Alfvén ionization in exoplanetary atmospheres

Stark, C.R., Helling, C., Diver, D.A. and Rimmer, P.B. (2013) Alfvén ionization in exoplanetary atmospheres. In: European Planetary Science Congress 2013, London, UK, 8-13 Sep 2013, p. 706.

Abstract

Observations of continuous radio and sporadic X-ray emission from low-mass objects suggest such objects harbour an atmospheric, localized plasma. For lowmass objects, the degree of thermal ionization is insufficient to qualify the ionized gas as a plasma, posing the question: what ionization processes can efficiently produce the required plasma? We propose Alfvén ionization as a simple mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficiently large degrees of ionization (≥ 10^-7) that they constitute plasmas. We outline the criteria required for Alfvén ionization to occur and justify it's applicability in the atmospheres of low-mass objects such as giant gas planets, brown dwarfs and M-dwarfs for both solar and sub-solar metallicities. We find that Alfvén ionization is most efficient at mid to low atmospheric pressures where a seed plasma is easier to magnetize and the pressure gradients needed to drive the required neutral flows are the smallest. For the model atmospheres considered, our results show that degrees of ionization ranging from 10^-6-1 can be obtained. Observable consequences include continuum Bremsstrahlung emission, superimposed with spectral lines from the plasma ion species (e.g. He, Mg, H2 or CO lines). Forbidden lines are also expected from the metastable population as a consequence of the Penning Effect. The presence of an atmospheric plasma opens the door to a multitude of plasma and chemical processes not yet considered in current atmospheric models.

• Item Type: Conference Proceedings
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Diver, Professor Declan
• Authors: Stark, C.R., Helling, C., Diver, D.A., and Rimmer, P.B.
• College/School: College of Science and Engineering > School of Physics and Astronomy