期刊
ASTROPHYSICAL JOURNAL LETTERS
卷 857, 期 2, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.3847/2041-8213/aabcc6
关键词
planets and satellites: atmospheres; planets and satellites: individual (Mars)
资金
- National Science Foundation of China [41525015, 41774186, 41525016]
- Science and Technology Development Fund of Macau SAR [039/2013/A2, 119/2017/A3]
- NASA MAVEN project through the Mars Exploration Program
- STFC consolidated grants [ST/K000977/1, ST/N000722/1]
Combining the Mars Atmosphere and Volatile Evolution measurements of neutral atmospheric density, solar EUV/X-ray flux, and differential photoelectron intensity made during 240 nominal orbits, we calculate the ionization efficiency, defined as the ratio of the secondary (photoelectron impact) ionization rate to the primary (photon impact) ionization rate, in the dayside Martian upper atmosphere under a range of solar illumination conditions. Both the CO2 and O ionization efficiencies tend to be constant from 160. km up to 250. km, with respective median values of 0.19 +/- 0.03 and 0.27 +/- 0.04. These values are useful for fast calculation of the ionization rate in the dayside Martian upper atmosphere, without the need to construct photoelectron transport models. No substantial diurnal and solar cycle variations can be identified, except for a marginal trend of reduced ionization efficiency approaching the terminator. These observations are favorably interpreted by a simple scenario with ionization efficiencies, as a first approximation, determined by a comparison between relevant cross sections. Our analysis further reveals a connection between regions with strong crustal magnetic fields and regions with high ionization efficiencies, which are likely indicative of more efficient vertical transport of photoelectrons near magnetic anomalies.
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