar flare theory of Alfvén and Carlquist, the possibility to interpret the dashes as the eruptive instability of the pinch-effect is discussed. The computation of flare-current magnetic field gives the value H ∼ 3·104 oersted, its order being in agreement with the observed value.

This review is concerned with the origin of the fine structure of the fields and their relationship to the heating of the solar chromosphere and corona, the structure of prominences and the production of energetic particles in solar flares. The dynamics of sunspot formation, and the large-scale structure of individual sunspots have not been dealt with, although the evolution of AR fields has been considered insofar as it affects the flare problem.

The fine structure features of the umbral magnetic fields (i.e. large field gradients and changes of polarity in local regions) are considered as evidence of strong non-linear interactions between magnetic and thermal or mechanical forces in the umbra. It is suggested that the umbral dots observed in white light are the optical manifestation of these interactions. A three-dimensional radiative transfer analysis of possible models for these bright features is discussed and this enables one to place limits on the geometry of these features and on the non-radiative energy requirements of the models. Of the several models considered those which were compatible with convection as the source of this energy were found to be quite inconsistent with the available data. The most likely model was found to have a diameter of 200 km, a height of 50 km and an average emission of non-radiative energy of 4 × 103 erg s−1/cm−3 throughout this region. It is shown that this is two orders of magnitude greater than the energy available from Joule heating by locally twisted magnetic fields. However, if the energy flux transported through the umbra by Alfvén waves is partially dissipated in regions of locally twisted fields it is shown that the emission into a volume of the above dimensions is of the right order of magnitude.

A possible statistical model of the solar magnetoplasma in solar active regions consisting of a totality of small-scale current vortex plasma elements (‘subgranules’) is discussed. Some results are given which naturally follow from such a model: the small value of effective conductivity in the macro-structures of the plasma (hence, the possibility of a greater mobility and changeability of field and plasma for a short time), the formation of filamentary-structural elements, the oscillating regime of the magnetoplasma in an active region. A principal scheme is given of the experiment on a solar magnetograph with very high resolution, which is able to reveal discrete fields of the subgranules. The dispersion equation is derived for macro-magnetic oscillations of the statistical ensemble of magnetized subgranules. The possibility is noted to reveal macro-magnetic oscillations and waves during observations of low-frequency modulation in the solar radio-emission. It is shown that at solar radioburst propagation in the corona, (according to the model under consideration, some regularly located plasma inhomogeneities should be present in the active region) Bragg's diffraction takes place. Typical properties of complex radiobursts may be received (extreme narrowness of the band, short lifetime, directivity).

The dynamical evolution of a weak, random, magnetic excitation in a turbulent electrically-conducting fluid is examined under varying kinematic conditions. It is found that the results of an earlier paper (Kraichnan and Nagarajan, 1967) can be reliably extended to a stage of evolution wherein the magnetic spectrum has reached local equipartition with the velocity. The transfer of the magnetic energy to smaller wavenumbers (larger scales) is considerable and significant. This result is highly pertinent to the turbulent dynamo question, which has been variously investigated recently. The relevance of the coupling of the rms magnetic field to the magnetic modes of all scales in deciding the efficiency of this transfer is discussed.

At the beginning the problem of constructing the three-dimensional magnetohydrostatic models of the photospheric layers in sunspots is discussed in some detail. It is pointed out that the construction of such models by solving the set of equations of magnetohydrostatics cannot be effectively carried out.

In order to solve the difficulties a suitable method of determining the distribution of the magnetic force in sunspots from measurements of the magnetic field has been worked out. Tentative results of the computations are presented.

General features of the distribution of the magnetic force in the photospheric layers of stable sunspots are discussed. It is pointed out that significant magnetic forces are necessary in the penumbra; they secure its transversal equilibrium, but are rather unimportant for its vertical structure. And it is quite probable that the magnetic field in the umbras of stable spots is nearly potential or force-free down to the photospheric level.

A filamentary magnetic structure is produced on the plasma current sheath of a coaxial accelerator operated with deuterium. Space and time analysis of X-ray, neutron and visible-light emission indicates that the magnetic energy of the filaments is transferred to the plasma during a process of decay of the filaments. X-ray photographs show very localized regions (diameter <0.5 mm) of strong emission. Some of these regions are also located where the plasma is not subject to a maximum of compression. Similar bright spots (Hβ) are observed by 5 ns image converter photographs. The detailed analytical description of the self-consistent fields is deduced. The localized regions of strong emission may well correspond with the explosive onset of an instability at a point on a filament (single filament decay) or at a point where two filaments with opposite fields coalesce with magnetic field annihilation. The similarities with solar flares are considered.

