Simulations of secondorder Fermi acceleration of electrons: Solving the injection problem
Abstract
The boosting of electrons from a Maxwellian distribution into a suprathermal powerlaw tail has long been recognized as an important bottleneck governing the subsequent acceleration of some of these electrons to relativistic energies. This is the seed or injection problem. I study this boosting process using a testparticle simulation code, following the full equations of motion of tens of thousands of electrons chosen from a thermal population as they move through general timedependent magnetic fields. Inhomogeneities in the magnetic field are provided by finite swarms of moving current loops with Maxwellian velocity distributions and powerlaw distributions of loop size and dipole moment strength. Whether bulk heating or boosting occurs is found to depend on the size of the swarm thermal speed compared to the electron thermal speed. When the swarm thermal speed is comparable to the electron thermal speed the entire electron population is heated by encounters with the rapidly moving current loops, approximately preserving the Maxwellian character of the electron distribution. On the other hand, at very low swarm thermal speeds there is no bulk heating; instead one percent or fewer of the electrons are boosted into a powerlaw suprathermal tail with a differential energy spectral index between 1 and 2. Individual boosts of 2000 and more have been observed in samples of 50,000 electrons. Most of the strongly boosted electrons have initial energies that are well below the peak of the initial Maxwellian.
 Publication:

Presented at the Particle Acceleration in Cosmic Plasmas Conference
 Pub Date:
 1991
 Bibcode:
 1991paac.conf....2G
 Keywords:

 Boltzmann Distribution;
 Cosmic Rays;
 Magnetic Fields;
 Particle Acceleration;
 Thermal Diffusion;
 Acceleration (Physics);
 Computerized Simulation;
 Electrons;
 Equations Of Motion;
 FokkerPlanck Equation;
 Atomic and Molecular Physics