Hinotori observed 720 flares through its operation February 1981-October 1981. General discussions of the results were given in two symposia: the Hinotori Symposium (ISAS 1982) and the U.S.-Japan Seminar (de Jager & Švestka 1983). The hard x-ray imaging made at the effective energy 20-35 keV showed a wide variety of morphology. Many flares (22 out of 30 events) showed single source structure, either compact (12) or extended (10) in the spatial resolution of 15 arc sec (Takakura et al. 1983a, Ohki et al. 1983, Takakura 1984). Evidences are given in some limb events that the main source is located in the high corona (1-4 x 104 km) (Takakura et al. 1983b). The extended single source could be the whole coronal loops which may include footpoints, but the maximum brightness is near the loop top. In some events (8 out of 30), weak subsource(s) which sould be identified with the footpoint(s) appear intermittently (Tsuneta et al. 1983). Takakura et al. (1984) found that the extended single source becomes compact and slightly shifts to higher altitudes in later phases of the impulsive burst (5 out of 10 events). Tanaka (1984), Takakura (1984), and Tanaka S Zirin (1984) argued that the hard x-ray morphology of the impulsive burst is consistent with the non-thermal electron beam model in high density corona. Sakurai (1983) investigated magnetic field structures of the hard x-ray sources based on the potential field calculations. Timing between the hard x-ray and microwave in the impulsive bursts was examined by Takakura et al. (1983c), who found correlated subsecond time structures and also by Takakura et al. (1983d), who found a long (5-10 s) delay of the peaks at 17 GHz and E > 300 keV to the peak at E < 100 keV. Kurokawa (1983) showed detailed coincidence between the hard x-ray spikes and Hα brightenings.

Several proceedings of scientific meetings on sunspots appeared during the 1981-1984 period [The Physics of Sunsots, Cram and Thomas (eds.) 1981; see also reports of regional meetings, e.g.. Third European Solar Meeting, Oxford 1981; Nordic Astronomy Meeting, O. Hauge (ed.), Oslo 1983; 11th Regional Consultation on Solar Physics, L. Dezsö and B. Kalman (eds.), Debrecen 1983]. New interest in sunspots was aroused through observations of EUV sunspot spectra from space and was also inspired by the growing number of observations of starspots and other stellar activities [IAU Symposium No. 102, Solar and Stellar Magnetic Fields: Origin and Coronal Effects, J.O. Stenflo (ed.) 1983; Colloquium IAU No. 71 Activity in Red Dwarf Stars, Catania 1982]. Other reasons for the increased interest in sunspots and their energetics were prompted by the correlation between sunspot occurrence and the variations of the solar constant (Hudson et al. 1982) and by the use of sunspot positions for determining solar differential rotation and its change with latitude, depth, and time (Howard et al. 1984, Godoli S Mazzucconi 1982, Balthasar et al. 1984, Tuominen & Kyrolainen 1982, Adam 1983, Koch 1984).

The past 3 years since the last solar maximum have witnessed an unprecedented number (>200) of published scientific papers on many aspects of solar radiophysics. These contributions are the result of an intense research effort mounted during the first Solar Maximum Mission of 1980 and continued until the present. Excellent x-ray, EUV, and visible light observations of the disturbed corona and transition region have been obtained from the SMM, Hinotori, P78-1, and ISEE-3 spacecraft. ISEE-3 also has provided very low-frequency radio observations of solar bursts in the interplanetary medium. Ground-based radio support for space experiments has been provided by many observatories throughout the world. In particular, many collaborative studies using x-ray and radio observations of solar flares have been reported. The outstanding radio instrument during this period has been the VLA, operating at 2, 6, and 20 cm with a time resolution of 10 s and both modes of circular polarization. The two-dimensional spatial resolution of the radio images is a few seconds of arc, almost as good as the best resolution obtained so far at any wavelength in the solar spectrum. To complement the rather poor time resolution of the VLA one-dimensional arrays such as the WSRT at 6 cm wavelength and the Nobeyama interferometer at 17 GHz have been used successfully. In addition, a number of very-high-time resolution radiometers have been built at different locations. At meter wavelengths two-dimensional arrays at Clark Lake, Culgoora, and Nancay and a one-dimensional array at Nobeyama have been employed. The little known region of the solar spectrum at decimeter wavelengths is being investigated by the Zurich spectrograph. It is pleasing to see Chinese participation in solar radiophysics.

The corona above active regions is now recognized as an assemblage of magnetically confined loops of plasma. This advance in understanding the active upper atmosphere is documented in the monograph resulting from the Third Skylab Workshop (Solar Active Regions, 1981, ed., F.Q. Orrall). International collaborative programs during the Solar Maximum Year (SMY) have further stimulated the study of active regions with emphasis on the search for the underlying causes of solar flares. Scores of analyses of individual regions, combining space- and ground-based observations, have been published. We have as a result an improved picture of interactions between active regions: from creation of shear in the magnetic topology to inter-region connections via the corona. A revived interest in the phenomena of recurrent active regions and sunspot decay has highlighted a basic problem for solar magnetism: the removal of magnetic flux from the solar surface. The interpretation of temporary dips in the solar irradiance caused by active regions continues to generate lively debate.

