The Mechanisms for Particle Acceleration and Heating in Multi-Island Magnetic Reconnection

Abstract:

Magnetic reconnection is a significant driver of energetic particle
events both within the heliosphere and the broader astrophysical
environment. How magnetic energy at the largest spatial scales of a
system is dissipated on the Alfvenic time-scales seen in observations,
however, remains an unsolved problem. In large-scale systems do
particles gain energy at a single reconnection site or at multiple
sites? What role does reconnection play in the dissipation of
turbulence at the smallest spatial scales? Answering such questions
requires a fundamental understanding of how the conversion of magnetic
to particle energy takes place. For collisionless systems where the
particles are magnetized there are three basic mechanisms for particle
energy gain: motion along parallel electric fields; and the magnetic
curvature and gradient B drifts along perpendicular fields. The latter
two produce the classical Fermi and betatron acceleration,
respectively. Observations in the magnetosphere and solar wind have
clearly established that most of the direct energy release during
reconnection takes place in the large-scale exhaust and not in the
more localized diffusion region. Ion bulk flow and the
counterstreaming ions that develop as a result of Fermi reflection
carry most of the released energy. Thus, unlike in a classical fluid,
energy dissipation does not take place at small spatial scales. The
observational evidence on the physics of electron heating is not
clear. Parallel electric fields are difficult to measure either
because they are very weak or because they are highly localized,
electron holes and double layers being the exception. Particle-in-cell
simulations of reconnection suggest that electron energy gain is
dominated by Fermi reflection except possibly in the large guide field
limit where parallel electric fields are important. Extensions of the
Parker transport model to describe reconnection-driven particle
acceleration will be discussed as will the role of the upcoming MMS
mission in sorting out electron acceleration.