Geoscience Reference
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midlatitude region (Chapman and Lindzen, 1970). Thus, as just noted, these
results are reasonably consistent with a tidal E-region source, in particular for the
daytime zonal electric field component. (The equatorial prereversal enhancement
does not show up very well in this early presentation.)
The meridional electric field component in the right-hand panels is generally
larger and is dominated by a diurnal variation at all four sites. It seems curious
that one component exhibits a semidiurnal modulation and the other a diurnal
one! It may be that the two dynamos (E and F regions) conspire to yield a diurnal
pattern for the meridional electric field (zonal drift) at all these latitudes. There is
an important difference to note between data from the three lowest latitude sites
(Jicamarca, Arecibo, and St. Santin) and the Millstone Hill data. In the 1600-
2400 local time period even the algebraic sign of the Millstone Hill meridional
component is different from the other three sites. That is, Millstone registers
a poleward field, while all three other facilities register an equatorward field
(“downward” at Jicamarca). We return to this point later when auroral zone
effects are discussed.
The electric field vectors in the plane perpendicular to the magnetic fields
deduced from barium cloud drift data shown in Fig. 5.9 bear out these last
comments as well. Notice that in the evening twilight periods the releases all
show equatorward electric fields and that the releases were all at latitudes less
than 35 geomagnetically. These data agree with St. Santin and Arecibo quite
nicely but disagree with Millstone Hill. Mozer (1973) and Gonzales et al. (1978)
have previously pointed out this difference (also see Section 5.2.4).
90°
60°
30°
230°
260°
1 mV /m
290°
00
03
06
09
12
15
18
21
24
Local time
Figure 5.9 Examples of electric field vectors measured by several midlatitude barium
releases. [After Mozer (1973). Reproduced with permission of the American Geophysical
Union.]
 
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