Heating up the upper atmosphere of close-in planets due to external time-varying magnetic fields

Exoplanets on close-in orbits are subject to intense X-ray and ultraviolet (XUV) radiation from their host stars, which can lead to significant atmospheric heating and even thermal escape. However, XUV is not the sole source of energy deposition in these atmospheres. Ohmic heating can also be acting in the upper atmosphere of exoplanets, and has been underlooked so far. 

 

Indeed, close-in exoplanets orbit around their host star by experiencing the influence of the stellar wind, where variations in the ambient magnetic field can induce electric currents in their upper atmosphere. These electric currents can dissipate into heat, depending on the atmosphere conductive properties, via a process known as Ohmic dissipation. We have developed a simplified formalism to quantify this Ohmic heating, and assess its significance compared to ‘classical' XUV heating. We have applied our formalism to idealised atmospheric profiles, as well as to cutting edge photochemical models of Trappist-1 b and π Men c. Our results show that Ohmic heating strongly depends on both the shape and strength of the conductivity profile in the upper atmospheres. In the most extreme cases, we show that Ohmic heating can reach up to 10−3 erg s−1 cm−3, i.e. volumetric heating rates comparable to and even surpassing standard photochemical heating rates.

 

These findings suggest that Ohmic heating could significantly affect the thermal evolution and atmospheric escape processes of hot exoplanets. In addition, this strong heating is associated with a screening of the external time-varying field. We identify the parameter regimes where the upper atmosphere can act as a shield that prevents external magnetic fields from penetrating deeper into the atmosphere or the planet's interior, thereby diminishing the potential magnetic coupling of deep atmospheres with external magnetic transients.