Geoscience Reference
In-Depth Information
the electric field, the region with the higher internal conductivity (lower internal
resistance) will determine the electric field.
Comparison of
V
N
shows a remarkable anticorrelation with a net
poleward horizontal ion flow prior to 0030 LT and an equatorward flow there-
after. Although the individual behavior of
V
and
V
||
⊥
N
can be accounted for
by invoking nonlocal effects, the strong anticorrelation of the velocity compo-
nents observed on this night cannot be explained using any of the conventional
mechanisms discussed above. Certainly the local F-layer dynamo mechanism is
not operating, as demonstrated by the ions moving oppositely to the local wind.
The external electric field/gravity mechanism is also not operating, as witnessed
by the lack of a net upward layer motion. (In fact, the F layer falls through-
out the period 2100 to 0230 LT.) What is responsible for the close coupling of
the
V
and
V
||
⊥
N
velocity components on this night? We do not know. A great
deal of careful comparison between optical data and radar data is likely to be
necessary before we arrive at an understanding of F-layer dynamics on a night
like this.
and
V
||
⊥
5.2.5 The Transition Zone Between Mid- and High Latitudes
The magnetic dip angle increases very rapidly with increasing latitude. One effect
is to decouple parallel and perpendicular dynamics. To some extent this simpli-
fies the physics, since gravity and pressure gradients are most important in the
vertical direction, while electric fields and neutral winds are most associated
with horizontal motions of the plasma and neutral constituents. Atmospheric
motions driven by solar heating still create E- and F-region dynamo electric
fields, of course, but these sources are in competition with other processes. We
have already pointed out the marked difference between the St. Santin/Arecibo
nighttime observations and those at Millstone Hill, even during relatively quiet
times. The strong implication is that some dominating high-latitude factor is
present at
L
2, even during very modestly active times.
This result is further emphasized in Fig. 5.19, where hourly averages of the
electric field/zonal drift obtained on days with
=
3
.
<
14 are plotted for the
evening period (filled circles) along with the “low”
K
p
Millstone Hill values
published by Richmond et al. (1980), which were presented earlier in Fig. 5.8.
The “superlow”
K
p
electric field is found to be equatorward in the evening
period, as is the normal evening case for both St. Santin and Arecibo. It seems
that even in what one might consider magnetically quiet times, electrodynamics
in the Millstone Hill area are strongly affected by high-latitude process.
Two possible explanations were advanced by Gonzales et al. (1978). One is
the direct penetration of the high-latitude electric field into the subauroral region.
As we shall see in Chapter 8, this field has the proper sign, since the auroral zone
electric field is poleward in the evening-to-midnight period, in agreement with
the line plot in Fig. 5.19. This effect was studied at midlatitudes by Blanc (1983).
K
p
Search WWH ::
Custom Search