How expanded ionospheres of Hot Jupiters can prevent escape of radio emission generated by the cyclotron maser instability

Abstract

We present a study of plasma conditions in the atmospheres of the Hot Jupiters HD 209458b and HD 189733b and for an HD 209458b-like planet at orbit locations between 0.2-1 AU around a Sun-like star. We discuss how these conditions influence the radio emission we expect from their magnetospheres. We find that the environmental conditions are such that the cyclotron maser instability (CMI), the process responsible for the generation of radio waves at magnetic planets in the solar system, most likely will not operate at Hot Jupiters. Hydrodynamically expanding atmospheres possess extended ionospheres whose plasma densities within the magnetosphere are so large that the plasma frequency is much higher than the cyclotron frequency, which contradicts the condition for the production of radio emission and prevents the escape of radio waves from close-in exoplanets at distances <0.05 AU from a Sun-like host star. The upper atmosphere structure of gas giants around stars similar to the Sun changes between 0.2 and 0.5 AU from the hydrodynamic to a hydrostatic regime and this results in conditions similar to solar system planets with a region of depleted plasma between the exobase and the magnetopause where the plasma frequency can be lower than the cyclotron frequency. In such an environment, a beam of highly energetic electrons accelerated along the field lines towards the planet can produce radio emission. However, even if the CMI could operate the extended ionospheres of Hot Jupiters are too dense to let the radio emission escape from the planets.

Estimating the Magnetic Field Strength in Hot Jupiters

“…The five to ten fold higher magnetic field we predict from our analysis may be able to change the plasma conditions around HJs to such an extent that radio emission may escape. To quantify the changes, more analysis similar to Weber et al (2017) should be carried out for HJs with the higher magnetic fields we predict.…”

Interacting fields and flows: Magnetic hot Jupiters

“…The right‐hand sides of Equations and reduce down to constants such that the resultant units are in MHz. For the emission process to be efficient, this ratio needs to be ≳ 2.5 (Weber et al ). In the following section, this ratio for the circumplanetary material of both models and discussed in the context of observational emission will be shown.…”

Hot Jupiter accretion: 3D MHD simulations of star–planet–wind interaction

“…It is the FUV and X-ray heating of the upper exosphere that results in temperatures an order of magnitude higher (Lammer et al 2003) than the effective temperature. This enhancement of the exosphere temperature is directly responsible for the hydrodynamic atmospheric expansion seen in multiple HJs (Lammer et al 2003;Weber et al 2017;Sairam et al 2018). Enhanced mass-loss is not the only a consequence of absorbed stellar radiation.…”

“…Values of the order 50 G are predicted by Yadav & Thorngren (2017), an order of magnitude larger than Jupiter's magnetic field. However, such field strengths would be detectable due to MHz radio emission via the Electron Cyclotron Maser Instability (ECMI) (Weber et al 2017;Daley-Yates & Stevens 2017.…”

“…This energy input has been proposed as a means to amplify the ECMI process and has been previously thought to result in detectable emission (Stevens 2005;Zarka 2007). This has however recently been brought into doubt by Weber et al (2017); Daley-Yates & Stevens (2018).…”