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

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 and 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 allow the radio emission to escape from the planets.