34th Solar Physics Division Meeting, June 2003
Session 19 The Changing Solar Interior I
Poster, Wednesday, June 18, 2003, 3:30-5:00pm, Mezzanine

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[19.02] Magnetic Modulation of Solar 304 Å Irradiance

J. L. Lean, J. T. Mariska, H. P. Warren (E. O. Hulburt Center for Space Research, NRL), T. N. Woods, F. G. Eparvier (LASP, U. Colorado), D. R. McMullin, D. L. Judge (Space Science Center, U. Southern California), J. S. Newmark (E. O. Hulburt Center for Space Research, NRL), R. A. Viereck (NOAA Space Environment Center)

Solar 304 Å irradiance is an important source of heating and ionization in the Earthís upper atmosphere. Because only intermittent observations exist prior to solar cycle 23, the absolute levels and solar cycle variability of 304 Å irradiance are uncertain by a factor of two, based on the range of estimates from four current EUV irradiance variability models. Large active regions are a significant source of 304 Å radiation but their characteristics are not well specified, with contrasts reported in the range of two to ten. Statistical quantification of the role of small scale active regions and network is also lacking. During solar cycle 23, three different instruments are observing the Sunís 304 Å radiation concurrently. The EIT on SOHO records the brightness distribution on the solar disk in a 20 Å band, SEM on SOHO monitors the disk-integrated emission in an 80 Å band, and the SEE grating spectrometer on the TIMED spacecraft recently began observing EUV irradiance spectra with 4 Å resolution. We calculate daily histograms of the brightness distributions of EIT images after adjustments for various instrumental effects. Deconstructions of the histograms permit statistical characterizations of magnetic sources of 304 Å irradiance variability during solar cycle 23, in terms of fractional disk areas and contrasts. We also study center-to-limb variations. The calculations provide independent irradiance variability estimates for comparison with the SEM and SEE direct irradiance observations, and the models. We utilize the source characterizations to revise the NRLEUV model, the present version of which estimates 304 Å emission by assuming that a bright active region has a contrast of ten, and that source region evolution is temporally similar to the Mg chromospheric irradiance index. Funded by NASA SEC GI Program.

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