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The signs of an expanding atmosphere of HD209458b have been observed withfar-ultraviolet transmission spectroscopy and in the measurements of transitabsorption by metastable HeI. These observations are interpreted using thehydrodynamic and Monte-Carlo numerical simulations of various degree ofcomplexity and consistency. At the same time, no attempt has been made to modelatmospheric escape of a magnetized HD209458b, to see how the planetary magneticfield might affect the measured transit absorption lines. This paper presentsthe global 3D MHD self-consistent simulations of the expanding upper atmosphereof HD209458b interacting with the stellar wind, and models the observed HI(Lya), OI (1306 A), CII (1337 A), and HeI (10830 A) transit absorptionfeatures. We find that the planetary dipole magnetic field with the equatorialsurface value of Bp = 1 G profoundly changes the character of atmosphericmaterial outflow and the related absorption. We also investigate the formationof planetary magnetosphere in the stellar wind and show that its size is moredetermined by the escaping atmosphere flow rather than by the strength ofmagnetic field. Fitting of the simulation results to observations enablesconstraining the stellar XUV flux and He abundance at ~10 erg cm2/s (at 1 a.u.)and He/H=0.02, respectively, as well as setting an upper limit for the dipolemagnetic field of Bp<0.1 G on the planetary surface at the equator. Thisimplies that the magnetic dipole moment of HD209458b should be less than 0.06of the Jovian value.

The impact of intrinsic magnetic field on the absorption signatures of elements probing the upper atmosphere of HD209458b