Ca II K and Mg II Index Data from AURA OMI: Cycles 23,24 S. Marchenko, M. DeLand Science Systems and Applications, Inc., Lanham, MD 20706, USA Goddard Space Flight Center, Greenbelt, MD 20771, USA Background. Over the past 3 decades the Mg II index (based on the Mg II doublet at 280 nm: Heath, D.F., Schlesinger, B.M. 1986, J. Geophys. Res., 91, 8672) gained reputation as a reliable, highly sensitive indicator of Solar activity. As defined (Fig. 1), the index has proven to be relatively insensitive to differences in the instrumental setup. A similar-designed Ca II H and K index (at 397 and 394 nm, respectively) may help to tie relatively recent satellite measurements to the century-long record of ground-based data. The Ozone Monitoring Instrument (OMI: Levelt, P. F., et al., 2006, IEEE Geosci. Trans. Rem. Sens. 44, 1093) on the Aura satellite allows to produce high-quality daily Mg II and Ca II indices since October Fig. 3. Left panel: the Solar Mg II index delivered by OMI in comparison to available time-series from various instruments. Right –hand panels: progres- sively smaller sub-sets of the Mg II time- series. Note the consistent representation of the small dips with amplitudes < 0.5% between days and Available data. In order to construct an accurate, long-term time-series of the Mg II and Ca II indices spanning the OMI lifetime, we use all the available Mg II data from different instruments, both of a relatively high resolution ( Δλ = nm: SOLSTICE, GOME, SCIAMACHY), as well as low-resolution data ( Δλ =1.1 nm: SBUV/2 at NOAA; cf. OMI’s at Δλ =0.63 nm). In addition, we incorporate the contemporaneous ground-based high-resolution measurements from the National Solar Observatory. The differences in spectral resolution call for an appropriate re-scaling of individual sub-sets of indices: Ind scaled = a 0 + a 1 * Ind orig. Results. Once the OMI data are appropriately scaled and adjusted for low-amplitude (<0.5%) instrumental trends, various sub-sets could be brought together and inter-compared. First of all, we find that the Ca II H and K indices closely follow the Mg II variability patterns (Fig. 2). Comparison to the independently acquired Mg II indices (Fig. 3) shows that the OMI data is now devoid of any obvious instrumental trends exceeding (both short- and long-term) ~0.5%. Conclusions OMI delivers robust, accurate Solar Mg II and Ca II indices which may serve as a link between the contemporaneous space-based UV data and the long-term ground-based observations. The overall precision of the OMI indices allows to follow Solar variability patterns with a full amplitude as low as ~0.3% in Mg II and ~0.5% in Ca II. So far, the development of Cycle 24 consistently points to a relatively lower Solar activity level (Fig. 4). Fig. 1. The Mg II Solar index derived on a series of the OMI irradiance spectra. 1 % Fig. 2. The Solar Ca II H, K and Mg II indices. Fig. 4. The unusual Cycle 23/24 (OMI Mg II, in black) compared to the Cycle composite index provided by M.Weber at bremen.de/gome/gomemgii.html. Note the lack of rotational modulation during the Cycle 23/24 minimum, as well as the relatively slow Cycle 23/24 ingress/egress. bremen.de/gome/gomemgii.html 2 %