Motion across Magnetic Discontinuities and Fermi Acceleration of Charged Particles.
Abstract
The motion of charged particles across stationary magnetic discontinuities is investigated. The constancy of the magnetic moment, a result of the guiding center approximation (GCA), is shown to hold for both weak shocks and shocks of arbitrary strength whose fronts are nearly parallel to the lines of force. For most other shocks, "scattering" of the pitch angle, relative to the change across the shock expected from the GCA, is limited to less than the angle between the lines of force at the shock. Only rather strong shocks can significantly randomize the pitch angles, and these result in non-uniform distributions in phase angle. Charged particles with Larmor radii large compared to the thickness of shock fronts but small compared to the length of the shock disturbances can be efficiently Fermi accelerated between series of converging shocks with equal amplitudes: A particle between diverging shocks cannot be trapped and decelerated, but, upon entering the region between converging shocks, it may be trapped and accelerated. The average efficiency of acceleration is estimated to be comparable to that of "first-order" Fermi acceleration.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1964
- DOI:
- 10.1086/148001
- Bibcode:
- 1964ApJ...140.1013W