Expert: James Gort - 4/22/2008

Question
Hi. Can you explain a bit about how Fermi acceleration works and how one can best quantify it? Thanks!

Answer
Hello Jani,

This is a difficult one.  I'm not sure on your background in physics.  So my answer might be too simplistic, and raise more questions than it answers, or may just confuse you.  But I'll try...

First, Fermi acceleration (as you probably know) is not well covered in most astrophysics texts.  Two reasons, I suppose.  To do it justice requires a pretty length explanation.  And no one is sure of the role Fermi acceleration actually plays (in astrophysical jets, for instance).

I'm sure you did a little research yourself, but just in case - check out  http://en.wikipedia.org/wiki/Fermi_acceleration and http://en.wikipedia.org/wiki/Magnetic_mirror  Not bad explanations, but they can be a little confusing, too.  A nice little paper (at a 3rd year university physics level) can be found at http://arxiv.org/abs/astro-ph/0610141v1  For a slightly more advanced treatment, I refer you to "High Energy Astrophysics" Vol. 2 by Longair (Chapter 21 - The Acceleration of High Energy Particles).

But since you asked the question, I'll give the 50,000-foot answer.  We know charged particles are accelerated in Electric and Magnetic fields.  In astrophysical situations, strong fields are often present, and the fields are generally not static,  When a charged particle enters a non-stationary field, the particle will be accelerated along the lines of the field gradient.  So the particle will accelerate along the field lines, and be "flung out" as its velocity exceeds the ability of the field to hold it.  From a macro level, this looks like the particle "bounces" off a magnetic "mirror" - the mirror is merely the region of non-static fields.

I hope that answers your question without sacrificing too much accuracy.  Interesting question, though!  

Prof. James Gort