GOES EUVS Magnesium II Index: The beginning of a new climate data record

The GOES-R series spacecraft include a new operational measurement for GOES satellites, the Magnesium II (MgII) core-to-wing index.  This solar activity proxy has been measured at a daily cadence beginning in 1978.  It is widely used in solar spectral irradiance models such as the NRLSSI CDR (Coddington et al. 2016).  The C Chanel of the Extreme and Ultraviolet Spectrograph (EUVSC) on GOES 16 began making operational MgII index measurements at high cadence in early 2017.  There are currently three such instruments in orbit on GOES 16, 17, and 18 as part of the EXIS suite of solar irradiance instruments.  In the past, the MgII index was only measured a few times per day, but EXIS makes measurements at a 3-second cadence to monitor rapid changes in the Sun.In this presentation, we will describe the available data product and how it compares with previous measurements.  On long timescales it can be used to extend the historical record, and on short timescales it reveals solar activity of interest to space weather practitioners.