High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 392

direct connection between the ionosphere and the solar wind magnetic fields.
This process, called “viscous interaction,” produces a relatively narrow region
of antisunward-flowing plasma just inside the magnetopause. This region is now
called the equatorial magnetospheric boundary layer, and in this region the anti-
sunward plasma flow occurs on closed field lines. The ionospheric flow pattern
we described above for the electrical connection model (often called reconnec-
tion) does not differ dramatically for a theory in which viscous interaction pro-
duces the plasma flow in the ionosphere and thus the electric field pattern. The
major difference is that the antisunward convecting plasma is on closed instead
of open field lines. Second, when the interplanetary magnetic field is northward,
the viscous interaction idea does not change very much but connection to the
interplanetary magnetic field and the solar wind potential is drastically different.
Since B z is northward roughly one-half of the time, it is not surprising that some
of the flow patterns shown below are quite different from the idealized two-cell
pattern discussed thus far.
We also need to discuss the physics at the inner boundary of the region domi-
nated by magnetospheric processes. We know from Chapters 3 and 5 that the
electric field pattern on field lines within 3 or 4 earth radii is controlled primarily
by the atmosphere. How does this transition take place? As the plasma in the
magnetosphere flows toward the earth, it encounters an increasing magnetic field
strength. Since the first adiabatic invariant is conserved (see Section 2.5.2), the
perpendicular energy of the plasma increases. Since the gradient and curvature
drifts of these particles depend on both their energy and charge, a zonal charge
separation occurs with positive charges at dusk and negative charges at dawn.
This creates an electric field pointed from dusk toward dawn in the inner mag-
netosphere, which tends to cancel out the applied dawn-dusk electric field. The
inner magnetosphere is therefore shielded from the magnetospheric electric field
and the plasma flows around this region. This shielding only operates on long
time scales, however, and fluctuations of the external field with periods shorter
than eight hours or so can penetrate (see Chapter 3).
Since the magnetospheric electric field is reduced to almost zero earthward
of the ring current, the electric field due to the rotation of the earth becomes
comparable to the magnetospheric field near this boundary. When this source of
the plasma motion is included, the ionospheric flow paths acquire a corotating
component and look like those shown in Fig. 8.5. At latitudes below about
50 ◦ (not shown on this figure), the plasma undergoes circular convection paths
around the earth at the corotation speed. On the dusk side near 60 ◦ latitude,
the corotation and the magnetospheric electric fields oppose each other, leading
to complex flow trajectories that involve a stagnation point, marked S . At still
higher latitudes, the two-cell convection pattern is preserved. The corresponding
plasma flow paths in the equatorial plane of the inner magnetosphere might look
like those shown in Fig. 8.6. Near the earth in the shaded region, plasma flows
in concentric circles. This region is called the plasmasphere because it contains
a cool, dense plasma. This region has a high plasma content since, once per