Ionization in Atmospheres of Brown Dwarfs and Extrasolar Planets. V. Alfvén Ionization

Abstract

Observations of continuous radio and sporadic X-ray emission from low-mass objects suggest they harbour localized plasmas in their atmospheric environments. For low-mass objects, the degree of thermal ionization is insufficient to qualify the ionized component as a plasma, posing the question: what ionization processes can efficiently produce the required plasma that is the source of the radiation? We propose Alfvén ionization as a mechanism for producing localized pockets of ionized gas in the atmosphere, having sufficient degrees of ionization (≥ 10 −7 ) that they constitute plasmas. We outline the criteria required for Alfvén ionization and demonstrate it's applicability in the atmospheres of low-mass objects such as giant gas planets, brown dwarfs and Mdwarfs 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 of 10 −6 − 1 can be obtained as a result of Alfvén ionization. Observable consequences include continuum Bremsstrahlung emission, superimposed with spectral lines from the plasma ion species (e.g. He, Mg, H 2 or CO lines). Forbidden lines are also expected from the metastable population. The presence of an atmospheric plasma opens the door to a multitude of plasma and chemical processes not yet considered in current atmospheric models. The occurrence of Alfvén ionization may also be applicable to other astrophysical environments such as protoplanetary disks.

Alfvén ionization in an MHD-gas interactions code

“…Now critical velocities are very large, of the order of several km/s, 16 but these are velocities frequently reached in laboratory experiments, 18 theoretically expected in Brown dwarf atmospheres 17 or observationally detected in the solar atmosphere. For example, Rubio 43 and Vitas et al 44 show supersonic flows in the solar photosphere that are easily in the velocity range required.…”

“…For example, the atmosphere of brown dwarfs and M-dwarfs have previously been shown to satisfy this condition for the majority of the extent of their atmosphere. 17 The solar photosphere, with a number density of %10 23 m À3 only requires a field greater than %10 À4 T, a number easily exceeded by magnetic features such as magnetic bright points (>0.1 T) 45 and sunspots (0.2-0.37 T). 46 …”

“…the flow direction, or is sufficiently weak then the AI mechanism is no longer observed. 17,36 • The plasma must initially have electron density large enough that the maximum electric potential (after all ions are removed from a Debye sphere) can accelerate electrons to the ionization energy of the neutral.…”

“…It is thought to play a role in the atmospheric composition and the ionization fraction of stellar atmospheres. 16,17 For a fluid composed of a single species, the velocity at which we would expect AI to take place is found by equating the ionization energy and kinetic energy. This defines a critical velocity, v c : the speed where the kinetic energy per gas molecule exceeds the ionization energy of that same gas 16 …”

“…27 It is also thought to be important in understanding the solar wind and its composition 28,29 and in the atmospheres of stars, both in the Sun 16 and in the cooler atmosphere of brown dwarfs. 17 Neither the limit of relative velocity past v c or the change in ionization fraction of a partially ionized plasma due to these conditions are replicated by any existing MHD codes. The inclusion of AI in an MHD framework allows the previously fixed ionization fraction to evolve in a nontransient fashion; the relative fraction of gas to plasma could always change dynamically as waves and bulk motions move through the gas-plasma mixture.…”