The Kinetic Physics of Ion Heating in Magnetic Reconnection

Colby Haggerty, Ph.D.

Magnetic reconnection is a fundamental plasma process that rapidly converts magnetic energy into bulk flow, heating, and non-thermal particles. It is often used to explain impulsive particle energization in various magnetized plasma systems across laboratory, solar, heliospheric, and astrophysical contexts. However, many of these systems contain plasmas where collisions are too infrequent to thermalize (or Maxwellianize) particle distributions; this means that a kinetic (collisionless) approach is required to accurately study heating and non-thermal particle generation in these reconnecting systems. With this in mind, I present recent results from both fully kinetic and hybrid Particle-in-Cell simulations of magnetic reconnection, focusing on the kinetic effects related to ion heating. Ions are shown to be efficiently heated during reconnection, with the temperature increase proportional to the ion mass times the square of the outflow velocity. This heating influences the reconnection process by creating a pressure gradient that opposes the magnetic tension force, thereby reducing the outflow jet speed below the standard prediction of the Alfvén velocity based on the reconnecting magnetic field. As a result, ion heating and the outflow speed balance each other until an equilibrium is reached in the exhaust region. The importance of this result is demonstrated across various types of reconnection with different initial conditions, including a guide field (relevant to solar wind turbulence), an increased helium abundance (for chromospheric/coronal heating), and a flow shear (related to switchbacks and magnetospheric flanks). These findings highlight the need for a kinetic approach to modeling reconnection to predict ion heating and particle energization accurately.

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