Quantifying heating by magnetic pumping through in situ spacecraft observations*

Superthermal electrons and ions in power-law tails are observed throughout the universe in a variety of astrophysical systems, but how these particles are energized is an open question. It is well known that plasma can be heated by waves, but most theories of particle energization are based on wave-particle resonances which are only effective at particle velocities near the phase velocity of the wave, v ~ ω/k. Starting from the drift kinetic equation, we have derived a magnetic pumping model, similar to the magnetic pumping well known in fusion research, where particles are heated by the largest scale turbulent fluctuations. We have shown that this is a complementary heating mechanism to the turbulent cascade, effective up to v ≤ ω/k, which results in power-law distributions like those observed in the solar wind [1]. However, compressional Alfvenic turbulence has the ability to magnetically trap superthermal particles. Magnetic trapping renders magnetic pumping an effective Fermi heating process for particles with v >> ω/k, and produces superthermal power-law distributions. To test this, we used satellite observations of the strong, compressional magnetic fluctuations near the Earth's bow shock from the Magnetospheric Multiscale (MMS) mission and found strong agreement with our model. Given the ubiquity of such fluctuations in different astrophysical systems, this mechanism has the potential to be transformative to our understanding of how the most energetic particles in the universe are generated.

[1] E. Lichko, J. Egedal, W. Daughton, and J. Kasper. Astrophys. J. Lett. 2, 850 (2017)