This Symposium was held at Surfer's Paradise, Queensland, Australia, from 7 to 11 September 1973. The Organizing Committee, chaired by J. P. Wild, consisted of A. Boischot, A. Bruzek, J. T. Jefferies, G. Newkirk, T. Takakura, and V. V. Zheleznyakov. We are indebted to the Local Organizing Commettee, chaired by S. F. Smerd and including R. G. Giovanelli, R. E. Loughhead, N. G. Seddon, K. V. Sheridan, and J. P. Wild, for advice in preparing this volume as well as for the smooth arrangement of the sessions. In addition, the session chairmen and reporters are to be thanked for their assistance in preparing the recorded discussions. It is a pleasure to thank Mrs R. Toevs and Mr A. Csoeke-Poeckh of High Altitude Observatory for assistance in editing these Proceedings. The financial aid for the Symposium afforded by the International Astronomical Union, the Ian Potter Foundation of Melbourne, and the Sunshine Foundation of Melbourne, as well as generous assistance of the CSIRO Divisions of Physics and Radiophysics is gratefully acknowledged.

Coronal disturbances are considered as consequences of the ejection of electric currents (or non-potential magnetic fields) from the photosphere and chromosphere into the corona. It may be that electric currents are generated near neutral lines in the photosphere and are later ejected into the corona.

Due to the strong coupling between the coronal material and magnetic fields, magnetic structures are probably intimately involved, either actively or passively, in virtually all coronal disturbances. The purpose of this paper is to put into perspective the various roles magnetic fields can play in both exciting and guiding these observed transient phenomena. A discussion of our present theoretical concepts relevant to this subject can be conveniently divided into four categories.

We begin by discussing our present understanding of how the gross magnetic structure of the corona is determined. Important considerations here are the tendency for the coronal field to seek its lowest energy state, the effect of convection on the field, and the influence of the solar wind. Secondly, we investigate magnetic structures which reside in elevated energy states (higher than the energy of an equivalent potential field) as well as disturbances which appear to be related to changes in these configurations. Thirdly, the role of the field in guiding coronal disturbances is considered. This is evident for bulk motions (sprays, surges, flare loops, green line events, etc.), wave motions (flare associated waves, Alfvén waves), as well as for individual particle phenomena. Lastly, a special class of magnetic structures which seem to be constantly associated with coronal activity are discussed. These are the magnetic discontinuities such as neutral sheets and current sheets. In this context, the magnetic neutral point and associated reconnection phenomena are considered.

(Solar Phys.). Two results were obtained recently at Meudon concerning the metric type III production by flares which both emphasize the importance of the local magnetic configuration involved.

In the first part of our communication, in a short summary of our recent results, it is demonstrated that over the last 15 years (the time interval for which the magnetic synoptic charts are available) the largest solar activity, usually connected with proton-flare occurrence, has been very closely related to a characteristic large-scale pattern in the magnetic field distribution with a life-time of the order of 8–10 solar rotations. These regular features are seen in the negative as well as in the positive polarity, although they can be seen better in the negative polarity where their forms are more pronounced, regardless of the activity cycle. The form alternates with the location in the northern or the southern solar hemispheres due to the mutual relations of both polarities, individual active regions, the influence of the differential rotations, the shift in longitude, etc. This pattern could be observed up to the large August 1972 proton-flare activity.

The large-scale magnetic patterns described are accompanied by characteristic large-scale features in the green (λ 5303) coronal emission, presented in the form of isophotes on the synoptic charts (reduced to a unified photometric scale).

In the second part of the presentation, preliminary results, concerning the correlation of the longitudinal distribution of the green coronal emission with the negative and the positive polarity fields for two time intervals (August 1960–September 1961 and January 1969–December 1969) are described. The existence of two ‘coronal active longitudes’ in both intervals, as well as the close relation of these longitudinal emission maxima to certain parts of the large-scale characteristic bodies of negative polarity, is discussed. Also, the existence of one heliographic longitude, connected with ‘coronal holes’ (minimal green corona emission), and its relation to the positive polarity large-scale pattern are proved.

