A classic filament disruption/coronal transient event ocurred on 10 January 1974. After the prominence liftoff, “gradual” x-rays were recorded by Solrad 9. A white light coronal ejection, interpreted as a loop seen edge-on, followed. During the mass outflow, Hα loops formed at the original site of the prominence. The loops appeared also in EUV spectroheliograms, and rose rapidly before vanishing abruptly. During the disintegration of the loops the apices showed great enhancements and vertical spike structures. The overall behavior of this loop prominence system is compatible with reconnection models.

Observations from SKYLAB have shown that coronal transients, which involve mass ejections occur quite frequently, possibly up to three times a day at solar maximum (Hildner et al., 1976). An estimated mass of ≃1015 − 1016 g (Stewart et al., 1974) and a total mechanical energy in excess of 2×1031 ergs (Webb et al., 1978) is expelled from the Sun during each event. The transients therefore play a major role in the dynamics of the outer corona and of the interplanetary medium. Joint radio and white light observations provide the best opportunity to derive the physical parameters, such as the electron density and magnetic field in different parts of the transients, and consequently to estimate the forces driving the ejecta.

Solar radio bursts of spectral type II provide a prime diagnostic for the passage of shock waves, generated by solar flares, through the solar corona. In this investigation we have compared radio data on the shocks with computer simulations for the propagation of fast-mode MHD shocks through the solar corona. The radio data were recorded at the Harvard Radio Astronomy Station, Fort Davis, Texas. The computer simulations were carried out at NOAA, Boulder, Colorado.

Twenty type II solar radio bursts were observed during the period 1968 to 1972 by a solar radio spectroscope (240-40 MHz) at Ahmadebad. Intensity variations in type II bursts as a function of frequency and time are sometimes observed in their dynamic spectra. This fine structure enables determination of the shock thickness of the order of a few hundred to a few thousand kilometers. In a few cases, an interaction between streams of fast electrons and propagating shocks is clearly evidenced by simultaneous observations of short duration narrow band structures in type III bursts and type II bursts.

A paper that has considerable relevance of the subject matter of this symposium is the following: “Evidence for Extreme Divergence of Open Field Lines from Solar Active Regions,” by G. A. Dulk (Division of Radiophysics, CSIRO, Sydney, Australia and Department of Astro-Geophysics, University of Colorado, Boulder, Colorado), D. B. Melrose (Department of Theoretical Physics, University of Sydney, Australia) and S. Suzuki (Division of Radiophysics, CSIRO, Sydney, Australia).

If one defines coronal transients as events which occur in the solar corona on rapid time scales (≲ several hours) then one would have to include a large variety of solar phenomena: flares, sprays, erupting prominences, X-ray transients, white light transients, etc. Here we shall focus our attention on the latter two phenomena; solar flares have been discussed at great length in a recent Skylab workshop and IAU Colloqium No. 44 was devoted to the study of prominences. Coronal transients, in the narrower sense, were first seen with the instruments on board of Skylab, both in the optical and the X-ray part of the spectrum.

Physical arguments are given indicating that solar flare magnetic energy storage may happen through force-free fields with helical symmetry (∂z + λ (r) ∂ɸ = o). The mathematical results turn out simple for helical fields whether general, in equilibrium or force-free. A preliminary stability analysis points to appropriate properties.

It has been a customary assumption that any force-free magnetic field represents an equilibrium field in the solar atmosphere under the extreme condition 8πp/B2 ≪ 1. An example of a force-free magnetic field is presented for which this assumption fails in the sense that no equilibrium is possible for the magnetic field if imposed with an arbitrary ambient pressure, however weak the pressure is. A simple mechanism is proposed for the onset of eruption in the course of otherwise quasi-static evolution of magnetic fields in the solar atmosphere.

Now it is known that the solar corona consists of many loops which are believed to represent the structure of the magnetic field. Since the plasma is very tenuous in the corona, the equilibrium of the magnetic field is approximated by the force-free field:

Filaments and prominences are classical examples of condensations of nongravitational origin, their formation being due to the thermal instability. The physics of this instability in a uniform plasma at coronal temperatures, which exhibits increasing radiative output as it cools, is well known (Field, 1965). Filaments, however, form in regions of sheared magnetic fields, as evidenced by their location above photospheric polarity-inversion lines and their occurrence after a period of increasing fibril inclination (Tandberg-Hanssen, 1974). Physically, the importance of the field structure is readily understood if one recalls the capability of magnetic field of strongly collimating the local heat conduction. Thus, the preferred locations for the development of the thermal instability will be those where the field configuration inhibits the stabilizing effects of thermal conduction on the growing temperature perturbation.

We describe a model for first stage electron acceleration based on quasi-linear interaction with Langmuir waves. The acceleration takes place in a MHD-unstable plasma region (length L, volume V) that contains many microscopically unstable current layers, which act as (quasi-stationary) sources of Langmuir waves, Figure 1. Our work does not depend on the details of the generation mechanism for Langmuir waves, nor on the details of the MHD instability. For the latter, one could think of multiple tearing instabilities.

