Non ideal Transport Processes in the Solar Atmosphere

TL;DR

This paper develops a comprehensive framework for transport in a partially ionized solar atmosphere composed mainly of hydrogen and helium, incorporating both resonant and nonresonant ion–neutral collisions. By deriving parallel, perpendicular, and cross transport coefficients for viscosity, conductivity, and diffusion, it shows that neutrals dominate thermal conduction and viscous transport in the photosphere and chromosphere, while ions become important mainly in the transition region. The study reveals that resonant charge exchange significantly reduces the ion Hall parameter compared to nonresonant estimates, alters diffusion regimes (Ohm in the lower atmosphere, ambipolar higher up, with Hall in between), and highlights the critical role of neutral-driven thermal diffusion as a potentially major source of chromospheric heating. These findings imply that accurate chromospheric and transition-region models must include perpendicular transport and neutral-ion coupling to capture wave damping and heating processes. The work also cautions against uncritically applying Braginskii-type formulas to partially ionized plasmas and provides a physically motivated set of transport coefficients tailored to solar conditions.

Abstract

Transport coefficients are calculated for a partially ionized plasma consisting of approximately 90% hydrogen and 10\% helium, representative of a model solar atmosphere with an assumed magnetic field profile. The ion Hall parameter, defined as the ratio of ion cyclotron to ion collision frequency, is determined by considering dominant resonance charge exchange processes alongside less significant nonresonant ion neutral collisions. Based on these calculations, we derive profiles for various transport coefficients. Our results demonstrate that thermal conductivity in partially ionized media, both parallel and perpendicular to the ambient magnetic field, is dominated by neutral particles. The perpendicular thermal conductivity components show weak dependence on the ion Hall parameter and remain comparable in magnitude to their parallel counterparts. Wave damping through neutral thermal conductivity may contribute significantly to solar atmospheric heating. These findings indicate that perpendicular thermal conductivity components are essential for accurate modelling of partially ionized regions, including photosphere-chromosphere transition layers, spicules, and coronal prominences.

Figures (11)

• Figure 1: The fractional ionization as a function of height is plotted for model C of F93 in the figure above.
• Figure 2: Nonresonant and resonant interactions in the target rest frame for $\hbox{H}^+-\hbox{H}$. (a) Nonresonant interaction arising from long-range polarization forces-distant collisions that dominant at low relative particle speed (i.e., low temperatures); (b) resonant interaction resulting from electron transfer-close collisions that dominant at high speed (i.e., high temperatures).
• Figure 3: Ratio of collision frequencies as a function of temperature for hydrogen atoms is shown in the figure above. Labels R and P corresponds to resonant (R) and polarization/nonresonant (P) frequencies, respectively.
• Figure 4: Height variation of the collision frequency ratio for hydrogen atoms in model C of F93.
• Figure 5: The height variation of the ion-Hall ($\beta_{i}$) and viscous-Hall ($\beta_{i,\nu}$) parameters are displayed in the above panels.