Magnetohydrodynamic turbulence and enhanced atomic processes in astrophysical plasmas

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Abstract
This article discusses a way in which enhanced atomic physics processes, including radiative energy losses, may occur in an astrophysical plasma containing magnetohydrodynamic turbulence. Two-dimensional (2D) magnetohydrodynamics (MHD) is adopted as a model. A major characteristic feature of 2D MHD turbulence is the development of strong current sheets on a dynamical time scale L/V0 where L is the spatial scale of the turbulent fluid and V0 is the scale of the velocity fluctuations. The current contained in the sheets will be carried by an electron drift relative to the ions. The case of a plasma containing minority atoms or ions with an excited state accessible to collisions from the tail of the electron distribution is considered. In the current carrying sheets or filaments, the electron distribution function will be perturbed such that collisional excitations will be enhanced relative to the current-free plasma. Subsequent radiative de-excitation of the atoms or ions removes energy from the turbulence. Expressions are presented for the electron drift velocity arising in 2D turbulence, the enhancement of collisional excitations of a trace atom or ion, and the energy lost to the plasma turbulence by radiative de-excitation of these atoms or ions. The mechanism would be most pronounced in plasmas for which the magnitude of the magnetic field is large, the outer scale of the turbulence is small, and the electron density and temperature are low. A brief discussion of the relevance of this mechanism to some specific astrophysical plasmas is given.