Why an intrinsic magnetic field does not protect a planet against atmospheric escape

Abstract

The presence or absence of a magnetic field determines the nature of how a planet interacts with the solar wind and what paths are available for atmospheric escape. Magnetospheres form both around magnetised planets, such as Earth, and unmagnetised planets, like Mars and Venus, but it has been suggested that magnetised planets are better protected against atmospheric loss. However, the observed mass escape rates from these three planets are similar (in the approximate (0.5−2) kg s −1 range), putting this latter hypothesis into question. Modelling the effects of a planetary magnetic field on the major atmospheric escape processes, we show that the escape rate can be higher for magnetised planets over a wide range of magnetisations due to escape of ions through the polar caps and cusps. Therefore, contrary to what has previously been believed, magnetisation is not a sufficient condition for protecting a planet from atmospheric loss. Estimates of the atmospheric escape rates from exoplanets must therefore address all escape processes and their dependence on the planet's magnetisation.

Planetary magnetic field control of ion escape from weakly magnetized planets

“…An analytic study by Gunell et al (2018) showed a similar peak in ion escape rates at the point where the standoff radius reaches the IMB; however, their model enforces a sharp transition in escape processes at this point rather than a gradual build up. This study also uses the typical assumption of linear dependence of ion escape with the polar cap angle, whereas we find a stronger dependence.…”

“…Global hybrid modeling work from Kallio and Barabash (2012) looked at ion escape from a Mars type planet with a global dipole of 0, 10, 30, and 60 nT, and found maximum ion escape from the 10 nT case. Gunell et al (2018) combined empirical measurements at Earth, Mars, and Venus with semi-analytic models to analyze the influence of planetary magnetic field over a large range of magnetic field values, finding multiple peaks in escape rate over the range that varied in strength and value by planet. Sakai et al (2018) found an increased escape rate for a weakly magnetized Mars (100 nT) compared to an unmagnetized Mars in a multi-species MHD model.…”

Stellar proton event-induced surface radiation dose as a constraint on the habitability of terrestrial exoplanets

“…The dose is reduced by a factor of about 30 corresponding to an increase in the magnetospheric strength by an order of magnitude. However, it should be noted that planetary magnetic field is crucial in maintaining a substantial atmosphere on a planet (Jakosky et al 2015;Grießmeier et al 2004;Khodachenko et al 2007;Lammer et al 2007;Driscoll & Bercovici 2013;Dong et al 2017;Gunell et al 2018). Although recent observations have given us good measurements of flaring rates of nearby stars (Maehara et al 2012;Davenport 2016;Notsu et al 2019), the main source uncertainty in this work is the lack in measurement of particles ejected by high-energy flares (10 32 -10 36 ergs) on other stars.…”

Erosion of an exoplanetary atmosphere caused by stellar winds

“…On the other hand, it has been found that the atmospheric mass escape rate occurs in two areas. The first corresponds to polar cap escape and is dominant for hydrogen, while the second area is dominated by cusp escape (Gunell et al 2018). In the polar cap of a magnetized planet, magnetic field lines are opened, which allows the penetration of charged particles that are transported by the stellar wind, including cosmic rays, which Article number, page 1 of 14 arXiv:1908.06695v1 [astro-ph.EP] 19 Aug 2019 A&A proofs: manuscript no.…”