The SORCE Magnesium II core-to-wing index is produced from SOLSTICE data using the definition in Snow et al. [2005b]. The spectral resolution of SORCE SOLSTICE (0.1 nm) allows the emission cores of the Mg II doublet to be fully resolved and modeled with Gaussians. To determine the wing irradiances, the spectrum is convolved with a 1.1 nm triangular bandpass and then measured at the four wavelengths used by NOAA as described in Heath and Schlesinger . This method has several advantages; most importantly it extracts a more precise measurement of solar activity [Snow et al., 2005b]. A simple linear correlation with the standard NOAA data product will scale the SORCE measurement to be compatible with the long-term composite Mg II index maintained by NOAA. The Mg II index and applications are described in Sections 220.127.116.11 and 2.1.3.
Since SOLSTICE observes the region of the spectrum near 280 nm many times per day, dozens of measurements can be produced per day. The most common users of the Mg II index only need a daily average rather than higher cadence measurements, so that is the default data product on the web page. Unlike the SSI data products, the Mg II index is a ratio. Many of the correction factors that go into the standard data processing appear in both the numerator and denominator of the index, and therefore cancel out. Consequently, a high-quality index can be produced shortly after spacecraft contact each day. Operational space weather users of SORCE Mg II index data automatically check the SORCE ftp site for updated data files twice a day to get the latest observations.
Instrument Calibration and Data Quality
Since the Mg II index is a ratio, most instrumental calibration factors cancel out. The wavelength scale is precisely determined by fits to the photospheric absorption lines near the Mg II doublet. Our index algorithm yields an uncertainty of 0.6% [Snow et al., 2005b].
Data Access and Availability
The SOLSTICE Mg II index is available in 2 forms: quicklook and calibrated. The quicklook version, suitable for near-realtime space weather applications, is available as a text file via anonymous ftp within 12 hours after data downlink from the spacecraft. These data have been re-scaled to be consistent with the NOAA Mg II scale. The calibrated version is constructed from the standard Level 2 data and is available on the LISIRD data access website about one week after observation.