1 GeV particles are not sensitive to these uncertainties, and a reasonable quantitative understanding of the physics has been attained.

The heliosphere is bathed by an essentially isotropic, uniform distribution of galactic cosmic rays that is expected to remain steady over periods of thousands of years. These cosmic rays have difficulty in traveling into the inner solar system, resulting in a depressed intensity there. The problem is to understand this quantitatively in terms of what is known of the solar wind and cosmic ray transport. Since the solar wind is supersonic, the inner part of the heliospheric plasma and magnetic field are not affected much by the uncertainties at the outer boundary. The transport of the cosmic rays is determined by solar wind and its embedded magnetic field, both of which are convected out by the supersonic wind flow.

The large-scale structure of the magnetic field has been clarified considerably by observations carried out on the Pioneer, Voyager, and Ulysses spacecraft, and by the inferred relationship to observed coronal structure. During the years around each solar sunspot minimum, the field is generally organized into two hemispheres, separated by a thin current sheet at low heliographic latitude across which the field reverses direction. In each hemisphere the field is generally assumed to be the Archimedean spiral, with the sense of the field being outward in one hemisphere and inward in the other. At sunspot minimum, the current sheet is nearly equatorial. The structure for the years near sunspot maximum is not simple, with transient solar activity causing significant, large-scale propagating disturbances.

One other aspect of the theory of modulation and transport in the heliosphere is the study of the anomalous component of the cosmic rays, which are an important component at energies around a few hundred MeV. It appears that these particles are freshly ionized interstellar particles accelerated at the termination shock by the mechanism of diffusive shock acceleration.

The spacecraft Voyagers 1 and 2 are at present studying the outer parts of the heliosphere, and Voyager 1 has actually crossed the termination shock. This extended Voyager mission, to study the outer heliosphere, has provided important insights into the shape of the heliosphere and the mechanism of energetic particle acceleration. Other spacecraft, including Ulysses and the Advanced Composition Explorer (ACE), are observing the inner heliosphere.

To summarize the present knowledge, the modulation of galactic cosmic rays and the anomalous component are understood well enough to enable a confident prediction that the intensity will continue to vary in antiphase with the sunspot cycle, with variations of the order of 30 percent or so at GeV energies from sunspot minimum to sunspot maximum. It is possible that unexpected solar phenomena could produce lesser or larger effects.

SOLAR ENERGETIC PARTICLES

Energetic particles with energies occasionally exceeding several GeV are often produced in sporadic events at the Sun associated with solar activity. Solar flares and coronal mass ejections (CMEs) produce the energetic particles by processes whose specific nature is still being studied. The energy spectrum of the solar energetic particles is softer than that of galactic cosmic rays, and the events typically last for periods of hours to days. The intensities during the events can be quite large, although at energies above several hundred MeV, the time-integrated galactic cosmic ray flux is larger than that of solar cosmic rays. The events occur sporadically, although less frequently near sunspot minimum, and cannot be easily predicted. The basic energy source for the particles is the solar magnetic field, which becomes unstable and dissipates magnetic energy very rapidly, producing an explosive event. The explosion produces transient effects in the surrounding plasma, which then accelerates the solar energetic particles (SEPs). In addition to observations of the particles themselves, solar energetic particles produce variable electromagnetic



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