Geoscience Reference
In-Depth Information
5.2 Electric Fields in the Tropical and Midlatitude Zone
We turn now to consideration of the electric field that exists in the earth-fixed
reference frame, where, of course, the corotation field vanishes. In some sense
this corresponds to the “electrodynamic weather” just as neutral atmospheric
weather-related winds are measured in the rotating frame.
5.2.1 Electric Field Measurements
The lowest line in Fig. 5.6 is a plot of the average value of the Sq (solar quiet)
dynamo electric field as a function of latitude. This electric field source has been
discussed briefly in Chapter 3 and originates from the divergence in the electric
currents driven by tidal motions of the neutral atmosphere in the highly con-
ducting E layer (Matsushita, 1967, 1971). The solar heating source dominates
the tides, although lunar gravitational effects are also important. Chapman and
Lindzen (1970) have discussed tidal theory in great detail, and several applica-
tions of this theory involving electrodynamic calculations have been made (e.g.,
Matsushita, 1967; Richmond et al., 1976). The mathematics in these models
is complicated but the physics is relatively straightforward, and we will not
discuss these models extensively here. To first order they yield reasonable agree-
ment with quiet-time ionospheric electric fields, particularly in the daytime low-
latitude ionosphere. The results are not at all good at night and also deteriorate
with increasing latitude. (A comparison of tidal theory with Jicamarca measure-
ments, which show some of these features, was given in Chapter 3.) In this text
we emphasize the processes that are less well understood and thus concentrate
on the nighttime period at low (but not equatorial) latitudes and the influence of
auroral zone effects at the interface between the high-latitude and midlatitude
zones.
To organize our study, we use incoherent scatter data from the Northern
Hemisphere sites Arecibo (18
5
◦
;31
◦
), St. Santin (44
1
◦
;40
◦
), andMillstone Hill
.
.
6
◦
;57
◦
). The first number in the parentheses is the geographic latitude and
the second the geomagnetic latitude. Other relevant details concerning these sites
are included in Appendix A. In the discussion we distinguish between the tropical
and the midlatitude ionosphere. The former is characterized by an intermediate
dip angle and a sufficient enough distance from the auroral zone that most of the
time the ionosphere is unaffected by high-latitude electrodynamics. The Arecibo
data set is clearly tropical (31
◦
geomagnetic latitude; 48
◦
dip angle). St. Santin
andMillstone Hill have similar midlatitude geographic positions, but the latter is
much farther north geomagnetically (57
◦
versus 40
◦
). These two sites will thus be
the prototypes for our study of the differences between tropical and midlatitudes
and will be closely investigated for evidence of auroral effects.
Our starting point is the empirical study by Richmond et al. (1980), some
results of which are presented here in Fig. 5.8a-d. Magnetically quiet periods
.
(42
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