(Solar Phys.) The progress of shock waves generated by solar flares through the corona is delineated in the radio band by radio bursts of spectral type II. Previous discussion of these bursts has been mainly concerned with their characteristics at frequencies below 200 MHz. This paper discusses the characteristics of the bursts in the range 2000–200 MHz, and the information that may then be deduced about the propagation of shock waves through the lower corona. Particular attention is paid to the type II burst emitted by the flare of 1972 August 7, 1500 UT. The shock from this flare was subsequently tracked (by radio equipment on the IMP-6 satellite) through the interplanetary plasma right to the Earth.

(Solar Phys.) When the Culgoora radioheliograph started operating at both 80 MHz (the initial frequency) and 160 MHz (from May 1972) it became possible for the first time to make simultaneous observations of the fundamental and second harmonic sources in type II bursts. Three such bursts, each having harmonically related split-bands, have been observed so far. The new 80 and 160 MHz heliograph observations are illustrated in Figure 1. Since all three events had split-band structure the results are presented separately for the lower- (l) and upper- (u) frequency components and differences between the source positions in the two components will be highlighted. For the burst of 1973 May 19, which was observed at large zenith angles, calculated corrections for ionospheric refraction – approximately 1′ to 2′ for the 80 MHz sources – have been applied; the remaining two events are presented as observed since the refraction effects are thought to be small.

The first type II radio burst to be observed at frequencies below the ionospheric cutoff was recorded by the Goddard radio astronomy experiment on the Interplanetary Monitoring Platform, IMP-6. It occurred on June 30, 1971; a tracing made from the intensity contour plot of the radio data is shown in Figure 1. On the original, intensity contours were plotted in dB above background as a function of frequency and time. The figure shows 32 discrete frequencies from 4.9 MHz to 30 kHz received by the IMP-6 experiment. The time scale covers a period of almost five hours. During this time, five individual type III bursts and one group of type III's occurred; they are shown by their outlines to be drifting rapidly from high to low frequencies. The relatively narrow-band, slowly-drifting radiation with clearly defined fundamental and second harmonic is the type II. The average drift rate, represented by the straight lines drawn through the burst, is approximately 2.5 kHz min−1 in this frequency range, and is roughly 100 times slower than that of the type III bursts. The ovals defining the type II emission were traced from the record and show modulation of the signal caused by the spin of the satellite and the resultant sweeping of the antenna pattern of the dipole across the source direction. On the original record, the outlines of the type III's are also filled with a pattern of modulation ovals that were omitted from the figure for clarity. The shaded portion of the record at the lower frequencies represents magnetospheric noise.

(Solar Phys.). It is known that the sources of metric type IV radio bursts generally move outwards with speed of a few to several hundred km s−1 in the solar atmosphere and envelope (e.g., Kundu, 1965; Smerd and Dulk, 1971; Wild and Smerd, 1972). These sources consist of magnetic lines of force being stretched out from the flare regions and relativistic electrons being trapped by these field lines. They are often observed as expanding magnetic bottles (e.g., Smerd and Dulk, 1971). As a result of this expanding motion, the frequency range of type IV burst extends with time to lower frequencies, because of the decrease of both magnetic field intensity and energy of radiating electrons (Dulk, 1970; Sakurai, 1973a). By considering the observed expansion rate of emission frequency range, it seems possible to estimate the pattern of expansion of these radio sources: in doing so, we have analyzed the onset time differences between microwave and metric emissions of type IV bursts by referring to the longitudinal positions of parent solar flares on the solar disk, since the dependence of these differences on parent flare positions in the solar longitude seems to give a clue to estimate a general pattern of the expansion of magnetic bottles. The result thus analyzed is shown in Figure 1. Furthermore, peak flux intensities at metric frequencies for these type IV bursts have been also analyzed as a function of the longitude positions of parent flares (Figure 2). The results shown in Figures 1 and 2 are explained by assuming that, while moving outwards, the magnetic bottle tends to expand a few 10 deg east of the meridian plane which crosses the flare region (Sakurai, 1973a).

