Solar EUV and Soft X-Ray Irradiance: Measurements, Insights, and Open Questions

The solar EUV and soft X-ray spectral irradiance is a primary energy input into the upper atmospheres of Earth and other planets. We’ve come a long way in our understanding of the absolute value and variability of the solar output at these wavelengths. Observations by TIMED-SEE, SORCE-XPS, SDO-EVE, MAVEN-EUVM, MinXSS, DAXSS, GOES-EXIS, and other missions, all with different wavelength, resolution, cadence, and temporal coverage, have closed many of the gaps in our knowledge of the EUV. Results from these missions have been incorporated into the latest irradiance models such as (but not limited to) FISM, the GOES EUVS spectral model, the SORCE XPS model, and their progeny. No longer are we limited to using reference spectra or simple scaling by single solar activity indices. Yet these observational and modeling efforts have also exposed areas where we still don’t capture the full structure and variability of this important quantity vital to understanding upper atmospheres. This presentation will be an overview of the data and key results from these missions and models followed by a discussion of their limitations and a summary of what still needs to be addressed.