title: Magnetic moment and plasma environment of HD 209458b as determined from Lyα observations.
abstract: Transit observations of HD 209458b in the stellar Lyman-α(Lyα) line revealed strong absorption in both blue and red wings of the line interpreted as hydrogen atoms escaping from the planet's exosphere at high velocities. The following sources for the absorption were suggested: acceleration by the stellar radiation pressure, natural spectral line broadening, or charge exchange with the stellar wind. We reproduced the observation by means of modeling that includes all aforementioned processes. Our results support a stellar wind with a velocity of ≈400 kilometers per second at the time of the observation and a planetary magnetic moment of ≈1.6 × 10(26) amperes per square meter.
authors.affiliation: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria. kristina.kislyakova@oeaw.ac.at.
authors.forename: Kristina G
authors.initials: KG
authors.lastname: Kislyakova
authors.affiliation: Swedish Institute of Space Physics, PO Box 812, SE-98128 Kiruna, Sweden.
authors.forename: Mats
authors.initials: M
authors.lastname: Holmström
authors.affiliation: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria.
authors.forename: Helmut
authors.initials: H
authors.lastname: Lammer
authors.affiliation: Institute of Physics, University of Graz, Universitätsplatz 5, A-8010 Graz, Austria.
authors.forename: Petra
authors.initials: P
authors.lastname: Odert
authors.affiliation: Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, A-8042 Graz, Austria. Skobeltsyn Institute of Nuclear Physics, Moscow State University, Leninskie Gory, 119992 Moscow, Russia.
authors.forename: Maxim L
authors.initials: ML
authors.lastname: Khodachenko