[1] The Mg II core-to-wing ratio is a measure of solar chromospheric variability. The Mg II Index, formed by combining various Mg II core-to-wing data sets, has been used in EUV, UV, and total solar irradiance models. It is one of the longest records of solar variability reaching back nearly 25 years. We present a single, continuous time series of the Mg II core-to-wing ratio extending from November 1978 to the present. The Mg II core-to-wing ratio is a measurement that is well suited to the creating of a single time series despite the fact that the seven different instruments measuring the solar flux near 280 nm have different spectral resolutions and sample rates. The Upper Atmosphere Research Satellite (UARS) Solar Ultraviolet Spectral Irradiance Monitor (SUSIM), UARS Solar Stellar Irradiance Comparison Experiment (SOLSTICE), ERS-2/Global Ozone Monitoring Experiment (GOME) and five NOAA solar backscatter ultraviolet data sets were used. Initially, the best data sets were selected to create a time series spanning from 1978 to the present. Then the gaps in the record were filled with data from various other Mg II data sets. Where no alternate data were available, a cubic spline function was used to bridge the missing data. In some cases the data gaps were too long for reasonable spline fits (more than 5 days), and for these gaps the F10.7 cm flux data were scaled to fill the gaps. Thus a continuous, uninterrupted time series of the Mg II core-to-wing ratio was created. The final Mg II time series is compared with other solar activity indices, such as the F10.7, He I 1083, and Sunspot number, to look for trends in the Mg II data.
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
Periodicity in the 13-14 day range for full-disk UV fluxes comes mainly from episodes of solar activity with two peaks per rotation, produced by the solar rotational modulation from two groups of active regions roughly 180 ~ apart in solar longitude. Thirteen-day periodicity is quite strong relative to the 27-day periodicity for the solar UV flux at most wavelengths in the 1750-2900 A, range, because the rapid decrease in UV plage emission on average with increasing solar central angle shapes the UV variations for two peaks per rotation into nearly a 13-day sinusoid, with deep minima when the main groups of active regions are near the limb. Chromospheric EUV lines and ground-based chromospheric indices have moderate 13-day periodicity, where the slightly greater emission of regions near the limbs causes a lower strength relative to the 27-day variations than in the above UV case. The lack of 13-day periodicity in the solar 10.7 cm flux is caused by its broad central angle dependence that averages out the 13-day variations and produces nearly sinusoidal 27-day variations. Optically thin full-disk soft X-rays can have 13-day periodicity out of phase with that of the UV flux because the X-ray emission peaks when both groups of active regions are within view, one group at each limb, when the optically thick UV flux is at a rotational minimum. The lack of 13-day periodicity in the strong coronal lines of Fexv at 284 A, and FexvI at 335 A during episodes of 13-day periodicity in UV and soft X-ray fluxes shows that the active region emission in these strong lines is not optically thin; resonant scattering is suggested to cause an effective optical depth near unity in these hot coronal lines for active regions near the limb.
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