Electrodynamics on extrasolar giant planets

TL;DR

This paper investigates the electrodynamics of close-in extrasolar giant planets, revealing that photoionization dominates their upper atmospheres, leading to high electron densities and significant ion drag effects above 10 mbar, with implications for their energy balance.

Contribution

It introduces a comprehensive model of ionization and electrodynamics in exoplanet atmospheres, accounting for photoionization and generalized Ohm's law, improving upon previous thermal ionization-based models.

Findings

Photoionization dominates upper atmospheres above 100 mbar.

Electron densities are higher than in any solar system planetary ionosphere.

Ion drag significantly affects energy balance above 10 mbar.

Abstract

Strong ionization on close-in extrasolar giant planets suggests that their atmospheres may be affected by ion drag and resistive heating arising from wind-driven electrodynamics. Recent models of ion drag on these planets, however, are based on thermal ionization only and do not include the upper atmosphere above the 1 mbar level. These models are also based on simplified equations of resistive MHD that are not always valid in extrasolar planet atmospheres. We show that photoionization dominates over thermal ionization over much of the dayside atmosphere above the 100 mbar level, creating an upper ionosphere dominated by ionization of H and He and a lower ionosphere dominated by ionization of metals such as Na, K, and Mg. The resulting dayside electron densities on close-in exoplanets are higher than those encountered in any planetary ionosphere of the solar system, and the conductivities are comparable to the chromosphere of the Sun. Based on these results and assumed magnetic fields, we constrain the conductivity regimes on close-in EGPs and use a generalized Ohm's law to study the basic effects of electrodynamics in their atmospheres. We find that ion drag is important above the 10 mbar level where it can also significantly alter the energy balance through resistive heating. Due to frequent collisions of the electrons and ions with the neutral atmosphere, however, ion drag is largely negligible in the lower atmosphere below the 10 mbar level for a reasonable range of planetary magnetic moments [abridged].