Effect of stellar wind induced magnetic fields on planetary obstacles of non-magnetized hot Jupiters

Abstract

We investigate the interaction between the magnetized stellar wind plasma and the partially ionized hydrodynamic hydrogen outflow from the escaping upper atmosphere of non-or weakly magnetized hot Jupiters. We use the well-studied hot Jupiter HD 209458b as an example for similar exoplanets, assuming a negligible intrinsic magnetic moment. For this planet, the stellar wind plasma interaction forms an obstacle in the planet's upper atmosphere, in which the position of the magnetopause is determined by the condition of pressure balance between the stellar wind and the expanded atmosphere, heated by the stellar extreme ultraviolet (EUV) radiation. We show that the neutral atmospheric atoms penetrate into the region dominated by the stellar wind, where they are ionized by photo-ionization and charge exchange, and then mixed with the stellar wind flow. Using a 3D magnetohydrodynamic (MHD) model, we show that an induced magnetic field forms in front of the planetary obstacle, which appears to be much stronger compared to those produced by the solar wind interaction with Venus and Mars. Depending on the stellar wind parameters, because of the induced magnetic field, the planetary obstacle can move up to ≈0.5-1 planetary radii closer to the planet. Finally, we discuss how estimations of the intrinsic magnetic moment of hot Jupiters can be inferred by coupling hydrodynamic upper planetary atmosphere and MHD stellar wind interaction models together with UV observations. In particular, we find that HD 209458b should likely have an intrinsic magnetic moment of 10-20% that of Jupiter.

Grid of upper atmosphere models for 1–40 M⊕ planets: application to CoRoT-7 b and HD 219134 b,c

“…Overall, the energy-limited formula reproduces well the escape rates obtained through detailed hydrodynamic upper atmosphere modelling, particularly for close-in gas giants with atmospheres in blow-off (e.g., Lammer et al 2009;Fossati et al 2015;Salz et al 2016;Erkaev et al 2016Erkaev et al , 2017. Because of its analytical form, hence allowing for fast computations, the vast majority of planetary evolution and population synthesis models employ the energy-and recombination-limited formalisms to model atmospheric escape for a wide range of planets subject to (very) different stellar irradiation levels (e.g., Jackson et al 2012;Batygin & Stevenson 2013;Jin et al 2014;Lopez & Fortney 2013;Owen & Wu 2017;Jin & Mordasini 2017;Lopez 2017).…”

Modeling the Lyαtransit absorption of the hot Jupiter HD 189733b

“…The position of the stellar wind stagnation point (R s ) is determined by the pressure balance condition, which means that the external stellar wind total pressure has to be equal to the momentum flux of the internal ionized atmospheric particles at the boundary. The ratio of the curvature radius of the obstacle to the distance between the stagnation point and the planetary center is taken as 1.3, similar to the value used by Erkaev et al (2017).…”

“…It is unknown if HD 189733b possesses an intrinsic magnetic field strong enough to affect its mass-loss. However, we do consider the generation of an induced magnetic field at the obstacle (magnetic barrier) in our MHD model (Erkaev et al 2017). An intrinsic planetary magnetic field may push the pressure balance distance with the stellar wind further out.…”

Transit Lyman-α signatures of terrestrial planets in the habitable zones of M dwarfs

“…Hydrogen-dominated atmospheres also expand if the XUV flux is increased. We always chose the inner boundary of our simulation, R ib , equal to the exobase height estimated in 1D simulations by Lammer et al (2008) for nitrogen-dominated atmospheres and the 1D model described in section 2.2 adopted from Erkaev et al (2017) for hydrogen-dominated atmospheres.…”