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Session 36 - Solar Activity.
Display session, Tuesday, June 11
Tripp Commons,

[36.17] A New Model of Solar EUV Irradiance Variability

H. P. Warren, J. T. Mariska, J. Lean (NRL)

Solar soft X-ray (SXR, 1-100\,Åand extreme ultraviolet (EUV, 100--1200\,Åradiation plays a central role in the energetics and dynamics of the Earth's upper atmosphere. Solar radiation at these wavelengths is strongly affected by solar magnetic activity and varies significantly during the solar activity cycle. Empirical models of solar irradiance variability essentially parameterize existing full-disk irradiance observations with proxies for solar activity. However, the limitations of existing EUV observations and absence of any current irradiance measurements at these wavelengths limits the utility of empirical irradiance modeling. Motivated by solar physics experiments on Yohkoh, SOHO, and TRACE we have developed a new, physics-based approach to modeling solar SXR and EUV irradiance variability. In this new model, the intensities of optically thin spectral lines are calculated using theoretically determined values of plasma emissivity coupled with emission measure distributions for features of the solar atmosphere: coronal holes, quiet Sun, and active regions. For emission lines with very complicated formation processes, such as the Lyman lines of hydrogen and helium, spatially and spectrally resolved solar observations are used in place of emission measure calculations. Information about the distribution of emitting regions on the Sun is inferred from full-disk images of the Sun, such as BBSO Ca\,II k-line and Yohkoh SXT images, rather than from proxies for solar activity. Comparison of the model with existing empirical irradiance models based on F_10.7 and other proxies for solar activity reveals disagreements in the absolute magnitude, the amplitude of the rotational modulation, and the solar cycle variability of the predicted fluxes at many wavelengths.

This research was supported by the NASA SEE program.

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