34th Solar Physics Division Meeting, June 2003
Session 26 The Changing Solar interior II
Oral, Friday, June 20, 2003, 9:00am-12:00noon, Auditorium

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[26.01] Irradiance Variability

J. L. Lean (E. O. Hulburt Center for Space Research, NRL)

From the solar interior emerges magnetic flux that alters the Sun’s electromagnetic radiation, producing irradiance variability on a wide range of time scales. The organization, outward extension and expansion of magnetic fields structure the solar atmosphere and cause the entire electromagnetic spectrum to vary in different ways, depending on wavelength. Surface magnetic fields produce variations in near UV, visible and near IR spectra which emerge from the photosphere, and comprise the bulk of total irradiance. Yet the correlation of daily total irradiance and total magnetic field strength is poor; rather, the net effect of two different magnetic features – dark sunspots and bright faculae – better account for the variations observed in total irradiance during the solar cycle. The relationship of UV irradiance with magnetic flux is more direct since bright active regions control the global variations in both these quantities. Connecting X-ray and short wavelength EUV irradiance variations to the solar interior requires the extrapolation of emerging surface magnetic fields to the corona and assumptions about the relationship of their strengths and topologies with coronal pressure. Knowledge of the emergence, evolution, transport and decay of magnetic flux is thus a key to understanding and forecasting solar irradiance variability at all wavelengths. Identification of sizeable magnetic regions on the side of the Sun far from Earth may enable EUV and X-ray irradiance forecasts and subsequent space weather effects on time scales of days to weeks. On multi-decadal time scales improved knowledge of the sub surface dynamo and surface transport processes may help constrain secular solar irradiance evolution, needed for climate change attribution. Also needed is quantification of the association between the closed flux that controls irradiance variability and the open flux that extends into the heliosphere. Although utilized frequently to infer irradiance variability, proxies of long-term solar activity in tree-rings and ice-cores actually reflect heliospheric modulation of galactic cosmic rays. Funded by NASA and ONR.

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Bulletin of the American Astronomical Society, 35 #3
© 2003. The American Astronomical Soceity.