Magnetohydrodynamic (MHD) theory for the initiation and development of solar flares has developed considerably over the past 3 years and represents one of the liveliest areas of solar physics (Hood & Priest, 1981a, Priest 1983a, b, Schindler 1982, Van Hoven 1982, Syrovatskii et al. 1983). This has been stimulated by a thorough analysis of the Skylab observations and also by the startling new observations from the Solar Maximum Mission (SMM). In addition, the realization that flares appear to form two basic types, namely, small simple-loop flares and large two-ribbon flares, has focussed the imagination of theorists (e.g.. Priest 1981, 1982), even though reality may be somewhat more complex. In the former type, a single-loop structure brightens up and decays without moving; whereas in the latter, an active region filament erupts and then two ribbons of chromospheric emission form and separate, with an arcade of hot and cool loops joining them.

The scope of Commission 12 has broadened somewhat in recent years, to include not only the structure of the solar atmosphere, but that of the solar interior as well. The scientific purview of this commission, and of the present report, are complementary to those of Commission 10 (solar activity). Rather than attempting to review all progress in solar structure studies over the past triennium, this report deals with six topics of great current interest, in which there is a great deal of current work.

Spectrocopy is the classical diagnostic tool of astrophysics. Intensities and line shapes of well identified emission and/or absorption atomic and molecular features are used to provide information on species concentrations, and degree of excitation, from which gas kinetic, rotational, vibrational, electronic and excitation “temperatures” can be inferred when LTE conditions exist. Departures from LTE can also be determined spectroscopically. Diagnostic interpretation of spectra in optically thin circumstances is fairly straightforward. However, in optically thick conditions when the photon mean free path is very much less than the geometrical path, the emission spectrum is controlled by the absorption coefficient (Armstrong and Nicholls, 1972), (see equation 4a).

Developments in the measurements of oscillator strengths, that are of current and future interest to astrophysics, have taken place in the past few years:

1. – there is increased emphasis on methods which are independent of level populations (i.e., of number densities and thus equilibrium in the vapour or plasma under investigation),

2. – clear progress has been made towards higher precision (and accuracy) and

3. – the dynamic range of astrophysically relevant oscillator strengths was further extended towards weaker lines.

The study of solar rotation using the Doppler effect in solar spectrum lines continues to be an active and fruitful area of research. Duvall (1982) has confirmed a slower rotation rate for the photosphere than for sunspots, which bears on an earlier controversy regarding this point between the Stanford Solar Observatory and some other observatories. The issue is not settled, however.

During the period 1981-1984, major developments have been made in the analysis of the shape and shifts of photospheric lines used as a diagnostic of the convection in the outer layers of the Sun. The first three-dimensional numerical simulation of the solar granulation has been developed, and it, is able to reproduce with some success most of the observed morphological and spectroscopic properties of granulation. It is becoming clear that the properties of the solar granulation are variable over the solar cycle. Many questions remain to be solved; further progress will rely heavily on expected improvements of the spatial resolution of the observations and the availability of larger computers.

In many low-charge ions of astrophysically abundant light elements, the spins of the ground and first-excited terms are different. Because of spin-orbit interactions, the states of these terms are mixtures of LS basis states, and as a consequence, transitions with ∆S = 1, i.e., ‘intersystem transitions’, can occur. The_transition probabilities (A-values) for such lines in low-Z ions are about 104 sec -1 to 102 sec-1. These values are of the same order of magnitude as the collisional de-excitation rates in many low-density astronomical objects. Consequently, intersystem lines, which are not readily seen in the laboratory, are significant features in many astronomical spectra. IUE spectra that show such lines in novae, hot stars, cool stars, symbiotic stars, binary stars, variable stars, Herbig-Haro objects, H II regions, planetary nebulae, quasars, and galaxies are discussed in Kondo et al. (1982). Moreover, because of the commensurate radiative and collisional de-excitation rates for the upper levels, the ratios of intersystem-line intensities to allowed-line intensities are density sensitive in many objects.

One of the more interesting aspects of the chromosphere-corona transition region is its tendency to exhibit large Doppler shifts. Both the non-thermal velocity component of line widths and the velocity displacement of line positions tend to maximize at temperatures near 105 K. The increase in velocity amplitudes with increasing temperatures below 105 K is readily understood in terms of the increasing sound speed and decreasing densities associated with the outwardly increasing temperature. Why the observed velocity amplitudes should decrease at still higher temperatures is not at all clear, however, and it seems very likely that this phenomenon is indicative of fundamental differences in the dynamics of the upper transition region and corona from those in the lower transition region and chromosphere.

Dielectronic recombination (DR) is an electron-ion process particularly effective in high temperature plasmas such as those observed in the Solar Corona, Supernovae remnants in fusion plasmas (Tokamak and laser produced plasmas). This process is a resonant capture process of projectile electrons by a target ion as one of the target electron is excited, thereby forming an intermediate autionising state which can decay radiatively to a singly excited state. DR plays an important role on the establishment of ionisation equilibrium in the plasma and is also responsible for spectral lines appearing as satellites of the resonance lines of the target ion. The analysis and interpretation of such satellite lines in terms of plasma diagnostics has been widely used in soft X-ray spectroscopy during the last decade, and has given reliable estimates of the physical parameters of the plasma (electron and ion temperatures and densities). In the case of H-like and He-like resonance lines, high resolution spectra have been obtained in Tokamak for Z = 14 - 28 and most of the satellites have been clearly identified. To help the reader to go further we give some references of solar studies 2, 3, 4, 0, Tokamak 6, 7, laser plasma 8, 9.

Recent texts dealing with background aspects of this topic include Priest (1982), Noyes (1982) and Giovanelli (1984). Here we concentrate on the equilibrium structure of coronal loops, their oscillations and instabilities, and their heating.