Particle acceleration in compressible reconnection layer

Particle acceleration in space and astrophysical magnetic reconnection sites is

an important unsolved problem. Earlier kinetic simulations have identified

several acceleration mechanisms that are associated with particle

guiding-center drift motions. Here, we show that, for sufficient large systems,

the energization processes due to particle drift motions can be described as

fluid compression and shear. By analyzing results from fully kinetic
simulations, we show that the compression energization dominates the
acceleration of high-energy particles in reconnection with a weak guide field,
and the compression and shear effects are comparable when the guide field is
50% of the reconnecting component. Based on this result, we then study the
large-scale reconnection acceleration by solving the Parker's transport
equation in a background reconnection flow provided by MHD simulations. Due to
the compression effect, particles are accelerated to high energies and develop
power-law energy distributions. The power-law index and maximum energy depend
on guide-field strength and diffusion model. This study clarifies the nature of
particle acceleration in reconnection layer, and may be important to understand
particle energization during solar flares.

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