The ejection of rapidly-moving solar material into interplanetary space in association with solar flares has been discussed since 1859, when geomagnetic disturbances and auroral displays followed shortly after the first observation of a flare by Carrington and Hodgson. Until the advent of in situ interplanetary observations in the early 1960's, such discussions were based upon the indirect information regarding interplanetary space that could be inferred from geomagnetic or cosmic ray data. The past decade of space exploration has provided a great deal of direct information regarding the interplanetary effects of solar flares and some quantitative implications regarding the nature of transient coronal disturbances.

The theory of shock waves in a hot plasma is reviewed, and the types of shock waves which can exist are summarized. The detailed application of shock wave theory to the explanation of laboratory experiments is described; a comparison is made between the results of these experiments and the observations made of type II radio bursts from the Sun.

(J. Geophys. Res.). The August 1972 events provided an excellent opportunity for synthesizing a variety of observations in a coordinated fashion for the purpose of improving flare-shock associations, and our understanding of interplanetary shock propagation and solar wind interaction with planets and comets. These observations included the usual sudden commencements of magnetic storms at Earth; preliminary shock data from Heos-2, Prognoz-1 and Prognoz-2 (at Earth) and the radially-aligned Pioneers 9 (0.77 AU) and 10 (2.2 AU) located about 45° east of the Sun–Earth axis; solar radio types II and IV (as reported in World Data Center A's UAG Report 28, 1973, and this Symposium); discrete radio source scintillations in the solar wind; and the more speculative ideas regarding the solar wind's interaction with planets and comets. In the last case, Jupiter and Comet P/Schwassmann-Wachmann I exhibited non-Io-associated radio emission and a brightness increase, respectively, as a possible result of shock waves from the flare and/or coronal ejection activity initiated on 1972 June 15. During the August events, Comet P/Giacobini-Zinner exhibited statistically-significant sudden brightness decrease following its perihelion on 1972 August 4 at 1 AU approximately 57° west of the Sun–Earth axis.

(Solar Phys.). The author has proposed a hypothesis that the type II burst-sources may be due to the local enhancement of the flare-produced MHD fast-mode shock wave on encountering the pre-existing low Alfvén velocity regions in the corona (Uchida, 1973). The purpose of the present paper is to show that this may actually be the case by comparing the behavior of the computor-simulated propagation and strengthening of MHD fast-mode (weak) shock with the observed characteristics of type II burst-sources.

(Astron. Astrophys.). In this paper it is shown that the Alfvén wave turbulence created behind a shock wave propagating along the ambiant magnetic field in a collision-less plasma, accelerates electrons of the high energy tail of the distribution up to mildly relativistic energies. The efficiency of the process depends upon the ratio of the acceleration time by the turbulence to the scattering time of the same turbulence. At high energies the accelerated particles have a power law spectrum, the steepness of which depends upon the turbulent energy.

(Astrophys. Letters). The measured amount of band-splitting, Δf, in the spectra of nine harmonic type II bursts is illustrated in Figure 1. Here, as in previous, smaller samples (Roberts, 1959; Maxwell and Thompson, 1962; Weiss, 1965) Δf is found to increase with frequency, f.

The possible validity of thermal bremsstrahlung models of flare hard X-ray bursts is investigated quantitatively. In particular, the problem of rapid thermal conduction in ‘multi-temperature’ models is adequately examined for the first time by using a continuous temperature distribution consistent with the observed X-ray spectrum. This distribution is obtained from a general analytic solution for the temperature structure required to mimick any ‘non-thermal’ spectrum, the method being equally applicable to cosmic sources.

It is concluded that the thermal interpretation might extend to X-rays of hundreds of keV, a result with important consequences for flare energetics. The relationship of such a model to observations of X-ray polarization and rapid time variations is also considered.

The charge and isotopic composition of solar cosmic rays potentially contain a wealth of information on the acceleration and confinment of energetic particles at the Sun. As the experimental techniques have improved and with the increasing number of measurements, this potential is now being realized. It is convenient to divide the composition studies into three areas: (a) helium-iron, with energies >10 MeV nuc−1; (b) helium-iron, and the trans-iron elements, with energies <2 MeV nuc−1; (c) the isotopic composition. In the first area at energies above 10 MeV nuc−1 the solar cosmic rays appear to provide a representative sample of the solar corona. At lower energies (b) complex enhancement effects are noted. These increase with Z and decrease with energy. This result as well as the high abundance of 3He suggest that the acceleration process is not a simple one and probably several stages are required.

