Solar EUV Irradiance Uncertainties for Planetary Studies

Solar extreme ultraviolet (EUV) radiation is the primary energy input to the upper and tenuous atmospheres of most planetary bodies. As such, accurate estimates of EUV irradiance are needed in order to understand the dynamical, chemical and plasma processes occurring in these regions. Most missions sent to explore upper or tenuous planetary atmospheres are not instrumented to measure solar irradiance in-situ, and instead rely on irradiance measurements made at Earth, which are extrapolated to the location of interest. The Mars Atmosphere and Volatile EvolutioN (MAVEN) orbiter includes the Extreme Ultraviolet Monitor (EUVM) to measure the solar EUV irradiance in-situ at Mars. These in-situ calibrated measurements provide an opportunity to quantify the error introduced when phase-shifting EUV measurements from Earth to other locations in the solar system. The EUVM solar soft X-Ray 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 to other points in the solar system. The phase-shifting error, ε_ps, measured in the EUVM bands is extrapolated to the full EUV spectrum by assuming proportionality to the 27-day solar rotation variability. Values for ε_ps as a function of wavelength are reported and used to find the typical phase shift error for estimates of photoionization frequencies of major planetary atmospheric species. Measuring EUV irradiance in situ reduces the random uncertainty by approximately half of that expected from phase shifting irradiances from Earth. Considering the impact on CO₂ photoionization frequency, we show that the typical extrapolation error for the CO₂ photoionization frequency is 5.7% of the solar cycle mean value, and 87% of the typical 27-day variability. These findings indicate that estimates of EUV induced variability in planetary atmospheres are highly uncertain at timescales of ~10 days for large phase angles.