Geoscience Reference
In-Depth Information
Now
E
z
is not a function of
y
(equipotential magnetic field lines), so if as a first
approximation we take
uB
constant with
y
until the field line enters the F-layer
valley, we can perform the
y
integrals in (3.10), giving
1
2
E
z
+
P
uB
d
dz
1
2
F
P
E
PN
F
+
=
0
F
P
E
where
P
are the field line-integrated conductivities discussed in
Chapter 2. A more realistic model would include the full
y
dependence of
u
and
and
, but some insights are possible, even in this simple case. Including the
Southern Hemisphere conductivity,
E
E
E
PS
E
PS
, and assuming
PN
=
=
P
, the
solution for
E
z
(
z
)
is thus
u
F
F
E
E
z
(
z
)
=−
(
z
)
B
P
(
z
)
P
(
z
)
+
2
P
(
z
)
(3.11)
In (3.11) we have dropped a constant of integration, since the slab geometry is
not physically accurate and would be replaced by a smooth transition to a two-
dimensional
(
J
x
and
J
z
)
equatorial electrojet current in a more accurate model
(see Section 3.4).
Equation (3.11) clearly shows that if the off-equatorial E region is insulating
0
,
E
E
B
everywhere. We can now use (3.11) to
explain some aspects of the diurnal variation in the vertical electric field. Dur-
ing the nighttime,
P
=
(
z
)
will be equal to
−
u
(
z
)
E
P
becomes quite small and therefore
E
z
≈−
uB
in regions
F
where
P
is large—for example, near the peak in the F-region plasma density.
During the daytime, however, the E-region conductivity is comparable to or
larger than the magnetic field line-integrated F-region conductivity. For large
E
P
the electrodynamic control of the ionosphere is vested in the E region. As
discussed following, a dynamo operates there as well, but it is driven by daytime
tidal winds, which are smaller than the thermospheric winds in the F region.
This explains the diurnal variation in the F-region zonal plasma drifts plotted
in Fig. 3.1. At night the E-region conductivity is low, and the high zonal winds
at several hundred kilometers altitude determine the vertical electric field and
thus the horizontal plasma flow. For
F
E
P
the electric field will be almost
equal to
uB
. The eastward plasma velocity thus nearly matches the neutral wind
speed. During the day, however, the F-region dynamo loses control of the elec-
trodynamics, and the resulting electric fields are determined by winds in the E
region. Since these winds tend to be weaker, the plasma drift is smaller during
the day. Curiously, nighttime E-region winds measured by chemical releases are
comparable to the F-region winds (see Chapter 5), but since
P
E
P
is small, they do
not seem to create large E-region dynamo electric fields.
An analogous electric circuit is shown schematically in Fig. 3.10. The three
batteries correspond to the two (north and south) E-region dynamos and
the F-region dynamo. Each battery has a finite internal resistance given by
Search WWH ::
Custom Search