The NOAA Mg II core-to-wing solar index: Construction of a 20-year time series of chromospheric variability from multiple satellites

The Mg II core-to-wing ratio is a measure of the amplitude of the chromospheric Mg II ion emission at 280 nm. It has been shown to have strong correlation with solar UV (150 - 400 nm) irradiance and other measures of solar activity and is relatively insensitive to errors introduced by variations in instrument sensitivity. For these reasons, the Mg II core-to-wing ratio has been used as a proxy for solar activity. Observations of the Mg II ratio started with the Solar Backscattered Ultraviolet spectrograph (SBUV) instrument on the Nimbus 7 spacecraft in November 1978. Since then, SBUV and SBUV2 instruments have flown regularly on the NOAA series of polar orbiting spacecraft, providing a nearly continuous set of daily solar observations. Measurements of the solar UV irradiance made from NOAA satellites ended in February 1998 when the NOAA9 satellite failed. Because it had been operating for nearly 12 years (at least 7 years beyond its expected lifetime), the NOAA 9 SBUV2 showed significant degradation near the end of its life. The most recent data required significantly more attention to extract the Mg II core-to-wing ratio. There have also been data gaps that require the augmentation of data from other sources. In particular, data from the UARS Solar Stellar Irradiance Comparison Experiment (SOLSTICE) instrument was used to bridge a 5 month gap in 1995. In this report we combine data from four different instruments to create the longest and most complete record of the Mg II core-to-wing ratio to date. We describe a modified analysis procedure that has allowed us to extend the Mg II index up to the beginning of 1998. Finally, we compare the time series of the Mg II ratio to similar measurements from the Solar Ultraviolet Spectral Irrandiance Monitor (SUSIM) instrument to determine the quality of the NOAA Mg II core-to-wing data set.