title: Using the transit of Venus to probe the upper planetary atmosphere.
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.
authors.affiliation: 1] Dipartimento di Fisica e Chimica, Università di Palermo, Piazza del Parlamento 1, Palermo 90134, Italy [2] INAF/Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, Palermo 90134, Italy.
authors.forename: Fabio
authors.initials: F
authors.lastname: Reale
authors.affiliation: Dipartimento di Fisica e Chimica, Università di Palermo, Piazza del Parlamento 1, Palermo 90134, Italy.
authors.forename: Angelo F
authors.initials: AF
authors.lastname: Gambino
authors.affiliation: INAF/Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, Palermo 90134, Italy.
authors.forename: Giuseppina
authors.initials: G
authors.lastname: Micela
authors.affiliation: INAF/Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, Palermo 90134, Italy.
authors.forename: Antonio
authors.initials: A
authors.lastname: Maggio
authors.affiliation: Universitè de Versailles-Saint-Quentin, ESR/DYPAC EA 2449, Observatoire de Paris, LESIA, UMR CNRS 8109, 5 Place Jules-Janssen, Meudon 92190, France.
authors.forename: Thomas
authors.initials: T
authors.lastname: Widemann
authors.affiliation: INAF-IAPS (Istituto di Astrofisica e Planetologia Spaziali), via del Fosso del Cavaliere 100, Roma 00133, Italy.
authors.forename: Giuseppe
authors.initials: G
authors.lastname: Piccioni