High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 411

point the open or closed state of the flux tube becomes an important issue. In
Fig. 8.17 the field line remains open and convects down the tail and toward
the earth-sun line until it assumes the position of the original field tube (1 -1).
The associated convection is labeled 1, 2, 3, 4, 1 in which the antisunward flow
toward the earth-sun line at the magnetopause corresponds to sunward flow
3 ,4 ,1 in the ionosphere. Alternative or additional convection paths in which
field tubes reconnect at point 3 and subsequently flow sunward in a plasma
sheet that thickens along the earth-sun line have been proposed to explain the
possible existence of closed magnetic field lines at the highest latitudes. It should
be emphasized that the precise location of the transition from open to closed
field lines in the ionosphere can be quite variable within the picture we have
drawn.
The convective motion described above exists in addition to the viscous inter-
action mechanism described earlier, and we must invoke both of them to explain
the four-cell convection pattern that is most frequently observed.We note that the
tendency of the flow of the inner two cells to be preferentially directed toward
dawn or dusk, depending on whether B y is positive or negative, can again be
explained by the effective “tension” in connection to the IMF (see Fig. 8.16).
8.4 Empirical and Analytical Representations of
High-Latitude Convection
As we shall see in Chapter 9, knowledge of the high-latitude ionospheric convec-
tion pattern is required to effectively calculate the distribution and composition
of plasma in the F region and to determine the F-region neutral wind field and
Joule heating rate. For such studies it is necessary to know the ionospheric flow
velocity at all points at any given time. This requirement has led to the develop-
ment of several empirical and semi-empirical models for the convection pattern
designed to mimic one or more of the features described in previous sections.
These models are derived from a synthesis or statistical analysis of satellite and
radar data. A global representation of the electrostatic potential distribution
derived from satellite electric field measurements is shown in Fig. 8.18. This
empirical model is valid for southward B z and is shown for the IMF with B y
negative (a) and positive (b). It depicts several of the major properties of the
convective flow that we have previously noted in individual cases. For example,
the ion flow or electric field is larger on the dawn side than on the dusk side in
the Northern Hemisphere when B y is positive (i.e., the potential lines are closer
together). Figure 8.18b also shows the existence of a crescent-shaped convec-
tion cell on the dusk side similar to that shown in Fig. 8.10d. A comparison
of Fig. 8.18a and b shows the change in the flow configuration near noon when
B y changes sign, as depicted in Fig. 8.8d. Data such as this can be useful in