An instrument aboard the Fifth Orbiting Solar Observatory has observed hard solar X-rays from January 1969 to May 1972. A large number of X-ray bursts generated by solar cosmic ray flares have been observed. The X-ray bursts consist, in general, of two non-thermal components. The earliest occurring non-thermal component, coincident with the explosive phase, consists of a group of one to about ten X-ray bursts that are, for each burst, approximately 10 s duration and symmetrical in rise and decay. The time structure and multiplicity of these bursts is remarkably similar to that found in type III radio bursts in the meterwave band. The spectra of these bursts steepens sharply at energies greater than 100 keV indicating a limit at this energy for electron acceleration during the explosive or flash phase of the flare. For several flares these multiple X-ray bursts have occurred in coincidence with a group of type III bursts.

The existence of non thermal radio bursts provide evidences for the acceleration of electrons in the solar atmosphere.

It is shown, from the characteristics of the bursts, that the electrons are accelerated in at least four different phases:

1. (1) An impulsive phase which gives μib and III bursts.

2. (2) A gradual phase which gives μIV and S1IV bursts.

3. (3) A quasi-continuous phase which gives S2IV bursts and noise storms.

4. (4) An acceleration by shock waves gives type II bursts.

5. (5) Eventually, another shock-wave acceleration giving the moIV burst.

A review of observational data supports the proposal that there are two distinct phases of particle acceleration in solar flares. ‘Phase 1’ is associated with the flash phase and is here interpreted as acceleration during field-line reconnection. ‘Phase 2’ is associated with type II and type IV radio bursts, and is ascribed to stochastic acceleration in the turbulent plasma behind a magnetohydrodynamic shock formed ahead of an ejected plasmoid.

Recent theories of solar flares are reviewed with emphasis on the aspects of pre-flare heating. The heating evident at 3.3-mm wavelength is analyzed in the form of daily maps of the solar disk and synoptic maps compiled from the daily maps. It is found that isotherms defining antenna temperature enhancements of 340 K correspond in shape and location to facular areas reported by Waldmeier. Maximum enhancements occur over sunspots or near neutral lines of the longitudinal magnetic fields which indicates heating associated with chromospheric currents. These enhancements are correlated with flare importance number and are observed to increase during several days preceding flaring. This evidence for a containment mechanism in the chromosphere is collated with current theories of solar flares.

We present the first observations of protons down to 44 keV and electrons down to ≲ keV emitted in an impulsive solar particle event. The observations are from the Apollo 15 Subsatellite during a flare event which began on September 1, 1971. We obtain a lower limit estimate of the energy contained in ≳ MeV protons in the flare. The effects of adiabatic deceleration in the interplanetary medium and dE/dx energy loss in the corona are discussed. We conclude that energetic protons may constitute a major energy release in large solar flares.

(Solar Phys.). Several models for pulsating type IV radio bursts are presented based on the assumption that the pulsations are the result of fluctuations in the synchrotron emission due to small variations in the magnetic field of the source. It is shown that a source that is optically thick at low frequencies due to synchrotron self absorption exhibits pulsations that occur in two bands situated on either side of the spectral peak. The pulsations in the two bands are 180° out of phase and the band of pulsations at the higher frequencies is the more intense. In contrast, a synchrotron source that is optically thin at all frequencies and whose low frequency emission is suppressed due to the Razin effect develops only a single band of pulsations around the frequency of maximum emission. However, the flux density associated with the later model would be too small to explain the more intense pulsations that have been observed unless the source area is considerably larger than presently seems reasonable.

**Magnetic fields in coronal and interplanetary space play a critical role in the propagation and storage of energetic particles from the flare site to 1 AU. The current investigation is designed to determine if the detailed configuration of the coronal magnetic fields below 2.5 R⊙ has a measurable influence on the escape of particles to 1 AU, can account for the observed broad longitudinal distribution of proton flares, and can provide a realistic site for storage of energetic particles.

(Solar Phys.). The observed enrichment of heavy nuclei in the composition of relativistic cosmic rays including solar, may be explained on the basis of a coherent accelerating mechanism. This mechanism requires the existence of high intensity relativistic electron beams or preferably blobs (with number density ≥1011 electrons cm−3) in the accelerating solar regions. The radius of each blob should be less than its plasma wavelength, i.e. of the order of cm.