Geoscience Reference
In-Depth Information
5.2.2 Neutral Wind Effects
The thermospheric wind vector can be measured using the Doppler shift of the
redline emission occurring at 630 nm, and the meridional wind component can
be found by using the radar technique. Early observations were compiled by
Burnside et al. (1980) for Arecibo. The most important feature for the nighttime
ionosphere is that prior to midnight, the meridional wind is usually southward.
Because of the finite dip angle, this means that the plasma is held up against grav-
ity until about midnight. After this the “midnight collapse” occurs, the F layer
falls, recombines, and sometimes nearly disappears (Nelson and Cogger, 1971).
In 1973, Perkins created a remarkably robust concept: the field line-integrated
ionosphere. The huge advantage of this idea is that pressure gradients disappear
from the equations. The entire ionosphere is a gigantic particle affected by winds,
gravitational forces, and electric fields. Figure 5.10 shows how the equator-
ward wind (southward in the Northern Hemisphere) supports this “particle”
against gravity. In the Northern Hemisphere, the steady state, the plasma veloc-
ity antiparallel to the magnetic field due to the action of a southward wind and
gravity is
−
g
/ν
in
sin
I
W
||
=
u
s
cos
I
where
I
is the inclination angle, which is equal to the dip angle. If the first
term dominates, the ionosphere will rise. But since
ν
in
exponentially decreases
with height, eventually the second term will cancel the first and the parallel
velocity will vanish. In general, we are most interested in the vertical component
of ionospheric motion. Projecting the parallel velocity to vertical, an equilibrium
holds when
W
||
=
0—that is, when
g
/ν
in
sin
2
I
=
u
s
cos
I
sin
I
(5.20)
This equality holds if and only if
ν
in
=
g
u
s
tan
I
/
(5.21a)
This equation represents a subset of Perkins' (1973) solutions for the height of
the ionosphere based on wind and gravitational forces but ignoring any electric
Magnetic field
line
Ionospheric layer
Wind
B
Figure 5.10
A ping-pong ball model for the midlatitude ionosphere. [After Kelley (2002).
Reproduced with permission of Elsevier.]
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