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Session 79 - Quiet Solar Photosphere and Chromosphere.
Oral session, Thursday, June 13

[79.01] Facular Origin of the Sun's 11-year Total Radiation Cycle

J. Lean, J. Cook (NRL), W. Marquette, A. Johannesson (BBSO), R. Willson (Columbia Univ.)

While the amplitude and temporal structure of the 11-year cycle in the Sun's total radiation is relatively well established from spacebased radiometry, the origins of the variations are not. Rotational modulation on shorter time scales is attributable to the changing presence on the Sun's Earth-facing disk of dark sunspots and bright faculae, both of which occur frequently during times of high solar activity but may be absent for days during solar minimum conditions. However, calculations of the energy changes in sunspots and active region faculae over the longer time scale of the 11-year cycle underestimate the observed solar cycle irradiance modulation by about a factor of two. This apparent lack of facular brightness has lead to speculation of a missing irradiance brightness component, and the possibility that a non facular -- perhaps global -- mechanism may be responsible for the observed total radiation energy changes.

Recent analysis of Ca K filtergrams, recorded with a 1 Angstrom Daystar filter at the BBSO, suggest a methodology with which to quantitatively assess the contribution of facular brightness to total solar irradiance variations. Following removal of instrumental effects and limb variations from the Ca K images, the resultant "flattened" spectroheliograms provide a surrogate for facular brightness in both active regions and in the surrounding chromospheric network. Evolution of the globally integrated signal from enhanced Ca K emission on the full solar disk is compared with concurrent total solar irradiance measurements made by the ACRIM II radiometer on the Upper Atmosphere Research Stellite from 1992 to 1994. This period covers most of the decline in solar activity from the cycle 22 maximum to the impending solar minimum, and suggest that facular changes can account for essentially all the measured variations.

Program listing for Thursday