The optical resolution in recent eclipse photographs is now sufficient to illustrate clearly many qualitative features of gas-magnetic field interactions in the solar corona. Close to the Sun where the field is strong, the coronal gas over certain regions of the solar surface can be contained within closed loop structures. However, since the field strength in these regions declines outward rapidly, the pressure and inertial forces of the solar wind eventually dominate and distend the field outward into interplanetary space. The overall configuration is determined by a complex interplay of inertial, pressure, gravitational, and magnetic forces. The present work is oriented toward the understanding of this interaction.

Of central importance to this general problem is the typical ‘helmet’ streamer generally overlying a large diffuse bipolar region on the solar surface. A helmet streamer consists of closed magnetic loops near the Sun and open field lines adjacent to and above the loops. The configuration possesses a ‘cusp-type’ neutral point (to be differentiated from a y-type neutral point) at the top of the closed loops. In addition to volume currents, a single sheet current separates the regions of opposite polarity above the neutral point. Below the neutral point, sheet currents also are present between the open and closed regions. These sheet currents are necessary to support the discontinuity in gas pressure which arises since the open region is expanding and the closed region is not. The neutral point problem is discussed in some detail and sample numerical examples are presented.

It is demonstrated that in principle the magnetic field may originate in the fine structure elements of the photosphere and the sunspots as a result of lack of coincidence of isobaric and isothermic surfaces in those elements.

The Joule dissipation of magnetic fields in the highest photosphere and spot layers can be considerably accelerated by the anisotropy of electric conductivity.

The general properties of large scale solar magnetic fields are reviewed. In order of size these are: (1) Active region, generally bipolar fields with a lifetime of about two solar rotations. These are characterized by fields of several hundred G and display differential rotation similar to that found for the photosphere. (2) UM regions which appear to be the remnants of active region fields dispersed by the action of supergranulation convection and distorted by differential rotation. These are characterized by fields of a few tens of gauss and have lifetimes of several solar rotations. (3) The polar fields which are built up over the solar cycle by the preferential migration of a given polarity towards the poles. The poloidal fields are of a few gauss in magnitude and reverse sign in about 22 yr. (4) The large scale sector fields. These appear closely related to the interplanetary sector structure, cover tens of degrees in longitude, and stretch across the equator with the same polarity. This pattern endures for periods of up to a year or more, is not distorted by differential rotation, and has a rotation period of about 27 days. The presence of these long enduring sector fields may be related to the phenomenon of active solar longitudes. The consequences of large scale fields are examined with particular emphasis on the effects displayed by the corona. Calculated magnetic field patterns in the corona are compared with the density structure of the corona with the conclusion that: (1) Small scale structures in the corona, such as rays, arches, and loops, reflect the shape of the field and appear as magnetic tubes of force preferentially filled with more coronal plasma than the background. (2) Coronal density enhancements appear over plages where the field strength and presumably the mechanical energy transport into the corona are higher than normal. (3) Coronal streamers form above the ‘neutral line’ between extended UM regions of opposite polarity. The role played by coronal magnetic fields in transient events is also discussed. Some examples are: (1) The location of Proton Flares in open, diverging configurations of the field. (2) The expulsion of ‘magnetic bottles’ into the interplanetary medium by solar flares. (3) The relation of Type IV radio bursts to the ambient field configuration. (4) The guiding of Type II burst exciters by the ambient magnetic field. (5) The magnetic connection between widely separated active regions which display correlated radio bursts.

We have made detailed comparisons of coronal structure, as photographed at λ5303 Å, with magnetic lines of force, as computed from measurements of the longitudinal component of the underlying photospheric magnetic fields. Coronal fields were computed under the assumption that the space above the active regions is current-free. Out of 36 regions for which there exist both magnetic and coronal data at the Sacramento Peak Observatory, we found only four suitable for analysis. Using a light-pen attachment to a digital computer, we were able to choose lines of force whose footpoints in the photosphere best matched those of the lines of force which were suggested by local intensity variations in the corona. The lines of force thus computed give excellent agreement with the apparent height and shape of coronal fieldlines. In particular the magnetic field in the loop structures of 2 November 1969 and 18–19 November 1968 falls from several hundred gauss at 10000–20000 km above the photosphere to 2–20 G at heights of 100000–150000 km. These computed values are in general agreement with other measurements. Spectra of the loops reveal that the direction of the line-of-sight component of the motion agrees with that anticipated from a comparison of the computed fieldline orientation and the observed motion in the plane of observation. We conclude that coronal magnetic fields above slowly changing active regions are nearly the same as the vacuum potential fields derived from underlying photospheric field sources (principally the larger sunspots in a region) and that calculations of this type may be relied upon to give the magnetic fields for studies of coronal dynamics.

