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Jicamarca eastward drift velocity
Summer
Equinox
Winter
200
100
Flux
,
120
0
100
Flux.120
0
2
100
08
12
16
20
Local time (75
00
04
08
8
W)
Figure 3.1
Seasonal variations of the zonal plasma drifts during periods of low and
high solar flux. [After Fejer et al. (1991). Reproduced with permission of the American
Geophysical Union.]
As just discussed, these drifts are directly related to the ambient perpendicu-
lar electric field (where, as usual, we mean the component perpendicular to
B
)
through
E
⊥
=−
V
i
×
B
(3.2)
10
−
5
tesla. Thus, a zonal
eastward drift is due to a vertically downward electric field component, with a
100m/s drift corresponding to a 2.5mV/m electric field. Likewise, an upward
drift of 40m/s corresponds to an eastward electric field of about 1mV/m.
Evidence for the repeatability of the average drifts just plotted is given in
Fig. 3.3, where a large number of individual 24 h measurement sets are superim-
posed. Except for the July 1968 event, which was associated with changes in the
interplanetary magnetic field, the data are remarkably well behaved. Such a plot
suggests that the temporal sequences plotted in these figures can be interpreted
equally well in terms of Universal Time measurements, at least in an average
sense. That is, if we take a snapshot of the instantaneous electric field pattern
around the earth's equatorial zone, it should look very much like the Jicamarca
equatorial measurements as a function of local time plotted here. One test of
this hypothesis uses the fact that for electrostatic fields, the Maxwell equation
∇×
The magnetic field over Jicamarca is about 2
.
5
×
E
=
0 implies
E
·
d
l
=
0
(3.3)
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