Comparisons of the NOAA-11 SBUV/2, UARS SOLSTICE, and UARS SUSIM Mg II solar activity proxy indexes

Abstract

A NOAA-11 SBUV/2 Mg II solar activity proxy index has been created for the period February 1989 through October 1994 from the daily discrete mode solar irradiance data using an algorithm that utilizes a thorough instrument characterization. This product represents a significant improvement over the previously released NOAA-11 SBUV/2 sweep mode-based Mg II data set. As measured by the NOAA-11 Mg II index, the amplitude of solar rotational activity declined from approximately 4-7% peak-to-peak near the maximum of solar cycle 22 in 1989-1991 to roughly 1% peak-to-peak by late-1994. Corresponding to this decrease, the 27-day averaged NOAA-11 MgII index decreased by 5.8% over this period. The NOAA-11 Mg II data set is compared with coincident data sets from the UARS SOLSTICE and SUSIM instruments. The impact of differences in instrument resolution and observation platform are examined with respect to both the absolute value and temporal variations of the MgII index. Periodograms of the three indexes demonstrate comparable solar variation tracking. Between October 1991 and October 1994 predominate power occurs near 27 days, with secondary maxima in the power spectra near 29 and 25 days. Overall, there is low power near 13.5 days during this period. Dynamic power spectral analysis reveals the quasi-periodic and quasi-stationary nature of the middle UV variations tracked by the Mg II index, and periods of significant power near 13.5 days in mid-1991 and late-1994 through mid-1995.