The role of partial ionization effects in the chromosphere

Abstract

The energy for the coronal heating must be provided from the convection zone. However, the amount and the method by which this energy is transferred into the corona depend on the properties of the lower atmosphere and the corona itself. We review: (i) how the energy could be built in the lower solar atmosphere, (ii) how this energy is transferred through the solar atmosphere, and (iii) how the energy is finally dissipated in the chromosphere and/or corona. Any mechanism of energy transport has to deal with the various physical processes in the lower atmosphere. We will focus on a physical process that seems to be highly important in the chromosphere and not deeply studied until recently: the ion-neutral interaction effects in the chromosphere. We review the relevance and the role of the partial ionization in the chromosphere and show that this process actually impacts considerably the outer solar atmosphere. We include analysis of our 2.5D radiative magnetohydrodynamic simulations with the Bifrost code (Gudiksen et al. 2011 Astron. Astrophys. 531, A154 (doi:10.1051/0004-6361/201116520)) including the partial ionization effects on the chromosphere and corona and thermal conduction along magnetic field lines. The photosphere, chromosphere and transition region are partially ionized and the interaction between ionized particles and neutral particles has important consequences on the magneto-thermodynamics of these layers. The partial ionization effects are treated using generalized Ohm's law, i.e. we consider the Hall term and the ambipolar diffusion (Pedersen dissipation) in the induction equation. The interaction between the different species affects the modelled atmosphere as follows: (i) the ambipolar diffusion dissipates magnetic energy and increases the minimum temperature in the chromosphere and (ii) the upper chromosphere may get heated and expanded over a greater range of heights. These processes reveal appreciable differences between the modelled atmospheres of simulations with and without ion-neutral interaction effects.

Keywords: atmosphere; magnetic field; magnetohydrodynamics; methods; numerical.

Figure 1.

Maps of ohmic diffusion (a), Hall term (b) and ambipolar diffusion (c) from a snapshot of a 2D radiative MHD simulation that includes partial ionization effects. The colour scheme is on a logarithmic scale. The magnetic field is drawn with white lines in (a).

Figure 2.

Temperature maps for the NGOL simulation (a), GOL-OS (b) and GOL-F (c) reveal differences in the thermal properties. The temperature is shown on a logarithmic scale.

Figure 3.

Joint probability density function of temperature (vertical axis) and density (horizontal axis) integrated over 30 min for simulations NGOL (a), GOL-OS (b) and GOL-F (c). The white contours correspond to the temperature and density regime of the simulation NGOL in dotted line, GOL-OS in solid line and GOL-F in dashed line in order to simplify the comparison between the simulations. The circles in (b) enhance the differences between the various simulations.