Collisions, magnetization, and transport coefficients in the lower solar atmosphere

¹ R. Vandervaerenlaan 16/402, 3000 Leuven, Belgium
e-mail: jvranjes@yahoo.com
² Institute of Physics Belgrade, Pregrevica 118, 11080 Zemun, Serbia
³ Joint Institute of Computational Sciences, University of Tennessee, Oak Ridge, TN 37831-6173, USA
e-mail: pkrstic@utk.edu

Received: 15 November 2012
Accepted: 5 April 2013

Abstract

Context. The lower solar atmosphere is an intrinsically multi-component and collisional environment with electron and proton collision frequencies in the range 10⁸−10¹⁰ Hz, which may be considerably higher than the gyro-frequencies for both species. Collisions between different species are altitude dependent because of the variation in density and temperature of all species.

Aims. We aim to provide a reliable quantitative set of data for collision frequencies, magnetization, viscosity, and thermal conductivity for the most important species in the lower solar atmosphere. Having such data at hand is essential for any modeling that is aimed at describing realistic properties of the considered environment.

Methods. The relevant elastic and charge transfer cross sections in the considered range of collision energies are now accepted by the scientific community as known with unprecedented accuracy for the most important species that may be found in the lower solar atmosphere. These were previously calculated using a quantum-mechanical approach and were validated by laboratory measurements. Only with reliable collision data one can obtain accurate values for collision frequencies and coefficients of viscosity and thermal conductivity.

Results. We describe the altitude dependence of the parameters and the different physics of collisions between charged species, and between charged and neutral species. Regions of dominance of each type of collisions are clearly identified. We determine the layers within which either electrons or ions or both are unmagnetized. Protons are shown to be unmagnetized in the lower atmosphere in a layer that is at least 1000 km thick even for a kilo-Gauss magnetic field that decreases exponentially with altitude. In these layers the dynamics of charged species cannot be affected by the magnetic field, and this fact is used in our modeling. Viscosity and thermal conductivity coefficients are calculated for layers where ions are unmagnetized. We compare viscosity and friction and determine the regions of dominance of each of the phenomena.

Conclusions. We provide the most reliable quantitative values for most important parameters in the lower solar atmosphere to be used in analytical modeling and numerical simulations of various phenomena such as waves, transport and magnetization of particles, and the triggering mechanism of coronal mass ejections.