How local trapping amplifies the effects of heating from magnetic pumping*

One of the outstanding problems across a variety of astrophysical phenomena is how power-law distributions with superthermal tails are generated. Most theories of particle energization rely on energy injection at a specific scale. In contrast magnetic pumping, a complementary heating mechanism to the turbulent cascade in which particles are heated by the largest scale magnetic fluctuations, results in power-law distributions like those observed in the solar wind. We have found that the ability of compressional Alfvenic turbulence to magnetically trap superthermal particles renders magnetic pumping an effective Fermi heating process for particles with v >> ω/k. This process can be further amplified by local trapping. Entropy is created when the plasma becomes untrapped, much in the same way that a gas diffusing into a new compartment creates entropy. This allows even a small amount of scattering to play a large role in heating the plasma. Here we will present an analytic description of the effect of local trapping, as well as a comparison of the analytic model with the spacecraft data obtained from the Magnetospheric Multiscale mission.