Ionospheric Response to Electric Fields - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 450

At high latitudes, the electric fields are impressed from outside the ionosphere/
thermosphere system and the possibility for a motor arises. In fact, the origin of
the force on the neutrals is again the J

× B force, only in the high-latitude case

the expression

J = σ · ( E + U × B )

is usually dominated by the impressed electric field, E . In the F region, where the
conductivity tensor is diagonal, the force becomes

σ

p E ×

F = σ B

where E is the electric field in the neutral frame of reference. A data set taken in
the evening auroral oval, which shows the importance of electrodynamic forcing,
is presented in Fig. 9.13a and b. Figure 9.13a is an all-sky camera picture showing
the location of several chemical releases made from the same rocket. The rocket
was launched from Poker Flat, Alaska, at 1810 local time on February 28, 1978.
Figure 9.13b is a schematic diagram that describes the various features in the
photograph. The two visible barium (Ba) ion clouds have been driven by the
E

B drift at high velocity toward the west, since E was the usual northward
electric field at that time period. The two circular strontium (Sr) neutral clouds
have hardly moved since their release (which was at the same time and altitude
(

×

200 km) as the ion clouds). However, the trimethyl aluminum (TMA) trail
shows a high-velocity region streaming in the same direction as the ion cloud
at about one-third the plasma cloud velocity. More quantitative data from the
same experiment are given in Fig. 9.14. The three solid tracks in the figure show
sequential locations of the barium ion clouds and are labeled I205, I268, and
I214, corresponding to the three ion (I) cloud altitudes in kilometers. Minute
markers show the temporal progression of the three clouds, whichmoved parallel
to the auroral oval at about 1 km/s. The two dashed lines markedN131 andN140
show the neutral (N) trail locations at the peak in the two neutral velocity profiles,
which occurred at 131 and 140 km, respectively. Notice that the southern trail
moved very nearly parallel to the E

≥

×

B direction with a velocity equal to about
27% of the velocity of the ion cloud nearest in horizontal location. The northern
trail had a speed equal to 44% of the nearest ion cloud velocity and a direction
rotated slightly poleward of the E B velocity. The high-altitude neutral clouds

(N210, N278, and N209) moved much more slowly and had a considerably
different direction of motion from the E

×

B direction.
Some of these results may be explained in a straightforwardmanner. First, note
that since the E field maps uniformly with altitude, the main altitude dependence
in F is in the

×

σ P term. The simultaneous Chatanika radar data showed that
this term (primarily due to σ P ) peaked at 140 km altitude, in excellent agreement

with the observed peaks in the neutral wind profiles. A crude estimate of the time