Post
by jjohnson » Fri Sep 17, 2010 10:30 pm
Hi, Shelgeyr,
I think I know what you are getting at, and you are in a way right. A Birkland current is a magnetically field aligned current; i.e., the electrons and ions have a translational direction aligned with the [imaginary] lines of magnetic force. They can also have spiraling trajectories whose radius, if viewed end-on to the magnetic field axis, is a function of their velocity, mass, charge, and the magnetic field strength. Moreover, the 3-D case usually has surfaces (cylinders) or sometimes sheets which we imagine as depicting "places" or areas in the field where the magnetic field potential is all of a single value. The electrons "like" to move with the magnetic field, because it takes more energy to cross field lines. Said crossing sets up a force on the charge perpendicular to both the velocity vector and the magnetic field - forcing the particle to curve. The charged particles travel "force-free" (magnetically speaking) when their vector points parallel to the magnetic field vector. Lower energy state, in a sense.
The fact that there is a magnetic field at all implies that there must be the underlying moving charges (current) which create it in the first place. Collisions in field-aligned currents are relatively rare, and because of that, thermal interaction rates (collisions) are low, and the random or thermal temperature may be quite low, even though the kinetic energy of the moving charges can simultaneously be very high. Extremely high.
This is why the math is so messy. It is not a very stable configuration, and small perturbations rin particle motions set off chains of charged particles' being accelerated in new directions, said changes affecting all the surrounding particles, resulting in different electric and magnetic fields in response. Hence the illusory "living" effect, as Langmuir noted, so he named the self-organizing ionized gas state "plasma", as in the blood.
In Steve's defense, it is a "which came first, the cart or the horse, the chicken or the egg?" Magnetic fields are the inevitable companions of electric currents, and an electric current can be set up any time there are charged particles within its field to be accelerated. Birkeland currents move within their magnetic fields, however things got set up to begin with, and those fields accompany the current flows. Being parts of circuits (the "wires") the currents cannot just start or end somewhere out in "empty" space. They are hooked into planets, stars, galaxies and galaxy clusters, connecting the Universe up and lighting its streets at stellar intervals.
