NOAA 16 and NOAA 17 operational satellites host the Solar Backscatter Ultraviolet (SBUV) spectrometer that has the objective of monitoring ozone in the Earth’s lower atmosphere. In its discrete operating mode, a diffuser screen is placed in front of the instrument’s aperture in order to scatter solar MUV radiation near 280 nm into the instrument. This solar spectral region contains both photospheric continuum and chromospheric line emissions. The chromospheric Mg II h and k lines at 279,56 and 280,27 nm, respectively, and the weakly varying photospheric wings or continuum longward and shortward of the core line emission, are operationally observed by the instrument.
On the ground, the Mg II core-to-wing ratio is calculated between the variable lines and nearly non-varying wings. The result is a measure of chromospheric and some photospheric solar active region activity is referred to as the Mg II core-to-wing ratio (cwr), and is provided daily by NOAA Space Environment Center (SEC). The ratio is an especially good proxy for some solar FUV and EUV emissions and it can represent very well the photospheric and lower chromospheric solar FUV Schumann-Runge Continuum emission. The daily Mg II cwr is used in a linear regression with F10.7 to derive the M10.7 index for reporting in F10.7 units and with a 5-day lag.