(Solar Phys.). The emission of circularly polarized radiation of reversed senses from the respective solar hemispheres at λ = 4.3 cm is discussed, referring to consistent eclipse results obtained in November 1966 (Figure 1), March 1970, and January 1973 (Figure 2). Further qualitative information were obtained by performing a series of 18 daily solar maps (Figure 3) at the same wavelength, with moderate angular resolution (i.e., 12′). We were not able to resolve individual centers' polarities, but have confirmed that reversed senses of circular polarization occur, tending to be right-handed at southern areas of the solar disk, and left-handed at northern areas. These senses correspond to preceding polarities at the hemispheres for the 20th solar cycle, and a ‘neutral line’ divide the two polarized hemispheres, showing a contour that depends strongly on the presence of active centres and on their displacement with solar rotation. This suggest that the polarized features are strongly controlled, if not completely, by the presence of photospheric underlying magnetic areas.

The calculation of coronal magnetic fields was first suggested by Gold (1958). Altschuler and Newkirk (1969) and Newkirk et al. (1968) used a Legendre polynomial fit to the photospheric observations of magnetic fields whereas Schatten (1968) with Wilcox and Ness (Schatten et al., 1969) use a magnetic monopole fit, first incorporated by Schmidt (1964).

(Monthly Notices Roy. Astron. Soc.). Rocket observations of the EUV solar spectrum obtained during the total eclipse of 7 March 1970 showed the presence of a large coronal condensation on the NE limb (Gabriel et al., 1971). The condensation shows the existence of loop structure, which defines the local magnetic fields, and is apparent in forbidden lines including those of Si viii, Si ix, S xi, Fe ix, Fe xi and Fe xii which lie between 1200 Å and 2050 Å (Jordan, 1971). These lines are formed in the temperature range 9.3 × 105K to 2.0 × 106K. The photographic observations of the active region show clearly that the spatial distribution of material varies considerably with its temperature.

(Solar Phys.). An exceptional variety of positions and polarizations was found for two type I storms and numerous sporadic bursts observed during 15 consecutive days with the Culgoora radio-heliograph at 80 and 160 MHz.

Recent observations of impulsive hard X-ray, microwave, EUV and optical emissions during solar flares are briefly reviewed in order to deduce the characteristics of the impulsive (flash) phase phenomenon in small solar flares particularly from the point of view of the acceleration of electrons and their role in producing the various impulsive phase emissions. Observed and deduced characteristics of the various electromagnetic emission sources are summarized (Table II). The deduced characteristics of the electron acceleration process (Table III) indicate a process with high acceleration efficiency. The observations are found to be consistent with a model in which electrons are accelerated in a series of short pulses each lasting for ≲ 1 s and the accelerated electrons provide the energy necessary for all the observed electromagnetic emissions produced during the flash phase of small solar flares. Models of the impulsive phase emissions in which energetic electrons play a prominant role are examined and crucial tests to check the accuracy of these models are indicated (Table IV).

(Solar Phys.). Microwave burst data from the August 7, 1972 event recorded on nine discrete frequencies between 245 and 35000 MHz at the Sagamore Hill Radio Observatory (Figure 1) provide a basis for correlation studies (especially timing information) of the associated white light flare, the high energy particle emission, type II bursts, and many other phenomena. This is perhaps the first time that sufficient radio coverage (i.e., data above 10000 MHz) was available to obtain the spectral slope information (related to electron-energy distribution) which is inherent in this part of the spectrum. Heretofore, timing was related to burst flux density profile variations. Improved correlations resulted from shorter centimeter wavelength data which supplied more accurate timing information than that derived from Hα observations. The shape and intensity of the burst peak flux density spectrum has also been used for qualitative analysis of energetic particle and white light events. The ultimate good may possibly come from spectral analysis of the minute by minute variation of the burst microwave radiation spectral slope α (above fmax) in the area between 15000 and 35000 MHz. This may be used alone or in relation to the position of fmax, where fmax is the frequency of burst maximum emission at a given time. This is basically our present investigation.

(Solar Phys.). The relationship between Hα absorption features, type III radio bursts and soft X-ray emission has been examined in order to determine the characteristics of the particle acceleration process operating when a Hα-flare may or may not be detectable. The Hα observations were made by Meudon Observatory with a Hα telescope fitted with a 0.75 Å band pass Lyot filter. During a 10 s period, three pictures were obtained – one at the Hα line center, one at Hα + 0.75 Å and one at Hα −0.75 Å. This sequence of three pictures was repeated every one minute. Each picture covered a rectangular area 18 × 24 mm2, the diameter of the complete solar image being 38 mm on this scale. In addition, Meudon Hα films of the whole solar disc were also used. The X-ray observations were made with the University of California (Berkeley) experiment aboard the OGO-5 satellite and the NRL experiment aboard Solrad-9. The wavelength range covered was 0.5–20 Å. The type III radio data was obtained from two sources: The 169 MHz radio-heliograph at Nancay which provided east–west position of the radio burst on the Sun with an accuracy of ~ 1′ and the radio spectra measured by various ground based observatories. The findings are as follows:

