Infrared Solar Physics Beyond 2.5 µm: Why and How

(Including a Program for the McMath-Pierce Telescope)

Scientific Staff

National Solar Observatory

National Optical Astronomy Observatories

July 1997

1 Summary

Infrared observations beyond 2.5 µm are expected to contribute uniquely to solving two key problems of solar physics: the role of weak surface magnetic fields in the solar cycle and the origin of the chromosphere. More generally, infrared measurements of magnetic field strength as well as temperature, density, and chemical composition are the most direct and sensitive of any known techniques.

Infrared solar observations from space with useful angular resolution are impractical, and only one ground-based telescope in the world currently accesses the full infrared spectrum. This unique capability, exploited using modern detectors, has given us a new window on solar physics. A larger aperture is needed to further develop the powerful diagnostic possibilities available in the infrared.

2 Two Key Scientific Goals
2.1 Weak Magnetic Fields: the Dark Matter of Solar Physics?

Since the invention of the solar magnetograph, our understanding of photospheric magnetic fields has undergone one revolution and may well be in the process of another. The first revolution established that most of the magnetic flux in plages and the magnetic network is in the form of kilogauss-strength, sub arc-sec field concentrations rather than the much apparent weaker field that corresponds to the measured flux. The second revolution concerns the importance of the “magnetic carpet” that covers the rest of the Sun outside active regions. Several indirect but independent estimates (Stenflo 1994) indicate that the flux in this component is comparable to the strong-field flux, even at solar maximum. The weak-field component continually renews itself on a time scale of a few days at most.

How do strong fields and weak fields interact? Does the weak-field component have large-scale structure? How is it generated? How does it disappear? Stenflo (1994) emphasized the importance of understanding the role of weak fields in the solar cycle: “A tiny, non-random component would however suffice for the IN [internetwork] emergence to be the dominating source of the large-scale pattern, since the IN flux emergence rate is 104 larger than that of AR

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