Geoscience Reference
In-Depth Information
To sun
B
E
2
1
2
2
1
2
2
2
1
1
1
Dusk
terminator
Dawn
terminator
2
2
2
2
1
1
11
2
2
1
1
2
1
E
B
Figure 3.5 Schematic diagram showing the zonal electric field component and its rela-
tionship to the charge densities at the terminators. Since the zonal electric field varies
slowly with height, the charge density is also a weak function of height from 100 to
500 km.
we expect the observed dayside and nightside fields to be the result of charge
buildup at the terminators, with the dusk terminator charged negatively and
the dawn terminator positively. Assuming a change of the electric field equal
to 2mV/m over a 2 h local time interval corresponding to a distance of about
3
m 3 . This is less than one excess
elemental charge per cubic meter, again illustrating how very small net charge
densities can yield significant electric fields in a plasma. A schematic diagram
based on the “charged terminator model” is given in Fig. 3.5 from the viewpoint
of looking down on the earth from above the Northern Hemisphere.
10 6 m at the equator,
10 21 C
×
ρ
7
×
/
c
3.2 The Equatorial F-Region Dynamo
Before discussing the F-region dynamo, we need to understand a little about
the thermospheric winds in the equatorial region, since they provide the source
of energy that maintains the electric field. Early observational data on thermo-
spheric winds came from studies of the drag exerted on artificial satellites by
the neutral atmosphere (King-Hele, 1970). Analysis of such data shows that the
change in satellite inclination is directly related to the angular velocity of the
atmosphere with respect to the rotating earth. The surprising conclusion was
that, on average, the low-latitude thermosphere superrotates; that is, there is
a net eastward average zonal flow of about 150m/s near 350 km altitude and
about 50m/s at 200 km altitude. The effect is most pronounced in the 2100-2400
local time period. Direct measurements of thermospheric winds are now avail-
able at night using the Fabry-Perot technique to determine the Doppler shift of
airglow emissions (Sipler and Biondi, 1978; Biondi et al., 1999). Some of these
data (which were taken over Kwajalein in the Pacific Sector) are reproduced in
Fig. 3.6a in the form of azimuth plots of the tip of a vector in the direction toward
which the wind is blowing (note that at Kwajalein 8 UT is equal to 2000 LT).
Search WWH ::




Custom Search