A mechanism for slow and enduring mass release well after the flare onset, which is inferred from the enduring enhancement in the interplanetary mass flux after flare activity, is discussed in the modified scheme of a “neutral sheet” model of flares. According to a possible new way of looking at the neutral sheet model as proposed by Uchida and Sakurai, it is argued that a slow and enduring mass release in the later phase of a flare may come from the reconnection in the region of interleaved opposite polarity fields produced in the interchange collapse which is proposed to be responsible for the highly dynamical phase at the start of a flare. Heated plasma, which is magnetically disconnected form the anchoring field in the photosphere through reconnection in this interleaved region, is continuously produced during the thermal phase, and is expelled from the flare region by the “buoyancy” in the gradient of the surrounding high magnetic pressure. The trajectory of this mass leakage will be along the steepest gradient of magnetic pressure, rather than along the field lines as expected in other picture, and this process can supply mass to the solar wind in open field regions.

This paper reports on the application of a siphon flow model to the late stage of a surge observed in the UV radiation by the S055 experiment on board Skylab. The MgX λ625 and OVI λ1032 emissions from the density distributions occurring in flows which become supersonic at the top of the loop agree with the observations, indicating a pressure drop, along the loop, of a factor of 4 for the plasma emitting the OVI line, and of a factor of 2 for the plasma emitting the MgX line.

We report on a comprehensive numerical simulation of flare dynamics, encompassing the corona, transition region and chromosphere. A coronal loop geometry, whose magnetic pressure dominates gas pressure, is assumed. These preliminary results assume optically thin radiation; we are currently incorporating optically thick radiative transfer in the chromosphere. We discuss difficulties in modelling the transition region under flare conditions, and suggest tentative solutions.

The association of coronal transients with two-ribbon solar flares is well established. During the Skylab period, every two-ribbon flare when observed close enough to the limb was accompanied by a coronal transient. Flares do not occur with all transients, however many of these transients are associated with soft X-ray enhancements in the corona similar to but less energetic than the intense X-ray loops that occur with two-ribbon flares [cf. MacCombie and Rust (1979) for a review]. The eruption of a filament seems to be the ingredient common to all these events - more so than flares. For these reasons, we consider this class of phenomena, regardless of whether a flare occurs or not, to be exhibiting a common physical process. To produce chromospheric emission requires a substantial amount of energy. Hence, one should expect chromospheric flares to be associated with only the most energetic phenomena. Nevertheless, the most comprehensive observations covering a wide range of wavelengths (Hα, EUV, X-ray, radio, white light) are available for the large two-ribbon flares, and the study of these events sheds the most light on the mechanism which produces coronal transients.

The acceleration of solar cosmic rays in association with certain solar flares is known to be highly correlated with the propagation of an MHD shock through the solar corona (Svestka, 1976). The spatial structure of the sources of solar cosmic rays will be determined by those regions of the corona which are accessible to the flare-induced shock. The regions to which the flare shock is permitted to propagate are determined by the large scale magnetic field structure in the corona. McIntosh (1972, 1979) has demonstrated that quiescent filaments form a single continuous feature (a “baseball stitch”) around the surface of the sun. It is known that helmet streamers overlie quiescent filaments (Pneuman, 1975), and these helmet streamers contain large magnetic neutral sheets which are oriented essentially radially. Hence the magnetic field structure in the low solar corona is characterized by a large-scale radial neutral sheet which weaves around the entire sun following the “baseball stitch”. There is therefore a high probability that as a shock propagates away from a flare, it will eventually encounter this large neutral sheet.

A 2-fluid time-dependent analytical model of the perturbed solar wind is presented. The expansion of newly emitted material, caused, for instance, by the outburst of a solar flare, is simulated by a spherical piston. For a given thermal conductivity in the limit of strong coupling, one fluid flow in a thermally conducting medium is recovered. A pattern of flow which resembles one-fluid flow in adiabatic medium may be recovered if heat is removed from the perturbed plasma into the propelling plasma. The perturbed flow consists of a thermal precursor which is followed by a shock across which electrons are isothermal while protons are compressed and heated. Finally, we show that the post-shock rise and fall of density cannot be used to distinguish piston-driven waves from blast waves.

I consider it a great honour to give this talk today in place of my friendand colleague, the late Dr. S. F. Smerd, who died last December. All who knew Steve miss his enthusiasm and keen insight into solar physics.

An overview is given of various phenomena observed by numerous spacecraft in the interplanetary medium. These phenomena are related to transient solar events such as flares and coronal holes. The effects of such transient solar events are extensive. At times, a solar event can affect the interplanetary medium out to distances as far as 17.2 AU and over a wide range of azimuthal angles. Also some phenomena, such as high frequency fluctuations (precursors of shocks) in the interplanetary medium, appear beyond 1 AU. Thus, transient phenomena are significantly modified by their passage through this medium. The conclusion is reached that transient events originating on the sun and passing through the interplanetary medium represent complex and significant physical problems.

A recent sounding rocket experiment has provided high spectral resolution line profiles across the solar disk. The objective of this experiment is to provide information on the systematic velocity fields at the base of the corona by observing the displacement, width and shape of EUV emission lines.