Hydro- and Electrodynamics of the Midlatitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 255

toward the west. This wind, coupled with a negative meridional conductivity
gradient, would then create a disturbance dynamo electric variation of field in
the poleward direction. Just such a disturbance dynamo electric field may be
responsible for the Millstone Hill measurements at very low Kp values.
During very active times there is good evidence for such a wind pattern.
Quiet-time neutral wind measurements at Fritz Peak, Colorado (39 9 ◦ N 5 ◦ . , 105 .

W , for six nights are gathered in Fig. 5.20a, along with the predictions

of a thermospheric global circulation model (Hernandez and Roble, 1984). The
data and model both show eastward winds in the evening sector. On the other
hand, measurements made on an active day and shown in Fig. 5.20b display
strong westward winds until 0200 LT and an equally strong equatorward wind
from 2300 until 0400 LT. The unusual wind pattern was detected only north of
the station.

( L = 3 )

5.3 Midlatitude Lower Thermosphere Dynamics

5.3.1 Tidal Effects
When the E-region conductivity is high, the electrodynamics are driven by tidal
modes in the E-region. (These have already been discussed in Chapter 3.) As
noted, the semidiurnal tidal mode becomes important at midlatitudes, and we
expect that the daytime electric field will be dominated by semidiurnal tides at
Millstone Hill and St. Santin. Both diurnal and semidiurnal tides should con-
tribute at Arecibo. However, since the F-region dynamo and the high-latitude
electric field sources are both primarily diurnal in form, we might expect the
composite picture to be quite complex. This is borne out by the electric field
data in Fig. 5.8. Arecibo, St. Santin, and Millstone Hill all display semidiur-
nal variations in the zonal electric field component and diurnal behavior in the
meridional field.
Some of the richness of lower thermospheric dynamics may be visualized using
motion of the layers that form in the ionosphere as natural tracers. Plasma density
profiles over Arecibo for the period including sunset, sunrise, and the nighttime
hours are presented in Figs. 5.21 and 5.22. The F layer slowly undulates with a
several-hour period, whereas the lower-altitude layers exhibit both lower- and
higher-period variations. E-region layers are very common over Arecibo, and
the tidal modes can be visualized quite well. In the next chapter we discuss how
the layers are formed, but for now we take their existence as an experimental
fact and study their behavior. These lower-thermospheric oscillations are more
obvious at night than during the day, since plasma production by sunlight tends
to wash them out during the daytime. However, long-period oscillations such as
diurnal and semidiurnal tides can be traced both day and night if some care is
taken. Notice that the valley between the E and F layers has more plasma on the
day with high Kp (Fig. 5.22) than on the previous night. This is due to energetic
particle precipitation (Voss and Smith, 1979, 1980).