Geoscience Reference
In-Depth Information
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Figure 4.18 Numerical simulation of collisional shear instability in the bottomside
F-region ionosphere. The upper and lower row of panels depict number density and
perturbed potential for three time steps shown, respectively. Results are shown both in
grayscale and relief formats. Time steps represent 100 s of real time and the simulation
size is 62
8 km on each side. The solid and dashed lines to the right of the lower panels
show the magnitude of the perturbed potential and the zonal plasma drift speed, respec-
tively, taken through the cuts indicated by the dotted lines. Interchange instabilities have
been suppressed by setting gravity and the background zonal electric field to zero. [After
Hysell and Kudeki (2004). Reproduced with permission of the American Geophysical
Union.]
.
corresponds to the wind-driven dynamo electric field. The perturbed potential
plotted in the lower panels is the difference between the current potential and
that of the initial time step. By time step 16, evidence of shear instability clearly
can be seen in the perturbed potential. Here, we find periodic islands or cells
of low and high potential straddling the altitude of the shear node. A dominant
horizontal wavelength is also clearly evident by time step 16. Lines of constant
potential are streamlines of the flow, and the plasma circulates clockwise and
counterclockwise around the cells of low and high potential, respectively.
By time step 64, the instability has grown to the point of being detectable in the
plasma density plot. Here, we find elongated regions of depleted and enhanced
plasma penetrating above and below the shear node, respectively. A rotational
pattern is suggested by the morphology of the enhancements and depletions, and
the circulation cells visible at time step 16 have merged into one or two main
cells by time step 64.
By time step 90, the unstable flow pattern exhibits a distinct “cat's eye” sur-
rounding the most prominent circulation cell. The primary density irregularity
is unstable to secondary, E
×
B instabilities (note that the electric field is upward
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