Exoplanets in our Backyard 2020 (LPI Contrib. No. 2195) 3044.pdf

Incorporating Saturation Behavior of Magnetosphere-Ionosphere Interaction in Radio Emission Estimates for Extrasolar Planets. A. M. Sciola1, F. R. Toffoletto1, D. Alexander1, and A. O. Farrish1, 1Rice University De- partment of Physics and Astronomy, 6100 Main St., Houston, Texas 77005; [email protected].

Introduction: Coherent radio emission is ob- orbiting exoplanets are expected to experience, as it is served at each magnetized solar system planet, sug- estimated that these exoplanets would exist well gesting that it is a fundamental product of interaction within the saturation regime. between a magnetized planet and the stellar wind of Results: For the unsaturated regime that solar its host star and therefore should also be produced by system planets generally reside within, we find good magnetized exoplanets. The dominant emission mech- agreement between our model and the powers pre- anism is the Electron Cyclotron Maser Instability dicted by RBL. For select exoplanetary systems where (ECMI) whose frequency is that of the local electron the planet is expected to experience saturation, our gyrofrequency, meaning that detection of ECMI emis- model predicts a radio emission of one to several or- sion from an exoplanet would provide information on ders of magnitude less than that predicted by RBL, the magnitude of the exoplanetary magnetic field. with a strong dependence on the magnitude of the Such information is valuable in estimating parameters ionospheric conductance. Additionally we find that such as ionospheric escape rates and helps constrain while the energy transmission becomes more propor- planetary formation and dynamo models. tionally limited for closer orbital distances, the total It has been found that the power of the radio emis- emission power that our model predicts still increases sion produced at each magnetized solar system planet with decreasing orbital distance. is linearly proportional to the incident solar wind References: [1] Desch M. D. and Kaiser M. L. power for average solar wind conditions, which is (1984) Nature, 310, 755-757. [2] Zarka P. (1992) known as the Radiometric Bode’s Law (RBL) [1-4]. It ASR, 12, 99-115. [3] Zarka P. (1998) JGR: Planets, has been proposed that Hot Jupiters, Jupiter-like exo- 103, 20159-20194. [4] Zarka P. et al. (2001) Astro- planets which closely orbit their host star, are likely to physics and Space Science, 277, 293-300. [5] Kivel- experience an incident stellar wind power orders of son M. G. and Ridley A. J. (2008) J Geophys. Res., magnitude greater than that seen at Jupiter and there- 113, A05214. fore should produce radio emission orders of magni- tude greater than Jupiter’s, which should be detectable by current radio telescopes in extreme cases. Despite this prediction, to date no such exoplanetary radio emission has been observed. The linear relationship of the RBL suggests that the processes which convert stellar wind energy into emission energy in the planet’s ionosphere all scale linearly with the stellar wind. On the contrary, it is found at Earth that the ionospheric potential, the mag- nitude of which is determined in part by the motional electric field of the solar wind, saturates for large val- ues of the magnetic field carried by the solar wind. Kivelson and Ridley (2008) (KR08 hereafter) sug- gested that Alfvén wave reflection due to the mis- match between the solar wind Alfvén conductance and ionospheric Pedersen conductance explains this satu- ration behavior [5]. Method: We adopt the wave reflection formalism of KR08 to quantify the limiting of energy which is transferred from the solar wind to the planetary iono- sphere, where we then assume an efficiency factor at which a proportion of the transmitted energy is con- verted to ECMI emission. While similar in form to RBL, this approach more readily accounts for the ex- treme conditions that a majority of the known closely-