Hydro- and Electrodynamics of the Midlatitude Ionosphere - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 220

5 Hydro- and Electrodynamics of

the Midlatitude Ionosphere

The classic aeronomy of the midlatitude ionosphere is discussed in the topics by
Rishbeth and Garriott (1969) and Schunk and Nagy (2000). Our goal here is to
treat what we feel are the most interesting hydrodynamical and electrodynamical
processes that arise in midlatitude ionospheric physics. In both a geographic and
dynamical sense, the midlatitude zone is a buffer between the low-latitude pro-
cesses discussed in Chapters 3 and 4 and the high-latitude phenomena presented in
later chapters. Both electric fields and perturbed neutral winds penetrate from high-
latitude sources, while equatorial plasma streams into the region along the magnetic
field lines. Atmospheric tides are quite important, since they grow in intensity with
altitude, as do the gravity waves that continually roll in from high latitudes and/or
up from stratospheric and tropospheric sources. These are just a few of the dynam-
ical interactions that continue to make the study of midlatitude ionospheric physics
challenging and interesting. We explore the dynamics and electrodynamics of the
midlatitude ionosphere here and its plasma physics in Chapter 6.

5.1 Introduction to the Tropical and Midlatitude
Ionospheres

5.1.1 Background Material
In this context we might define the tropical zone as that region where the mag-
netic field has a significant dip angle yet cannot be considered to be nearly
vertical, as is the case in the auroral zone and polar cap. The latitudes of the
Arecibo and, now closed, the St. Santin Radar Observatories are ideally suited
for study of this zone (see Appendix A for position data on several sites dis-
cussed in the text). The dip angle at Arecibo, for example, is 46 7 ◦ , which allows

approximate geometric equality between forces parallel and perpendicular to B .
Important information also comes from the Millstone Hill Observatory, which is
equatorward of St. Santin in geographic coordinates but poleward of St. Santin
geomagnetically. Millstone is thus in the transition zone between high and low

.