Geoscience Reference
In-Depth Information
atmosphere, nature seems to abhor a vertical velocity, even for the plasma and an
equilibrium height is sought, as just indicated theoretically. At least three expla-
nations have been put forward to explain this observation: ion drag (Dougherty,
1961), the F-region dynamo (Rishbeth, 1971), and parallel ion diffusion (Stubbe
and Chandra, 1970).
In the first of these theories, the atmosphere is assumed initially to be at rest
and an external zonal electric field is applied which puts the plasma in motion
via the E
B drift. (The reader is warned that this mechanism is not thought to
be important at tropical or midlatitude regions but does work at high altitudes,
as seen in Chapter 9.) For example, a zonally eastward electric field would cause
the plasma to move northward and upward at a near 45 angle over Arecibo with
velocity
×
a y is a unit vector perpendicular to B in the magnetic
meridian (see Fig. 5.15). We ignore the declination of the magnetic field. The
moving ions would, after a while, set the neutral gas in motion via the ion drag
effect. Scaling arguments (Holton, 1979) show that the neutrals cannot have a
very large vertical velocity so they would eventually attain a poleward horizontal
velocity U
(
E e /
B
) ˆ
a y , where
ˆ
a y due to momentum transfer from the ions. Once this horizontal
flow begins, it entrains the plasma flow with its component downward, parallel
to B , which would oppose the upward velocity component. We estimate the
acceleration time below. Note that we are using the standard meteorological
notation with components ( u
=
v
ˆ
w ) positive in the directions east, north, and
up, and use primes for the geomagnetic coordinate system. The plasma carries a
zonal current, J
,
v
,
, where we can use a scalar Pedersen conductivity, since
we are considering perpendicular motion in the F region where
= σ P E
σ
is diagonal.
The J
B force due to this zonal current is, in fact, the origin of the poleward ion
drag force on the neutrals. Once the neutrals begin to move with some velocity
U the current is given by J
×
= σ P (
E
+
U
×
B
)
. In the final equilibrium force-free
Up ( a z )
East ( a x , a xz 9
)
( a y ) North
I
a z 9
a y 9
E 3 B /B 2
U · B
v i
E
E
v i
U · B
E 3 B /B 2
B
B
(a)
(b)
Figure 5.15 Schematic diagram of motor (a) and dynamo (b) electromagnetic phenom-
ena in the tropical F-region ionosphere.
 
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