A new and alternative look at NonLinear Alfv\'enic States

Abstract

A new formalism for the nonlinear Alfv\'enic states sustainable in Hall Magnetohydrodynamics is developed in a complete basis provided by the circularly polarized Beltrami Vectors, the eigenstates of linear HMHD. Nonlinear HMHD is, then, reduced to a rather simple looking set of scalar equations from which a model problem of three interacting Beltrami modes is formulated and analytically solved. The triplet interactions span a variety of familiar nonlinear processes leading to a redistribution as well as periodic exchange of energy. The energy exchange processes (whose strength is measured by an energy exchange /depletion time) will, perhaps, play a dominant role in determining the spectral content of an eventual Alfv\'enic state. All nonlinearities (sensitive functions of the interacting wave vectors) operate at par, and none is dominant over any substantial region of k-space; their intricate interplay prevents a "universal" picture from emerging; few generalizations on the processes that may, for instance, lead to a turbulent state, are possible. However, the theory can definitely claim: 1) the energy tends to flow from lower to higher $k$, and 2) the higher $k_z$ (in the direction of the ambient magnetic field) components of a mode with a given $k$ are depleted/oscillate faster -in some cases much faster. It is noteworthy that the mode coupling is the strongest (with the shortest depletion time) when the participating wave vectors are nearly perpendicular; perhaps, an expected consequence of the curl (cross product) nonlinearities. Numerical simulations will be necessary to help create a fully reliable picture.