Comparison of Magnesium II Core-to-Wing Ratio Measurements J. Machol 1,2*, M. Snow 3, R. Viereck 4, M. Weber 5, E. Richard 3, L. Puga 4 1 NOAA/National Geophysical Data Center, Boulder, Colorado, USA; 2 Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA; 3 Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA; 4 NOAA Space Weather Prediction Center, Boulder, Colorado, USA; 5 Institute of Environmental Physics, University of Bremen, Bremen, Germany The Mg II index as a proxy for solar variability The Mg II index is a ratio of solar spectral features that provides a robust measure of solar chromospheric activity. It is based on the ratio of irradiances of highly variable chromospheric lines to stable photospheric emissions near 280 nm. The Mg II index has been shown to be a good proxy for solar UV, EUV, and the facular brightening component of the Total Solar Irradiance. As a ratio, it is robust against instrument degradation. A composite Mg II index is created by created by linearly scaling data sets from different instruments to each other and selecting the best data for each time period based on trends and noise. Variations in data sets are due to instrument resolution, systematic errors (such as uncorrected light scatter), and degradation. The challenge is that there is no "truth" with which to compare. Solar EUV and the thermosphere/ionosphere At Earth, solar irradiance at various wavelengths is absorbed preferentially at various altitudes. The primary energy input into the upper atmosphere is EUV which heats the thermosphere and creates the ionosphere. Variations in EUV flux drive the chemistry and dynamics of the thermosphere and ionosphere. EUV has high variability on many time scales (secs to years). EUV data is needed for thermospheric/ ionospheric models which provide information about satellite drag, communications, navigation systems, and upper atmospheric chemistry. GC51C-0975 The Mg II index is an important proxy for solar activity. In particular, it correlates well with the extreme ultraviolet (EUV) which is important for understanding the upper atmosphere. We compare data from all of the instruments that made Mg II measurements during the recent solar minimum and find that they are in good agreement throughout the interval. Our primary goal is to create a composite Mg II data set building on previous data sets. Atmospheric absorption as a function of wavelength (reds and blues). Also shown are the solar spectrum (white line on a log scale) and Earth's atmospheric temperature profiles (orange and yellow lines). EUV nm XUV (soft x rays) nm Acknowledgements We thank Matt DeLand and Dave Bouwer for useful conversations. Conclusion Multiple Mg II data sets in good agreement with each other are available for extending the Mg II composite. We are working on a final evaluation and will publish the results in Goal: composite data set of Mg II index This project is to create a composite daily Mg II index from 1978 to 2013 and has error bars. It will extend previous composites with 5 data sets. Data sets The NOAA Mg II composite (Viereck, Puga) uses Tiros SBUV and NOAA 9, 11 and 16 SBUV/2 data with 1.1 nm resolution. It is the basis for the early part of the other two Mg II composites. After 1992, the Bremen Mg II composite (Weber) uses UARS and ESA satellite data. Resolutions are 0.2 nm (UARS SOLSTICE), 0.17 nm (GOME), 0.2 nm (SCIAMACHY) and 0.17 nm (GOME-2). SORCE SOLSTICE (Snow) measurements at 0.1 nm resolution which are used to produce both low and high resolution Mg II time series. (See GC51C-0979, Snow, regarding a data correction.) After 1992, the SET Mg II composite (Bouwer, Tobiska) is generated from NOAA 16 and 18 [DeLand algorithms) using different algorithms than the NOAA composite. Aura OMI (DeLand) measures the Mg II index with a resolution 0.63 nm. The Penticton F10.7 index is a daily measure of solar radio flux at 10.7 cm and is a measure of solar variability. The Lyman-α composite (Woods) provides daily values of the irradiance from the H line at nm which is a measure of solar variability. Comparisons Bremen (GOME2), SOLSTICE, and OMI agree to within about ± Does OMI have a different trend? All Mg II measurements agree to within ± There is a downward trend (by 0.01) of the Bremen and SET composites relative to the NOAA composite. Why? Agreement is excellent (±0.002) between NOAA and Bremen (SUSIM and GOME) composites Composites agree to ± fit over 6/08-7/09 fit over 1/05-7/05 An 81-day smooth has been applied to the ratios in these plots. (Allen et al., 1978) Wavelength (nm) h ( nm) k ( nm) Relative Intensity SBUV Scan Data Solar UV Spectrum from NOAA SBUV Wings Core Mg II = Core Wings Method We compare Mg II data sets with each other and with Lyman-α and F10.7. The data sets are fit to each other, during quiet times where possible, and then we ratio, normalize, and smooth it to look for trends and anomalies. Care must be taken in the comparisons since there are distinct glitches in some of the composites when they switch instruments. We assume that the composites each use the best options among their overlapping data sets. SOLSTICE plotted versus Ly-α and F10.7 for 2005.