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

TL;DR

This study uses 3D MHD simulations to explore how a planetary magnetic field influences atmospheric escape and absorption signatures in HD209458b, providing new constraints on its magnetic field strength and stellar environment.

Contribution

It presents the first self-consistent 3D MHD modeling of HD209458b's magnetized atmosphere and its effects on observed transit absorption features.

Findings

Magnetic field significantly alters atmospheric outflow and absorption signatures.

The magnetosphere size is mainly determined by atmospheric escape flow, not magnetic strength.

Constraints on stellar XUV flux, helium abundance, and planetary magnetic field strength.

Abstract

The signs of an expanding atmosphere of HD209458b have been observed with far-ultraviolet transmission spectroscopy and in the measurements of transit absorption by metastable HeI. These observations are interpreted using the hydrodynamic and Monte-Carlo numerical simulations of various degree of complexity and consistency. At the same time, no attempt has been made to model atmospheric escape of a magnetized HD209458b, to see how the planetary magnetic field might affect the measured transit absorption lines. This paper presents the global 3D MHD self-consistent simulations of the expanding upper atmosphere of HD209458b interacting with the stellar wind, and models the observed HI (Lya), OI (1306 A), CII (1337 A), and HeI (10830 A) transit absorption features. We find that the planetary dipole magnetic field with the equatorial surface value of Bp = 1 G profoundly changes the character of atmospheric material outflow and the related absorption. We also investigate the formation of planetary magnetosphere in the stellar wind and show that its size is more determined by the escaping atmosphere flow rather than by the strength of magnetic field. Fitting of the simulation results to observations enables constraining 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 dipole magnetic field of Bp<0.1 G on the planetary surface at the equator. This implies that the magnetic dipole moment of HD209458b should be less than 0.06 of the Jovian value.