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- Title
Solar Extreme Ultraviolet Irradiance Uncertainties for Planetary Studies.
- Authors
Thiemann, E. M. B.; Eparvier, F. G.; Knoer, V.; Al Muharrami, Abdulla; Lillis, R. J.
- Abstract
Accurate 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-a 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 phaseshifting error, eps, measured at SXR and Lyman-a 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 eps 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. Plain Language Summary Solar extreme ultraviolet (EUV) radiation is the major source of energy to the upper atmospheres of the planets, and varies with time and location on the Sun's surface. This study uses direct measurements of EUV radiation made by the Mars Atmosphere and Volatile Evolution probe at Mars to measure the accuracy of approximation techniques for estimating solar EUV radiation at other planets. This study finds that estimates of EUV radiation induced change in planetary atmospheres may be inaccurate by 6% of its average value, leading to nearly a factor of two inaccuracy compared to typical 30 day changes.
- Subjects
EXTREME Ultraviolet Explorer Satellite; PLANETARY atmospheres; MARTIAN atmosphere; SOLAR system; PLANETARY systems
- Publication
Journal of Geophysical Research. Space Physics, 2021, Vol 126, Issue 1, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2020JA028184