AAS Meeting #194 - Chicago, Illinois, May/June 1999
Session 36. Hale Prize Lecture: Why is the Sun Active?
Invited, Oral, Monday, May 31, 1999, 3:40-4:25pm, Grand Ballroom

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[36.01] Why is the Sun Active?

J. Harvey (NSO/NOAO)

This fundamental, centuries-old astrophysics question prompted G. E. Hale to create modern solar research. The question (of practical importance to humans) endures and motivates most solar work. What would we tell Hale about recent work if he reappeared among us? Great progress is being made in three areas, all of which he either initiated or strongly influenced. First, Hale would probably be surprised that we can now `look' at activity beneath the surface by studying the oscillations discovered in 1959 using the 60-foot tower telescope that he built in 1907. Such helioseismology research points to the interface between the differentially-rotating convection zone and the almost-uniformly-rotating radiative interior as the nursery of solar activity. Next, Hale's discovery of magnetic fields at the surface of the Sun has led to today's idea that magnetism is at the root of virtually all solar activity phenomena. His interest in vortical (or helical) patterns associated with magnetic fields has recently been reawakened and is now a vigorous area of solar research. The twist of a magnetic field may be almost as important as the field itself in producing violent solar activity. Finally, The study of the solar atmosphere against the solar disk, initiated in Chicago by Hale in 1891, has allowed us to observe the interactions of mass motions and magnetic fields that produce gigantic explosions such as solar flares and coronal mass ejections. These observations strongly suggest that magnetic reconnection is a fundamental and nearly ubiquitous process associated with the activity of the solar atmosphere. Hale vigorously promoted simultaneous advances in observation and theory. Today, a detailed answer to the title of this talk is probably in the literature, but we lack adequate observational tools to test the many published theories and ideas. Hale made revolutionary breakthroughs by applying the latest technology to make new observations that were stimulated by theory and conjecture. We are in a position to apply modern technology, as Hale did a century ago, to make breakthrough observations again. Two exciting next-millennium examples, in the tradition of Hale, are the use of space to get steady, sub-arc-second image quality (the Solar-B mission), and a large-aperture, adaptive-optics-equipped, ground-based telescope for detailed studies of solar MHD phenomena (the Advanced Solar Telescope).

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