The Field of Solar Physics Review and Recommendations for Ground-Based Solar Research (1989) / Chapter Skim
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3. Principal Science Opportunities and Initiatives for Ground-Based Solar Research
3. Principal Science Opportunities and Initiatives for Ground-Based Solar Research
Pages 16-41
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From page 16... ...
Neutrino astronomy; helioseismology; high-resolution observations of the solar surface; radio, infrared, UV, X-ray, and gamma-ray observations of the outer atmosphere; vector magnetic field observations; and spacecraft observations of the secular changes in the solar luminosity have all uncovered new and puzzling aspects of the Sun. These fundamental investigations have been possible only because of the proximity of the Sun.
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From page 17... ...
Geological and climatological evidence suggests that the change in solar luminosity has been much smaller. One proposed solution to this problem is to mix the solar interior to provide fresh fuel to the energy-generating core.
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From page 18... ...
Initial results from helioseismology indicate that the subsurface structure of sunspots and active regions does not agree with that described by current models. New models of the solar magnetic dynamo, which is thought to generate the solar activity cycle, are under development based on helioseismology.
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From page 19... ...
Continue operation of the 37C1 experiment through the next solar maximum expected in 1991, and continue support of the Kamiokande II experiment, whose results are an important consistency check for the 37C1 experiment. This will allow tests of suggested correlations of neutrino flux with the solar activity cycle and, more speculatively, with the Earth's heliocentric latitude.
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From page 20... ...
project. This is a community project initiated by the NSO to provide continuous solar oscillation data for a period of 3 years.
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From page 21... ...
The dashed line marks the base of the solar convection zone. The picture is based on measurements of oscillations of the Sun's surface, which are a manifestation of sound waves traveling through the solar interior.
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From page 22... ...
The impact of these observations will be a definitive determination of the stratification and motions of the upper layers of the convection zone, where our current understanding of the physics is quite uncertain. Investigation of the Interiors of Other Solarlike Stars The study of the solar interior gives us information about one star.
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From page 23... ...
THE PHYSICS AT SMALL SPATIAL SCALES The Basic Issues It is now well known that magnetic fields play a central role in the dynamics of the solar surface layers (for example, by ordering local transport coefficients such as thermal conductivity in an anisotropic fashion, by blocking convective transport, and by carrying the "mechanical" energy and momentum flux required for coronal plasma heating and acceleration of the solar wind) ; hence solar magnetic activity largely defines the interaction between the Sun's interior and atmosphere, and between the Sun's atmosphere and the heliosphere and terrestrial magnetosphere.
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From page 24... ...
. Unfortunately, solar magnetic fields are relatively strong, so that it is dubious whether kinematic theories are an appropriate description of the physics underlying the solar dynamo; furthermore, the solar convection zone is far from laminar in behavior (the Rayleigh number is far above critical, and the Reynolds number exceeds unity by many orders of magnitude)
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From page 25... ...
boundary conditions; and study of field line stochasticity. Rapid Magnetic Field Reconnection The role played by collective effects in the solar atmosphere was first appreciated in the impulsive phenomenon known as the solar flare, commonly believed to occur when oppositely directed magnetic fields in the solar corona "reconnect," thereby releasing energy in the form of heat, particle acceleration, and induced rapid flows.
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From page 26... ...
, also allows direct measurement of vector magnetic fields in the solar atmosphere and hence determination of the magnetic field topology in the solar corona. At very high photon
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From page 27... ...
spectroscopy takes advantage of the fact that atomic line Zeeman splitting is proportional to the square of the line center wavelength to enable exploration of the magnetic field structure in the lower photosphere and chromosphere. Initiatives and Impacts It is evident from the foregoing discussion that studies of the physics of the Sun's outer layers will very likely involve substantially greater interaction with the laboratory and magnetospheric plasma physics communities and increasingly greater contact with observers and plasma theorists dealing with astrophysical plasmas in general.
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From page 28... ...
. In both cases, many of the scientific issues discussed above- including the structure of magnetic field concentrations and of convective overshoot, and the interaction between convection and magnetic fields are directly addressed.
