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Session 2 - Everything Else.
Display session, Friday, June 27
Ballroom C, Chair: Richard Canfield

[2.75] SUSIM Measurements of UV Variations During the Decline of Solar Cycle 22

L. E. Floyd, P. C. Crane, L. C. Herring (Interferometrics, Inc.), D. K. Prinz, G. E. Brueckner (Naval Research Laboratory)

The Solar Ultraviolet Spectral Irradiance Monitor (SUSIM) aboard the Upper Atmosphere Research Satellite (UARS) has measured the solar spectral irradiance for wavelengths 1150-4100 Åon every available day from October 11, 1991 to the present. The record of spectral irradiances obtained during this period clearly shows the decline associated with the last half of solar cycle 22. Superimposed on this solar cycle variation is a 27-day solar rotation modulation which, for the most part, maintains coherence across the wavelength spectrum and with SUSIM's own Mg II core-to-wing ratio index. However, for wavelengths between the Si I edge at 1682 Åand the Mg I edge at 2513 Åand for some time periods, 13.5-day variations dominate.

SUSIM observations began when solar activity was near maximum and now extend through minimum. Generally, the measured peak-to-peak variations are larger for shorter wavelengths and for emission or absorption lines. The wavelength dependence of the UV variability apparently corresponds to the solar atmospheric emission heights given by radiative transfer models. The largest measured variation, that for H I Ly-\alpha, exceeds a factor of two. The variation in the continuum just shortward of the Al I edge at 2076 Åis about 10%; just longward, about 5%. This latter variation continues up to the Mg I edge and then declines to approximately zero measured variation at about 3000 Åand above.

Based on common proxies of solar UV variation, such as the Mg II core-to-wing ratio and He I 10830 Åequivalent width, SUSIM irradiance measurements have ranged over more than 85% of the entire solar cycle 22 variation. Through the separate use of each index as a UV proxy, we extend the SUSIM measurements to estimate the wavelength-dependent peak-to-peak UV variability over the whole of solar cycle 22.

SUSIM is supported under NASA-Defense Purchase Request S14798D.

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