Abstract
During a planetary transit, atoms with high atomic number absorb short-wavelength radiation in the upper atmosphere, and the planet should appear larger during a primary transit observed in high-energy bands than in the optical band. Here we measure the radius of Venus with subpixel accuracy during the transit in 2012 observed in the optical, ultraviolet and soft X-rays with Hinode and Solar Dynamics Observatory missions. We find that, while Venus's optical radius is about 80 km larger than the solid body radius (the top of clouds and haze), the radius increases further by >70 km in the extreme ultraviolet and soft X-rays. This measures the altitude of the densest ion layers of Venus's ionosphere (CO2 and CO), useful for planning missions in situ, and a benchmark case for detecting transits of exoplanets in high-energy bands with future missions, such as the ESA Athena.
Highlights
During a planetary transit, atoms with high atomic number absorb short-wavelength radiation in the upper atmosphere, and the planet should appear larger during a primary transit observed in high-energy bands than in the optical band
We describe how observations of this last transit made by space missions at ultraviolet, extreme ultraviolet and X-ray wavelengths give us new insight into the upper atmosphere of the planet
While the optical radius of B6,130 km is in agreement with previous measurements, the values retrieved in the extreme ultraviolet (B100–300 Å) and X-ray bands (B10 Å) are significantly larger than the optical radius by 70–100 km, while the altitude in the ultraviolet band (B1,500 Å) is in between (40–50 km)
Summary
Atoms with high atomic number absorb short-wavelength radiation in the upper atmosphere, and the planet should appear larger during a primary transit observed in high-energy bands than in the optical band. We measure the radius of Venus with subpixel accuracy during the transit in 2012 observed in the optical, ultraviolet and soft X-rays with Hinode and Solar Dynamics Observatory missions. While Venus’s optical radius is about 80 km larger than the solid body radius (the top of clouds and haze), the radius increases further by 470 km in the extreme ultraviolet and soft X-rays This measures the altitude of the densest ion layers of Venus’s ionosphere (CO2 and CO), useful for planning missions in situ, and a benchmark case for detecting transits of exoplanets in high-energy bands with future missions, such as the ESA Athena. We describe how observations of this last transit made by space missions at ultraviolet, extreme ultraviolet and X-ray wavelengths give us new insight into the upper atmosphere of the planet.
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