Particle Heating and Acceleration during Magnetic Reconnection

How the magnetic energy released during reconnection is transferred to

hot electrons and ions and nonthermal components is a topic of broad

importance both in the heliosphere and the broader universe. I will

review observations and ideas about the mechanisms

that drive both heating and nonthermal particle production with an

emphasis on non-relativistic reconnection. An organizing parameter is

the magnetic energy released per particle $W_B=m_iV_A^2$. In regions

where $W_B$ is large such as in the Earth's magnetotail, solar flares

and the solar wind near the sun, observations reveal hot thermal

and nonthermal, powerlaw components. Single x-line models fail to

explain the generation of the nonthermal component. However,

simulations reveal that reconnection becomes turbulent in the high

$W_B$ environment. Magnetic energy release and particle acceleration

therefore take place in a multi-x-line environment. The energy gain of the most energetic particles is dominated

by Fermi reflection in growing and merging magnetic islands rather

than the parallel electric fields in kinetic scale boundary layers. On the

other hand, the large-scale parallel electric potential that develops to

maintain charge neutrality controls the heating of the

hot thermal electrons during reconnection. The

kinetic scale boundary layers that control the parallel electric field

are not important in energy release in large-scale

systems. Particle-in-cell simulations are revealing powerlaw

distributions of both electrons and protons. However, the PIC models

fail to produce the extended powerlaws seen in some observations

because of inadequate separation of kinetic and macroscales. A new

computational model, {\it kglobal}, has been developed that blends MHD

dynamics with electron and ion particles but eliminates all kinetic

scales. Simulations of reconnection in a macro-scale system reveal

powerlaw distributions that extend nearly three decades in energy and

that the dominant control parameter is the ambient guide magnetic

field.