The effects of magnetic fields on observational signatures of atmospheric escape in exoplanets: Double tail structures

Abstract

Using 3D radiative MHD simulations and Lyman-α transit calculations, we investigate the effect of magnetic fields on the observational signatures of atmospheric escape in exoplanets. Using the same stellar wind, we vary the planet’s dipole field strength (Bp) from 0 to 10G. For Bp < 3G, the structure of the escaping atmosphere begins to break away from a comet-like tail following the planet (Bp = 0), as we see more absorbing material above and below the orbital plane. For Bp ≥ 3G, we find a “dead-zone” around the equator, where low velocity material is trapped in the closed magnetic field lines. The dead-zone separates two polar outflows where absorbing material escapes along open field lines, leading to a double tail structure, above and below the orbital plane. We demonstrate that atmospheric escape in magnetised planets occurs through polar outflows, as opposed to the predominantly night-side escape in non-magnetised models. We find a small increase in escape rate with Bp, though this should not affect the timescale of atmospheric loss. As the size of the dead-zone increases with Bp, so does the line centre absorption in Lyman-α, as more low-velocity neutral hydrogen covers the stellar disc during transit. For Bp < 3G the absorption in the blue wing decreases, as the escaping atmosphere is less funnelled along the line of sight by the stellar wind. In the red wing (and for Bp > 3G in the blue wing) the absorption increases caused by the growing volume of the magnetosphere. Finally we show that transits below and above the mid-disc differ caused by the asymmetry of the double tail structure.

Possible origin of the non-detection of metastable He I in the upper atmosphere of the hot Jupiter WASP-80b

“…The stream behind the planet, also composed of planetary escaped gas, is a typical characteristic of close-in giant planets with an escaping atmosphere (e.g. Bourrier et al 2016;Esquivel et al 2019;Shaikhislamov et al 2018;McCann et al 2019;Debrecht et al 2020;Carolan et al 2021;MacLeod & Oklopčić 2022). Figure 3 shows the absorption profiles obtained from time averaging (see Dos in the −0.1 to +0.1 planetary orbital phase range -the same range taken into account to extract the observed transmission spectrum from the data (see Figure 1 of Fossati et al 2022)-as an example, considering a He/H abundance ratio of 0.01.…”

“…In regards to the polar regions, the simulations indicate that the planetary magnetic field reduces the outflow velocity, resulting in a decreased density over the poles (see Trammell et al 2014;Khodachenko et al 2015;Carolan et al 2021, for a detailed discussion on the effect of a planetary magnetic field on the outflow velocity at the polar regions). However, because of the lower velocity, the planetary outflowing gas is more strongly photoionised, resulting in larger electron densities, which lead to a larger population of metastable Hei atoms produced through recombination of Heii.…”

Mean field models of flux transport dynamo and meridional circulation in the Sun and stars

“…The global magnetic field distribution in the Sun and stars, its parity, and the structure of coronal magnetic fields are governed by the dynamo mechanism (Dash et al, 2023) and surface emergence and evolution of magnetic flux (Nandy et al, 2018;Kavanagh et al, 2021). The magnetic field topology in turn determines the global stellar magnetosphere and magnetized stellar wind (Réville et al, 2015;Vidotto et al, 2014b) that play critical roles in star-planet interactions (Das et al, 2019;Basak and Nandy, 2021;Carolan et al, 2021) and the forcing of (exo)planetary space environments (Nandy et al, 2021;Hazra et al, 2022). Also, the stellar magnetic cycle alters the total X-ray and EUV (XUV) radiation from host stars affecting exoplanetary atmospheres (Hazra et al, 2020).…”

A Synergic Strategy to Characterize the Habitability Conditions of Exoplanets Hosted by Solar-Type Stars

“…A direct measurement of the exoplanet magnetic moment is a difficult task. Different methods have been proposed, involving bowshock observations during transits (Vidotto et al, 2011), radio signature (Hess and Zarka, 2011) or atmospheric escape structure detections (Carolan et al, 2021). All proposed techniques have a bias towards giant planet magnetospheric detection.…”