Michigan State University
Department of Physics & Astronomy

Spring Semester 2006 Colloquium
March 2, 2006

The Dynamics of Plasma and Field Around Planets, Stars and Galaxies

Eugene Parker
University of Chicago

The atmospheres of planets, stars, and galaxies vary from un-ionized, through partially ionized to fully ionized, and often collision-less, at large distance. A diverse lore has arisen over how to treat the large-scale dynamics of ionized gases with, or without, collisions and partially, or fully, ionized. One popular approach has been to declare the electric current rather than the magnetic field to be the basic field variable, so that the dynamics is treated in terms of the electric field and electric current, called the (E,j) paradigm. Unfortunately,it is not possible to give a general formulation of Newton's and Maxwell's equations in tractable form in terms of E and j. The absence of a complete set of workable dynamical equations has turned some investigators to invent diverse fantasies to enable them to proceed with the dynamical situations encountered in astrophysics, and much voodoo physics has been the result. It is unfortunate that the standard textbooks do not address these issues.

As a matter of fact, starting with Newton and Maxwell, the dynamical equations come out quite simply in terms of the magnetic field and the bulk plasma velocity, the (B,v) paradigm. If there are enough particles to provide a well defined local number density, then the dynamics is immediately described by the familiar hydrodynamic equations for conservation of particles, momentum, and energy. If there are enough free electrons and ions that the plasma cannot support any significant electric field in its own frame of reference, then it follows directly from Faraday's induction equation that B is carried bodily with v, and the hydrodynamics becomes magnetohydrodynamics.

In summary, the large-scale dynamics of magnetized plasma represents the mechanical interaction between the momentum of the bulk plasma motion and the magnetic stresses. The convoluted mysteries of currents and electric fields are best ignored. E and j are too remote from the basic dynamical variables B and v to serve as useful proxies for time dependent fields and motions.