On Mar 17, 2006, at 3:04 PM, Bob Fickle wrote:
Yes, they will follow the field lines; but there's not much large- scale order to the galactic magnetic field, so it's more a diffusion process, once the particles leave the supernova's immediate area. There's no significant recombination- not enough electrons moving close to the same speed, and even those that did combine would be broken apart again by collisions with atoms in the interstellar medium.
With an interstellar density of 1 atom/cm^2 this does indeed appear to be true. The pulse will remain in plasma form. However, this then also negates the effect of any ambient magnetic field.
As the fast electrons fly away from the nuclei a large E gradient develops and the electrons and nuclei eventually rejoin. Their relative motion is thermalized, but the net outward kinetic energy remains. Further, the initial ambient magnetic field is cancelled by the plasma action. As long as the plasma density is much larger than the interstellar matter density the plasma pulse should travel fairly unimpeded. I haven't done the calculation, but I would expect this distance to be a lot less than 150,000 ly.
Suppose we view the explosion from the north pole direction of the ambient magnetic field. The initial ambient magnetic field lines point toward us. The plasma that comes toward us is unimpeded by the ambient magnetic field. Plasma traveling in (or with a component motion in) a plane perpendicular to the ambient magnetic field has an ambient field canceling current generated in it. Outbound nuclei are bent in a clockwise manner by the Lorentz force, electrons are bent in a counterclockwise direction. This creates a clockwise current from our perspective. A clockwise current, by the right hand rule, generates a magnetic field that cancels the ambient field that points at us. Some of the lateral motion due to the induced current is lost to thermalization, but for the most part the plasma will retain the bulk of its outbound velocity and kinetic energy. The tiny interstellar magnetic field is cancelled by a nominal current density, thus little heating and little loss of kinetic energy occurs.
Last I heard, cosmic rays were believed to have an average age in the galaxy of a few million years- based on ratios of Li/Be/B isotopes produced in transit.
But this fact implies very little thermalization of, a very long mean free path for, the cosmic rays. They only have to travel 150,000 years to get here, not millions.
Since the LMC is actually outside our galaxy, I think it would be safe to add a few million more.
If nothing thermalizes the plasma pulse outbound motion, or at least some component of it, then some kind of material shock wave should immediately follow the light pulse and build.
Horace Heffner
