Science Seminars

Probing Atmospheres with EUV Irradiance, from the Solar Corona to the Thermospheres of Earth and Mars

Speaker: Ed Thiemann (LASP)
Date: Thursday, Nov 09, 2017
Time: 4:00 PM
Location: SPSC N100

Seminar Abstract:

Solar extreme ultraviolet (EUV) radiation spans the wavelength range from 10 to 120 nm. The EUV spectrum is dominated by atomic line emissions from some of the hottest plasma in the Solar System, the intensity of which is dependent on the temperature and density of the source regions, as well as the surrounding atmosphere through which it propagates as it leaves the Sun. Once out in the Solar System, EUV radiation ionizes and dissociates the major species of planetary atmospheres, leading to the total absorption of incident EUV radiation in the upper atmospheres of the terrestrial planets: Earth, Mars and Venus. It follows that given the appropriate conditions and geometry, EUV radiation can be used to probe regions of the atmospheres of the Sun and planets that are otherwise difficult to measure.
This seminar presents new results found from using EUV irradiance measurements to probe the solar corona and the thermospheres of Earth and Mars. Although the regions of study are drastically different, similar radiative transfer methods are used to characterize densities in both by characterizing the observed spectral absorption. At the Sun, it is shown that the generally valid assumption of solar coronal emissions being optically thin, breaks down during solar flares, resulting in limb darkening of flare emissions forming in the typically tenuous corona. Further, by looking at limb darkening statistics from hundreds of flares, the typical absorbing column density is determined. At Earth and Mars, the practice of using EUV solar occultations to probe the upper atmosphere is revived, enabled by a better understanding and modeling capability of solar EUV irradiance, and recent measurements by the NASA MAVEN EUVM and ESA PROBA2 LYRA missions. It is shown that full-disk, broadband EUV irradiance measurements are suitable for accurately characterizing these planets’ thermospheres. New extensive datasets of thermospheric neutral densities for both planets are presented, and interesting aspects of observed thermospheric variability are discussed.