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Solar Wind Dynamics
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Click on image for larger version. |
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Polar plumes are being modelled by Steve Suess, Shyamsundar
Parhi, and Martin Sulkanen. Plumes are bright rays in coronal holes, visible between
one and several solar radii. They are denser and slower compared to interplume plasma and
are modelled here as jets or wakes. The plasma is considered compressible and ideal (no
viscous dissipation, thermal diffusion, or electrical resistance).
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A jet is introduced
with a perscribed flow speed and an internal magnetic field. The surroundings are at rest
and are also permeated by a magnetic field but with a different field strength. The
simulations are described in a paper titled "Can Kelvin-Helmholtz instabilities of
jet-like structures and plumes cause solar wind fluctuations at 1 AU?" by Parhi,
Suess, and Sulkanen. The paper appears in the July 1, 1999 issue of the Journal of
Geophysical Research 104 No. A7, 14,781.
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Click on image for animation. |
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Click on image for animation. |
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The animations presented here are performed with the
computer code Zeus-3D which solves the equations of ideal magnetohydrodynamics explicitly
on a Eulerian grid. Open boundary conditions are considered at all boundaries of the
simulation domain. The evolution of the Kelvin-Helmholtz instability is studied when a
velocity perturbation of less than 1% of the jet flow speed is imposed at the origin of
the preexisting jet. In the upper animation, where magnetic shear is low, the effect of
the instability is less apparent than in case of strong shear as in the lower panel. This
instability helps set up a process of mixing between plumes and interplumes, accompanied
with the formation of shocks and plasma entrainment. This suggests that the instability
thus generated at a few solar radii can account for the smooth fast solar wind and reduced
velocity shear between plumes and interplumes as observed beyond 0.3 astronomical units
(about the orbit of the planet Mercury).
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