Formation of energetic powerlaw distributions through magnetic pumping with trapped electrons

Energetic particle generation is an important component of a variety of astrophysical systems, from seed particle generation in shocks to the heating of the solar wind. It has previously been shown that magnetic pumping is an efficient mechanism for heating thermal particles [1]. Extending the theory of magnetic pumping to include magnetic trapping we find that pumping also becomes an effective energization method for particles moving faster than the speed of the waves and naturally generates power-law distributions [2,3,4]. Without trapping, fast parallel streaming along magnetic field lines causes the electron pressure to be isotropized and homogeneous along the magnetic field lines. In contrast, trapping permits spatially varying pressure anisotropy to form along the magnetic field lines, and through a Fermi process this pressure anisotropy in turn becomes the main ingredient that renders magnetic pumping efficient for energizing superthermal electrons [3,4]. The theory is validated by spacecraft observations of the strong, compressional magnetic fluctuations near the Earth’s bow shock from the Magnetospheric Multiscale mission [2]. Given the ubiquity of magnetic 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] Lichko E, Egedal J, Daughton W, and Kasper J, “Magnetic Pumping as a Source of Particle Heating and Power-law Distributions in the Solar Wind,” (2017) Astrophys. J. Lett., 850.

[2] Lichko E, Egedal J, “Magnetic pumping model for energizing superthermal particles applied to observations of the Earth’s bow shock”, (2020) Nature Communications, 11, 2942.

[3] Egedal J, Lichko E, “The fast transit-time limit of magnetic pumping with trapped electrons”, (2021) Jour. Plasma Phys. 87, 905870610.

[4] Egedal J, Schroeder J, Lichko E, “Parallel velocity mixing yielding enhanced electron heating during magnetic pumping”, (2021) Jour. Plasma Phys. 87, 905870116.