The Flare Irradiance Spectral Model (FISM) is an important tool for estimating solar variability for a myriad of space weather research studies and applications, and FISM Version 2 (FISM2) has recently been been released. FISM2 is an empirical model of the solar ultraviolet irradiance created to fill spectral and temporal gaps in the measurements, where these measurements are infrequent as they need to be made from space due to their absorption in the planetary atmospheres. FISM2 estimates solar ultraviolet irradiance variations due to solar cycle, solar rotation, and solar flare variations. The major improvement provided by FISM2 is that it is based on multiple new, more accurate instruments that have now captured almost a full solar cycle and thousands of flares, drastically improving the accuracy of the modeled FISM2 solar irradiance spectra. FISM2 is also improved to 0.1 nm spectral bins across the same 0-190 nm spectral range, and is already being used in research to estimate space weather changes due to solar irradiance variability in planetary thermospheres and ionospheres.

The MAVEN/EUVM solar soft x-ray (SXR) and Lyman-α measurements are compared with analogous measurements made from Earth to characterize the typical error introduced when phase-shifting solar EUV irradiance measurements made from Earth to other points in the solar system according to the 27.27 day synodic solar rotation period. The phase-shifting error, ε, measured at SXR and Lyman-α are extrapolated to the full EUV spectrum by assuming it is proportional to the variability that occurs over the 27-day timescale of solar rotation. Values for ε as a function of wavelength are reported and used to find the typical error for estimates of photoionization frequencies of some major species found in planetary upper atmospheres derived by phase-shifted EUV irradiance. This study finds that the typical extrapolation error for the CO photoionization frequency is 5.7% of the solar cycle mean value, and 87% of the typical 27-day variability.