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From page 29... ...
Upper wavelength limit set by instrumentation, not by optics. High-Spatial-Resolution Microwave Instrumentation The near-term potential for extremely high-resolution imaging of coronal and chromospheric structures is nowhere as great as at radio wavelengths; this is of course a consequence of the coherence of radio wavelengths over very large baselines, so that ground-based interferometric observations can relatively easily reach subarcsecond spatial resolution; in addition, with the aid of spectral resolution and polarimetry, it is possible to infer the structure of magnetic fields in the atmosphere overlying the solar surface.
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From page 30... ...
MEClIANISMS UNDERIXING THE SOILER CYCLE The Basic Issues The longer time scales of solar variability reflect the presence of illunderstood phenomena in the deep interior that link rotation, convection, and magnetism. Cyclic variations of magnetic activity occur in many other solar-type stars, but we still lack satisfactory theoretical explanations of the origin and development of stellar magnetic fields.
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From page 31... ...
as observed bar the ACRIM instrument on board the Solar M~Darrnun Mission satellite since 1980. The data show striking dips of a few days' length due to the presence of large sunspots on the visible hemisphere.
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From page 32... ...
The Influences of the Sun on the Earth Solar magnetic activity produces hard radiation that affects the Earth's atmosphere and has significant social and economic consequences. These effects include the inflation of the Earth's upper atmosphere in proportion to the degree of solar activity, with attendant orbital and pointing disruptions of low-altitude satellites, the disruption of electrical power distribution caused by ionospheric surges, disturbances of navigation systems, and hazards for spacecraft and astronauts via solar flare energetic particles.
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From page 33... ...
Initiatives and Impacts Solar global observations include synoptic data, in which various tracers of solar activity are followed through the years in a semiquantitative manner. The conduct of such observations tends not to interest research-oriented solar physicists (nor most astronomers)
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From page 34... ...
Much of this energy appears in the form of highenergy particles and hot plasma. It is believed that the flare energy comes from the dissipation of- the nonpotential components of strong magnetic fields~oronal current systems in the solar atmosphere, possibly through magnetic reconnection, but the details of the energy release process as well as the mechanism of particle acceleration are still only poorly understood.
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From page 35... ...
Many of these observations remain to be carried out. Abundances Observations of X-ray and gamma-ray lines from solar flares have provided new techniques for determining abundances in the solar atmosphere.
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From page 36... ...
In addition to pure electromagnetic waves, the corona generates several other types of radiation, including hydromagnetic waves, Langmuir waves, and whistler waves. These radiations, although not propagating to Earth, still have important roles in energy transport and possibly in particle acceleration for many of the phenomena of solar activity.
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From page 37... ...
These constitute some of the most dramatic forms of solar activity and have yet-unresolved associations with the physics of classical solar flares. In some cases, a powerful flare will follow a white-light coronal transient and produce a clearly defined blast wave that propagates into the interplanetary medium.
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From page 38... ...
However, neutron monitors in Europe, the Soviet Union, Japan, and China will be used for solar flare study during this solar maximum. Radio Obse~vadons Radio observations allow us to study the solar atmosphere from approximately the temperature minimum region out to 1 AU, roughly corresponding to the wavelength range from 1 mm to 100 km.
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From page 39... ...
The impact of this initiative will be a better understanding of the chromospheric and upper photospheric parts of flares, improved quantitative information about energy and momentum balance in flares, and new diagnostics of thermal conduction along magnetic flux tubes. Vector Magnetographs It is evident that magnetic fields are responsible for nearly all solar activity.
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From page 40... ...
These capabilities offer the potential of predicting solar flare activity with far higher reliability than is currently possible.' Advanced Coronal Observations As noted previously, it is now suspected that coronal transient'phenomena result from the relaxation of stressed coronal magnetic fields. The coupling between this relaxation and the occurrence of associated solar flares or eruptive phenomena is currently unclear.
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From page 41... ...
The impact of these observations will be to illuminate the nature of the evolution of the solar large-scale magnetic field and the role of that evolution in generating solar activity.
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