Particle Acceleration in a Solar Jet – Niels Bohr Institute - University of Copenhagen

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Particle Acceleration in a Solar Jet

Master's Thesis by Joakim Rosdahl

Abstract: For many years MHD simulations have been used to investigate a vast
variety of dynamic phenomena taking place in plasmas. These represent
almost everything from tokamak physics, over the solar atmosphere to
the engine in active galactic nuclei. In this talk we focus on one
particular phenomenon, the formation of a high velocity jet created by
a magnetic reconnection process that takes place in the upper solar
atmosphere. The large scale dynamical evolution is investigated using
a numerical MHD experiment, and the result gives a good correspondence
to observations. But, as the MHD approximation is a macroscopic
description it totally ignores individual particle behaviour. From
solar observations of magnetic reconnection events, indications are
that a large fraction (up to 50%) of the energy released in the
reconnection process is transported away from the reconnection site by
nonthermal particles, and the MHD approach therefore can not correctly
account for the energetics in the process. As a first step on the way
to investigate particle acceleration in realistic 3D scenarios, one
can take a snapshot from the MHD experiment and use this as the
environment into which passive test particles are injected, and their
reactions due to the stressed MHD configuration are followed in time.

There are different approaches to investigate the particle
evolution. The choice depends on which characteristic one wants to
follow and to which parameter regime the underlying MHD experiment
belongs. In this talk we discuss the implementation of two different
approaches of a passive tracing scheme, the various problems encounted
and the solutions to these. The final product is a fast and reliable
scheme for tracing individual particles in gridded MHD data sets. The
result of the particle tracing is a series of energy distribution
functions. To compare these with observations they must be converted
into photon spectra. In this particular case we show how the particles
that hit the solar photosphere emit a bremsstrahlung spectrum and
compare this with observations.

The results clearly show that this type of investigation is only a first
crude approximation that provides far to energetic particle distributions.
A short discussion on how to improve the approach in the future is given.

Supervisor: Klaus Galsgaard