Geoscience Reference
In-Depth Information
Millstone Hill
4
50
2
0
0
2
2
2
50
24
SK
p
# 14 (4 days)
2100
26
0
3
6
9
12
15
Local time
18
21
24
3
6
Figure 5.19
Average Millstone Hill equatorward electric field component measurements
at equinox reproduced from Fig. 5.8 along with 1-hour averages of the same component
during times of very low activity [Figure courtesy of C. Gonzales.]
However, the situation is not so simple since the high-latitude field changes sign
near midnight, switching from poleward to equatorward, which is not observed
at Millstone Hill. Also, for
Kp
around 2 it is unlikely that high latitude electric
fields can penetrate to such low L values.
A second possibility is that the electric field is, after all, created by an F-region
dynamo but that the local solar-drivenwinds aremodified by high-latitude energy
and momentum sources. The heat sources stem both from Joule heating and
from particle precipitation into the atmosphere. The momentum source arises
from the
J
E
. In our previous discussion of
this term, we showed that at low latitudes it yields the ion drag effect. In the
neutral wind dynamo region
J
×
B
forcing term where
J
=
σ
·
0 and the electric field is generated by
the winds. At high latitudes the electric field is imposed on the ionosphere and
J
·
E
<
0. This shows that electrical energy is available for Joule heating as
mentioned previously and that the
J
·
E
>
B
force is not so much a frictional drag on
the neutrals as it is a mechanism for accelerating the thermospheric neutrals. At
high latitudes the applied electric fields to first order form a two-celled plasma
flow pattern which is virtually always present. It is not surprising then that the
high-latitude neutral atmospheric motions are greatly affected by electrodynamic
forcing. Suppose, as discussed in Chapter 9, that winds in the thermosphere
are indeed driven across the polar cap by the plasma flow. The plasma turns
to follow the auroral oval, but the wind has no such constraint. A flywheel-
like effect may then occur with disturbance winds blowing out of the auroral
oval even during relatively quiet times. Such winds would be reinforced by the
equatorward pressure gradient due to Joule and particle heating in the auroral
oval. Once equatorward of the oval, the Coriolis force will deflect the wind
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