Magnetic
Reconnection
What is magnetic reconnection?
Magnetic reconnection is the process by which magnetic field lines tear and
reconnect to each other. The basic phenomenon of magnetic reconnection was
originally devised to explain the energy release of solar flares, and is most
simply described by the Sweet-Parker model. In this 2-D model the reconnection
configuration consists of three parts: the inflow region, the outflow region,
and the thin central diffusion region (see figure). Magnetic field lines are
convected inward towards the diffusion region, where they reconnect. You can
imagine field lines like rubber bands. When they tear the "snap back", and
accelerate trapped plasma in the outflow region. In solving the resistive MHD
equations for the diffusion region they obtained flow patterns
around a
stagnation point (x point) with an outward flow at speeds the order of the
Alfvén speed. The difficulty with this model is the reconnection rate
is too small to account for observed phenomena.
A modification of the model
introduced two pairs of shocks near the boundaries of a large
outflow region and a small diffusion region
to significantly enhance the reconnection rate. These shocks consist of a standing
wave pattern of slow-mode Alfvén waves, where
the slow-mode is a branch
of the compressional Alfvén wave. Once the shock structure is established,
steady-state reconnection proceeds, and a standing wave shock structure remains.
In general, rotational discontinuities (sheared magnetic field) associated with
shear Alfvén waves, slow-mode shocks, slow expansion waves, and contact
discontinuities (density/temperature) may be present in the reconnection layer.
Reconnection studies on HelCat
Intended studies on HelCat including
investigations of the link between magnetic reconnection and the expansion
of a dense, magnetized plasma in to a diffuse background plasma. The work
is relevant to the formation of astrophysical jets. The wave magnetic field
will
be measured using magnetic "B-dot" probes.
Density and electron temperature will be measured using standard Langmuir probes.
In
addition, the steady state nature of the helicon environment is ideal for microwave
reflectometry. The planned reflectometry system will consist of an 18 GHz source
and detection system for measuring density fluctuations associated with compressional
waves. The existing 4 mm microwave interferometer will also be installed to
measure the chord averaged density. The
University of New Mexico has a unique diagnostic,
a Laser Induced Fluorescence (LIF) system, which has been used to measure plasma
flow velocities and ion temperatures. This system will be installed on the
proposed experiment to measure ion temperatures and ion flow velocities both
of the Alfvén waves and in the magnetic reconnection region.
The characteristics of Alfvén waves generated during a magnetic reconnection
event will also be investigated, including propagation during the onset of
reconnection and the possible formation of standing wave shocks.
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