ground and space high-resolution studies of helioseismology, and space observations of the global magnetic activity of the Sun.

The Yohkoh spacecraft has provided a dramatic demonstration that the solar magnetic cyle affects not only the low-latitude sunspot zone, but the entire Sun. Comparison of the full-disk Yohkoh X-ray images from 1991 to 1998 shows that the entire corona, from the equator to the poles, waxes and wanes with the solar cycle. Interpretation of this important fact requires extensive ground-based follow-up. We need to know how the magnetic field and the velocity fields at the surface of the Sun and beneath the surface (insofar as helioseismology can infer them) vary with the solar cycle. This includes the torsional oscillations discovered nearly two decades ago and closely related to the solar cycle. A combination of synoptic data and high-resolution exploratory studies may turn up further clues to the nature of the solar dynamo, or perhaps upset the present picture altogether. Whatever the result, the solar studies will profoundly affect other areas of astrophysics.

ENERGETIC PARTICLES

The acceleration of electrons and ions to high energies in the magnetic activity of the Sun, in blast waves in interplanetary space, and in the terrestrial magnetosphere when impacted by a solar blast wave, is all part of the general solar outreach. The high efficiency of the particle acceleration is such that it is estimated that 10 to 50% of the total flare energy appears as fast particles in some cases. The isotopic abundances of the escaping fast particles can be directly determined with state-of-the-art spaceborne instruments, providing information of the matter density at the acceleration sites. The elemental abundances and total particle intensities define the hazards to humans and electronics in space.

The universal presence of fast particles, all the way to extreme relativistic energies, in active astronomical objects and throughout interstellar space illustrates the importance of the acceleration physics. The nuclear interactions initiated by fast ions in the solar chromosphere and corona provide otherwise rare isotopes, and it has been speculated that this may be a major source of the anomalous surface abundances in some active stars. Ground-based neutron monitors indirectly detect the larger outbursts of energetic solar particles by the prompt arrival of the neutrons along straight-line paths, while the energetic nuclei come along the spiral interplanetary magnetic field. The relative number of neutrons and the various exotic nuclear isotopes provides a direct measure of the density of matter at the acceleration site.

SOLAR INTERIOR

Helioseismology is the tool for exploring the basic hydrostatic structure and internal rotation of the Sun. In particular, it has established that the standard solar model is basically correct and has provided the curious internal rotation profile shown in the illustration on the front cover of this report.



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