Unusual signals from pulsating radio sources have been recorded at the Mullard Radio Astronomy Observatory The radiation seems to come from local objects within the galaxy, and may be associated with oscillations of white dwarf or neutron stars 1968 saw the first report of a curious class of astronomical radio sources, distinguished by their rapid and extremely regular pulsations Hewish et al associated them with unusually stable oscillations in compact stars They are now understood to be rapidly rotating, magnetized neutron stars, or pulsars

Observation of a Rapidly Pulsating Radio Source

ISEE-3 measurements extending down to 2 keV energy have provided a new perspective on energetic solar electrons in the interplanetary medium. Impulsive solar electron events are observed, on average, several times a day near solar maximum, with ~40% detected only below ~ 15 keV. The electron energy spectra have a nearly power-law shape extending smoothly down to 2 keV, indicating that the origin of these events is high in the corona. These coronal flare-like events often produced 3He-rich particle events.

Energetic Solar Electrons in the Interplanetary Medium

This paper reviews the present state of knowledge concerning the relativistic particles of solar origin. The analytic techniques for obtaining information regarding the distribution of particles in the interplanetary space from ground-based observations are described. The general features of pitch angle anisotropy of high-energy particles are outlined. The concepts of relativistic particle propagation in the corona and interplanetary space are discussed with special reference to the relevant new ideas that have emerged from studies of energetic particle events. The experimental and theoretical arguments that indicate that particles can be accelerated to high energies by shock waves in the heliomagnetosphere are reviewed. Finally, a brief summary of the well-established ideas and unsolved problems is presented.

Relativistic solar cosmic rays

We consider the possibility that white light flares are due to heating of the photosphere by a flux of energetic ions and electrons impinging on it from above. Particles with energy in the range 10 MeV to 1 BeV release most of their energy to the ambient gas at about optical depth unity in the photosphere. This increase in energy produces a temperature perturbation of several hundred degrees in the layer and results in a re-radiation of the energy with a radiative relaxation time of several seconds. The consequences of this model are applied to a study of the great flare of May 23, 1967 and to the very fast event of August 11, 1954. Large numbers of very energetic electrons or protons must be produced in the first few minutes of the primary flare event if our interpretation (or one based on synchrotron emission) is correct.

White light events as photospheric flares

Solar phenomena produce cosmic ray intensity variations over a wide range of time scales. The observed flux is modulated and rendered anisotropic as the particles propagate in the solar wind, and it is occasionally enhanced by the sporadic emission of solar cosmic rays. Both quasi-static phenomena (the long-term omnidirectional intensity variation and the steady state diurnal anisotropy) and transient fluctuations (disturbed daily variation and Forbush decrease) as well as the spatial distribution of solar flare particles are represented by a theoretical model that prescribes the role of the several solar-controlled parameters that characterize the electromagnetic properties of the interplanetary medium. Considerable information concerning the ambient conditions has been obtained with spacecraft. However, the in situ measurements are confined to a limited region near the ecliptic plane. Consequently, in some cases, theoretical predictions based upon them are not in accord with observations of cosmic ray intensity variations. Thus the modulations and anisotropies must be treated in a three-dimensional framework. It is therefore reasonable to attempt to deduce the properties of the relevant inaccessible regions of the heliosphere from observations of the sun itself. These properties can be determined by relating the various cosmic ray phenomena to changes during a solar cycle and from one cycle to another. To this end, data covering at least two solar cycles are studied to determine the solar cycle dependence of the following effects: long-term variations in the omnidirectional intensity, Forbush decreases, solar diurnal variations, and solar cosmic ray events (ground level enhancement and polar cap absorption).

The Sun and cosmic rays

Solar cosmic rays, discussing flare association, solar particle acceleration, recurrence and low energy solar particle events

F. B. Mcdonald

Papers

Energetic Particles from the Sun