Modeling the thermal conduction in the solar atmosphere with the code MANCHA3D

Thermal conductivity is one of the important mechanisms of heat transfer in the solar corona. In the limit of strongly magnetized plasma, it is typically modeled by Spitzer's expression where the heat flux is aligned with the magnetic field. This paper describes the implementation of the heat conduction into the code MANCHA3D with an aim of extending single-fluid MHD simulations from the upper convection zone into the solar corona. Two different schemes to model heat conduction are implemented: (1) a standard scheme where a parabolic term is added to the energy equation, and (2) a scheme where the hyperbolic heat flux equation is solved. The first scheme limits the time step due to the explicit integration of a parabolic term, which makes the simulations computationally expensive. The second scheme solves the limitations on the time step by artificially limiting the heat conduction speed to computationally manageable values. The validation of both schemes is carried out with standard tests in one, two, and three spatial dimensions. Furthermore, we implement the model for heat flux derived by Braginskii (1965) in its most general form, when the expression for the heat flux depends on the ratio of the collisional to cyclotron frequencies of the plasma, and, therefore on the magnetic field strength. Additionally, our implementation takes into account the heat conduction in parallel, perpendicular, and transverse directions, and provides the contributions from ions and electrons separately. The model also transitions smoothly between field-aligned conductivity and isotropic conductivity for regions with a low or null magnetic field. Finally, we present a two-dimensional test for heat conduction using realistic values of the solar atmosphere where we prove the robustness of the two schemes implemented.Comment: 11 pages, 8 figure

Fast electrochemical doping due to front instability in organic semiconductors

The electrochemical doping transformation in organic semiconductor devices is studied in application to light-emitting cells. It is shown that the device performance can be significantly improved by utilizing new fundamental properties of the doping process. We obtain an instability, which distorts the doping fronts and increases the doping rate considerably. We explain the physical mechanism of the instability, develop theory, provide experimental evidence, and perform numerical simulations. We further show how improved device design can amplify the instability thus leading to a much faster doping process and device kinetics.Comment: 4 pages, 4 figure

Ultra-fast spin avalanches in crystals of molecular magnets in terms of magnetic detonation

Recent experiments (Decelle et al., Phys. Rev. Lett. 102, 027203 (2009)) discovered an ultra-fast regime of spin avalanches in crystals of magnetic magnets, which was three orders of magnitude faster than the traditionally studied magnetic deflagration. The new regime has been hypothetically identified as magnetic detonation. Here we demonstrate the possibility of magnetic detonation in the crystals, as a front consisting of a leading shock and a zone of Zeeman energy release. We study the dependence of the magnetic detonation parameters on the applied magnetic field. We find that the magnetic detonation speed only slightly exceeds the sound speed in agreement with the experimental observations.Comment: 4 pages, 4 figure

Evolution of the magnetic field generated by the Kelvin-Helmholtz instability

The Kelvin-Helmholtz instability in an ionized plasma is studied with a focus on the magnetic field generation via the Biermann battery (baroclinic) mechanism. The problem is solved by using direct numerical simulations of two counter-directed flows in 2D geometry. The simulations demonstrate the formation of eddies and their further interaction and merging resulting in a large single vortex. In contrast to general belief, it is found that the instability generated magnetic field may exhibit significantly different structures from the vorticity field, despite the mathematically identical equations controlling the magnetic field and vorticity evolution. At later stages of the nonlinear instability development, the magnetic field may keep growing even after the hydrodynamic vortex strength has reached its maximum and started decaying due to dissipation

A model for the dynamics and internal structure of planar doping fronts in organic semiconductors

The dynamics and internal structure of doping fronts in organic semiconductors are investigated theoretically using an extended drift-diffusion model for ions, electrons and holes. The model also involves the injection barriers for electrons and holes in the partially doped regions in the form of the Nernst equation, together with a strong dependence of the electron and hole mobility on concentrations. Closed expressions for the front velocities and the ion concentrations in the doped regions are obtained. The analytical theory is employed to describe the acceleration of the p- and n-fronts towards each other. The analytical results show very good agreement with the experimental data. Furthermore, it is shown that the internal structure of the doping fronts is determined by the diffusion and mobility processes. The asymptotic behavior of the concentrations and the electric field is studied analytically inside the doping fronts. The numerical solution for the front structure confirms the most important predictions of the analytical theory: a sharp head of the front in the undoped region, a smooth relaxation tail in the doped region, and a plateau at the critical point of transition from doped to undoped regions.Comment: 13 pages, 11 figure