Transient Hα activity observed in the absence of reported flares is associated with production of type III radio and soft X-ray emission. Since such optical phenomena are much more frequent than flares themselves, we conclude that instabilities generating fast particles may be produced in the corona in a quasi-continuous way with coincident perturbations in the lower solar atmosphere.

The soft X-ray component is not necessarily the direct product of fast particles, but is probably associated with some type of heating since both the soft X-ray emission and the Hα features exhibit a comparable evolution. The type III bursts, when they are produced, occur near the maximum of this perturbation.

We identify the transient Hα activity (emission or absorption) with the existence of a metastable situation which may or may not lead to the triggering of a flare.

Some examples are presented of fast variations of the structure and the intensity of coronal condensations. For the observations the lines λ 5303 and λ 5694 have been used. Most of the observed phenomena are flare-connected, but all of them are related to some kind of Hα-activity. Sudden brightening of the emission lines may occur after the disappearance of a filament. The events have a duration of up to several hours and consist of hot clouds of high density that are slowly rising from the upper chromosphere into the lower corona.

Plasma diagnostics of the active corona require data on transition probabilities and collision cross sections for relevant coronal emission lines. Atomic parameters pertaining to a number of coronal transitions have been derived as follows:

Multiple hard X-radiation, along with associated emission at optical and radio wavelengths, is discussed for three events in particular: December 13, 1970–1832 UT; December 12, 1970–1843 UT; June 28, 1970–2001 UT. Characteristics of these events as well as other multiple hard X-ray bursts observed by the author (e.g., Vorpahl, J.: Solar Phys.29, 447, 1973) are given at the end. Data originated from hard X-ray experiments on the OGO-5 (Kinsey Anderson) and OSO-5 (Ken Frost) satellites and were compared so that the effect of pulse pile-up in the OGO-5 data could be observed. The two December flares occurred in different active regions separated by about 100000 km yet both produced hard X-ray bursts with similar structure. This suggests that the two regions were joined by extended field lines with the acceleration mechanism somewhere in between. Although nothing was visible in Hα (such as surges or post flare ejections) that would indicate connecting field lines, higher coronal structures could explain the similarity in hard X-rays from two separate regions.

Observations over the past 20 yrs or so are reviewed, with emphasis on recent high spatial resolution observations. The results lend support to earlier ideas on the propagation of type III electron streams through coronal regions of weak magnetic field strength but have not as yet settled the question whether the electrons propagate along the axes of coronal streamers. Several important burst properties appear to be significantly affected by ray scattering on small-scale size density irregularities in the corona.

Properties of solar radio bursts observed by spacecraft at frequencies below several MHz are reviewed. In this frequency range most of the observed bursts are type III events (associated with particles) but several cases of type II emission (associated with shocks) have been reported. The analyses which lead to emission levels of type III solar bursts out to beyond 1 AU from the Sun also indicate that the low frequency radiation is observed at the harmonic of the emission region plasma frequency. Simultaneous particle and radio measurements imply that the bursts are generated near the leading edge of impulsive streams of solar electrons with energies extending from several hundred keV to several keV. Recent experiments measuring the direction of arrival of the radio emission allow the exciter particles to be tracked along the interplanetary magnetic field from regions near the Sun out to 1 AU.

Satellite observations of solar electrons bearing on flare particle acceleration and the generation of radio and X-ray emission are reviewed. The observations support a two stage acceleration process for electrons, one stage commonly occurring at the flare flash phase and accelerating electrons up to ~ 100 keV, and a second stage occurring only in large proton flares and accelerating electrons up to relativistic energies. The location of the acceleration region appears to be no lower than the lower corona.

The accelerated non-relativistic electrons generate type III radio burst emission as they escape from the Sun. Direct spacecraft observations of the type III emission generated near 1 AU and the energetic electrons, provide quantitative information on the characteristics of the electrons exciting type III emission, the production of plasma waves, and the conversion from plasma waves to electromagnetic radiation.