In article , "Ron Hardin"
wrote:

John Doty wrote:
Differential rotation winds the field up. When the fields get really
wound up and tangled, reconnection becomes more frequent, breaking long
tangled field lines into shorter loops. Solar flares are the result of
reconnection events. If you see them, the field is untangling.

Reconnection comes from resistance, which produces heating.

That's the magnetohydrodynamic story. The trouble is that MHD is a poor
model for real plasmas. Also, the resistivity of astrophysical plasmas is
much too low to produce the reconnection phenomena we see.

Without
dissipation charged particles cannot cross field lines (actually
reversing cause and effect here; the field lines take into account
charged particles not crossing them; their motion is a current.

On the scale of the cyclotron radius they cross field lines all the time.
In a uniform magnetic field, with no electric field, the motion is helical
with the axis parallel to the field, so a particle must stay *near* a
particular field line. This is the sense in which "particles don't cross
field lines". However, if the field isn't uniform on the scale of the
cyclotron radius a particle can readily move away from a particular field
line.

The
prohibition of crossing field lines disappears if the particle
velocities can be interrupted by something other than electrical
effects, like collisions. This produces heating.

In astrophysical plasmas particle collisions are generally too infrequent
to have an effect. Wave-particle interactions are much more important.
These tend not to be very effective at entropy generation, but they can
exchange free energy between the particles and waves in the plasma.
Laser-like phenomena can occur: the presence of waves stimulates the
emission of more wave energy. Wave amplitudes can grow until the helical
motion of the particles is severely disturbed. The macroscopic effect is
similar to collisions (look up "Bohm diffusion" in your favorite plasma
physics text). There's not as much heat as collisions would produce,
however: the energy tends to wind up in waves and accelerated particles.

--
| John Doty "You can't confuse me, that's my job."
| Home:
| Work: