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This estimate ignores many other terms in the equation of motion, but it does
show that J
B forcing is significant. Viscosity is particularly important in the
dynamics since it spreads the strong height-dependent J
×
B forcing to other alti-
tudes. Mikkelsen et al. (1981a, b) have studied this event with a two-dimensional
numerical model using the local forcing deduced from the radar and rocket data.
They found good agreement with the calculated and observed zonal wind (see
Fig. 9.15). Their model included the pressure gradient, a curvature effect due
to the shape of the auroral oval and advection of momentum in the vertical
and meridional directions. The agreement was not quite as good between the
observations and the model wind in the meridional component. Furthermore,
when applied to a second data set, obtained three days later on March 2, 1978,
the agreement was poor in both components. The primary difference in the
two events was the location of the forcing in longitude. In the February 28
data, shown in Figs. 9.13-9.15, a substorm occurred near College, Alaska, and
local forcing was an adequate approximation. On March 2, however, the sub-
storm was far to the east of College. Since the observed winds were very similar,
Mikkelsen et al. concluded that advection of momentum in the zonal direction
was crucial and that a three-dimensional model was essential for progress in
understanding electrodynamic forcing of the upper atmosphere.
×
250
Observations
Model integration
200
Southern
trail
150
Northern
trail
100
2300
2200
2100
0
100
200
2300
2200
2100
0
100
200
Zonal wind (m/s)
Zonal wind (m/s)
Figure 9.15 Comparison of the observed zonal wind profiles at the northern and south-
ern positions on February 28 with the results of the modeling. [After Mikkelsen et al.
(1981b). Reproduced with permission of the American Geophysical Union.]
 
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