We present polarization measurements obtained in 1970 in the green coronal line with a new coronameter located at the Pic du Midi. The analysis of these data has been conducted with the theory given by the writer in 1964 and 1965. It appears that magnetic field orientations in the Corona can be deduced from the above measurements. First results showing large scale magnetic structures are presented.

The six years of data from the Mt. Wilson Magnetic Atlas were analyzed in terms of surface harmonics. Between 1959 and 1962 the dominant harmonic corresponded to a dipole lying in the plane of the equator (2 sectors). There was also a significant zonal harmonic in which both solar poles had the same magnetic polarity, opposite to that at the equator. From the end of 1962 through 1964, the harmonic corresponding to 4 sectors was dominant. In 1965 and 1966, the harmonic of the north-south dipole became significant.

The structure of the magnetic field of the active solar corona is discussed with reference to optical and radio observations of the solar atmosphere. Eclipse observations provide evidence of fine scale structures in the solar atmosphere that appear to relate to the coronal magnetic field. The coronal magnetic field used for comparison is that field calculated from potential theory: the influence of solar activity upon the potential theory field is discussed with reference to observations of the Faraday rotation of a microwave signal from Pioneer 6 as it was occulted by the solar atmosphere. Evidence has been found suggesting the existence of expanding magnetic bottles located at 10 R⊙ above flaring active regions. The dynamics of these events is discussed. It is further suggested that these magnetic bottles are an important component in the solar corona.

It is possible to deduce information concerning large scale coronal magnetic field patterns from the knowledge of the location of radioburst sources.

As the method concerns active centers responsible for corpuscular emission, the knowledge of these structures may have important implications in the understanding of corpuscular propagation in the corona and in the interplanetary medium.

The characteristics of 12 moving type IV bursts observed with the 80 MHz radioheliograph at the Culgoora Observatory between February 1968 and April 1970 are summarized.

Three classes of moving sources can be recognized; they are described as: (1) Expanding arch; (2) Advancing front; (3) Isolated source.

The first class has been identified (Wild, 1969) with the expansion of a magnetic arch or loop; the second class is here identified with an advancing MHD disturbance which may accelerate the radiating electrons in situ when moving at greater than Alfvén speed; the third with solar ejecta in the form of magnetized plasma clouds, or plasmoids. In all cases the radiation mechanism is probably synchrotron radiation from mildly relativistic electrons; energies in the range ∼0.1 to ∼1 MeV could account for the observed strong circular polarizations.

With an expanding magnetic arch, source and magnetic-field movement are inseparable; the field remains a closed loop throughout the event. The MHD front probably moves largely along and the plasmoids between the open magnetic-field lines of unipolar regions or helmet structures. In the latter case it is the internal magnetic field – possibly toroidal – of the moving plasmoid that determines the polarization of the synchrotron radiation. A preliminary comparison of moving type IV sources with Newkirk-Altschuler maps of coronal magnetic fields shows suitably located closed loops for 2 events identified as expanding magnetic arches and unipolar open field lines along the path of a moving source identified as a plasmoid.

Some properties of solar active regions at 9 and 3.5 mm wavelengths are discussed. The regions have excess brightness temperatures of up to 1000 and 700K at 9 and 3.5 mm wavelengths respectively. The background radiation at 3.5 mm is often seen to be ‘absorbed’ in regions closely coincident with Hα dark filaments on the disk. Interpretation of this ‘absorption’ as due to the large optical thickness of the overlying filamentary material leads to an estimate of electron density in the filaments. The active regions at millimeter wavelengths show almost one-to-one correspondence with the Ca-plage regions as well as with the regions of longitudinal magnetic fields on Mt. Wilson magnetograms. A comparison of the mm-λ maps with the magnetograms ‘smoothed’ with the beams of mm observations shows this correspondence in a striking manner. This relationship suggests the possibility of measuring chromospheric magnetic fields from the measurement of polarization at millimeter wavelengths.