High precision, high time-cadence measurements of the Mg II index of solar activity by the GOES-R Extreme Ultraviolet Irradiance Sensor 1: EUVS-C design and preflight calibration

EUVS-C is one component of the Extreme Ultraviolet Irradiance Sensor (EUVS) instrument. EUVS together with the X-ray sensor (XRS) comprise the Extreme Ultraviolet and X-ray Irradiances Sensors (EXIS) investigation aboard the GOES-R satellite series which includes GOES-16, -17, -18, and -19. From their vantage points in geostationary orbit, the EUVS-C instruments make high-precision (better than 1 part in 104), high-time-cadence (3 seconds) measurements of the solar Mg II Index with moderate (0.1 nm) spectral resolution. The Index, also referred to as the Mg II core-to-wing-ratio, is a proxy for chromosphere activity that correlates with solar extreme ultraviolet (EUV) irradiance. Mg II produces two bright chromosphere emission lines in the Sun’s spectrum at 279.55 nm and 280.71 nm (Mg II k and h) that appear in the cores of their respective photospheric absorption lines. Measuring the ratio of emission from the core (chromospheric) to that from the wings (photospheric) provides an index that is relatively insensitive to changes in radiometric performance that often occur when scientific instruments observe the Sun. EUVS-C design specifications were informed by earlier research reporting index variability of approximately 0.2% on time scales of 6 – 10 minutes, increasing to approximately 0.3% and approximately 0.55% for 30 and 80 minutes, respectively. This paper describes the EUVS-C instrument design and implementation, its ground calibration and characterization, and anticipated measurement performance. A companion paper describes initial EUVS-C flight measurement performance.