Solar Extreme Ultraviolet Irradiance Uncertainties for Planetary Studies

Abstract

AbstractAccurate estimates of extreme ultraviolet (EUV) irradiance are needed in order to understand the dynamical, chemical, and plasma processes occurring in planetary upper and tenuous atmospheres. Most studies 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) instrument to measure the solar EUV irradiance in situ at Mars, which is used in this study to quantify the error introduced when phase‐shifting EUV measurements from Earth to other locations in the solar system. 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, εps, measured at SXR and Lyman‐α wavelengths 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 εps 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. Measuring EUV irradiance in situ reduces the random uncertainty by approximately half of that expected from phase shifting irradiances to the point of interest from Earth. These findings indicate that estimates of EUV induced variability in planetary atmospheres are highly uncertain at timescales of ∼10 days for large phase angles.