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P.C. Crane, L.E. Floyd (Interferometrics Inc.)
Knowledge of the variations in solar ultraviolet irradiances is essential to understanding both the Sun and the behavior of the Earth's upper atmosphere. Since the solar ultraviolet radiation is absorbed by the atmosphere, the requisite measurements must be done from space. Reliable, approximately daily measurements of solar ultraviolet irradiances have been made since November 1978 by a variety of instruments (Numbus-7 SBUV, NOAA-9 and NOAA-11 SBUV/2, UARS SUSIM and SOLSTICE, and ERS-2 GOME).
To overcome differences in spectral coverage and resolution and the challenges involved in the long-term calibrations of the instruments, a solar index suitable for use as a proxy for the solar ultraviolet (i.e., 100-400 nm) irradiances has been sought. The most popular indices for this purpose are the several Mg II indices based upon the Mg II k and h doublet near 280 nm; starting with the core-to-wing index developed by Heath and Schlesinger (J. Geophys. Res. 91, 8672, 1986) for the Nimbus-7 SBUV, they are calculated by taking the ratio of adjacent parts of the Mg II feature: one originating in the upper chromosphere that exhibits solar ultraviolet variations and the other, in the upper photosphere which is insensitive to solar variations. Because the ratio is of irradiances at nearby wavelengths, the Mg II indices mostly are not affected by temporal and spectral variations in the instrument responses.
While there is an ongoing effort to combine the available Mg II indices into a single 20-year time series (i.e., Viereck and Puga, J. Geophys. Res., in press), we are investigating an alternate approach. We report here on the first step in that study: the characterization of the signal and noise properties of the several Mg II indices available. We use Fourier analysis to determine the amplitudes of a common signal (the ~27-day variations) and of the high-frequency, day-to-day errors. Ultimately, the corresponding signal-to-noise ratios may be used to derive statistical weights to be used in combining the several Mg II indices into a composite index.
This work was supported by NASA-Defense Purchase Requests S14798D and S10108X.
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