Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes^⋆

¹ Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
e-mail: desiveri@astro.uio.no
² Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315, Oslo, Norway
³ Instituto de Astrofisica de Canarias, Via Lactea, s/n, 38205 La Laguna, Tenerife, Spain
⁴ Department of Astrophysics, Universidad de La Laguna, 38200 La Laguna, Tenerife, Spain
⁵ Bay Area Environmental Research Institute, NASA Research Park, Moffett Field, CA 94952, USA
⁶ Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA 94304, USA

Received: 17 October 2019
Accepted: 27 November 2019

Abstract

Context. Magnetic flux emergence from the solar interior has been shown to be a key mechanism for unleashing a wide variety of phenomena. However, there are still open questions concerning the rise of the magnetized plasma through the atmosphere, mainly in the chromosphere, where the plasma departs from local thermodynamic equilibrium (LTE) and is partially ionized.

Aims. We aim to investigate the impact of the nonequilibrium (NEQ) ionization and recombination and molecule formation of hydrogen, as well as ambipolar diffusion, on the dynamics and thermodynamics of the flux emergence process.

Methods. Using the radiation-magnetohydrodynamic Bifrost code, we performed 2.5D numerical experiments of magnetic flux emergence from the convection zone up to the corona. The experiments include the NEQ ionization and recombination of atomic hydrogen, the NEQ formation and dissociation of H₂ molecules, and the ambipolar diffusion term of the generalized Ohm’s law.

Results. Our experiments show that the LTE assumption substantially underestimates the ionization fraction in most of the emerged region, leading to an artificial increase in the ambipolar diffusion and, therefore, in the heating and temperatures as compared to those found when taking the NEQ effects on the hydrogen ion population into account. We see that LTE also overestimates the number density of H₂ molecules within the emerged region, thus mistakenly magnifying the exothermic contribution of the H₂ molecule formation to the thermal energy during the flux emergence process. We find that the ambipolar diffusion does not significantly affect the amount of total unsigned emerged magnetic flux, but it is important in the shocks that cross the emerged region, heating the plasma on characteristic times ranging from 0.1 to 100 s. We also briefly discuss the importance of including elements heavier than hydrogen in the equation of state so as not to overestimate the role of ambipolar diffusion in the atmosphere.