GOES-R solar extreme-ultraviolet irradiance: requirements, observations, and products

High cadence and high resolution solar spectra are needed as inputs for models of the Earth's upper atmosphere. Solar extreme ultraviolet (EUV; 10-122 nm) irradiance heats the thermosphere and creates the ionosphere. These processes impact satellite drag, communications and navigation systems. EUV irradiance (and x-ray emissions) vary on time scales ranging from the ~11-year solar cycle to the 27-day solar rotation period to hours and minutes for solar flares. It is important to monitor the variability of the solar EUV spectra for thermosphere / ionosphere models which are used for a myriad of space weather applications. The Extreme Ultraviolet and X-ray Irradiance Sensors (EXIS) aboard the future GOES-R series of satellites will produce improved measurements of EUV spectra. EXIS is composed of the X-ray Sensor (XRS) and the Extreme Ultraviolet Sensor (EUVS) and is designed and built by the Laboratory for Space and Atmospheric Physics (LASP) at the University of Colorado.

The layers in the solar atmosphere, namely the photosphere, chromosphere, transition region and corona, each have different compositions, characteristic temperatures and corresponding emissions. The EXIS design is based on observations that the solar EUV spectrum is a combination of emissions from the chromosphere to the corona. A full EUV spectrum can be recreated from a set of sparse measurements which are representative of each of the layers. The GOES-R EUV spectral algorithm is being developed at LASP and will reconstruct full EUV spectra from 5 to 127 nm from 10 narrowband EXIS measurements. It will fulfill the NOAA requirements for the output spectra of 5-nm resolution, 20% accuracy, 2% precision, and 1-minute cadence. The equation to create the spectra will be a non-linear combination of measurements, with each wavelength having a unique set of coefficients representing the relative contribution from each layer of the solar atmosphere. The coefficients and errors will be determined from studies using long-term satellite data. We will also investigate how well the same technique can be used to generate other EUV spectral products that are tailored to work as inputs to particular atmospheric models.