High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 385

We showed in Chapter 3 that, for an electrostatic field, ∇ ⊥ · ( E 1 × s ) is very

small in the F region and above (incompressible flow), and thus

E I · ∇ ⊥ P + E I ׈ s · ∇ ⊥ H

J

|| = P ( ∇ ⊥ ·

E I ) + (8.15)

This expression shows explicitly that field-aligned currents are intimately
related to spatial variations in the ionospheric electric field and conductivity. It
is important to realize, however, that the field-aligned current in (8.15) is driven
by some divergence of current in the generator, but modified by conductivity
gradients and polarization field in the ionosphere.
Next, consider the electric field in the closed field line region of the magne-
tosphere. (We are still considering the southward IMF case.) In Chapter 1 we
saw that the magnetic field lines in the boundary layer and the plasma sheet are
distorted from a dipole shape to produce a magnetic tail extending away from
the sun. This magnetic geometry has a tension (see Chapter 2) that exerts a force
on the plasma. Together with the pressure gradient and the potential difference
applied across the magnetosphere by the flowing solar wind, these forces pro-
duce motion of the magnetospheric plasma on closed field lines and an associated
dawn-to-dusk magnetospheric electric field in the tail. Further, since the gradient
and tension forces are equivalent to the J B force, the geometry requires the

existence of electric currents and vice versa. Figure 8.2a shows the configura-
tion of electric fields and currents in the ionosphere and magnetosphere. The tail
(or neutral sheet) current, J T , flows across the tail near the magnetic equatorial
plane to support the curl implied by the stretched magnetic field geometry. The
tail current is closed primarily in the magnetosheath by currents flowing on the
magnetopause (which are not shown in the figure). The ring current, J R , to first
order flows in closed loops around the earth at distances between 2 and 10 earth
radii

×

and is driven primarily by pressure gradient forces. Any divergences in
these currents must be closed by field-aligned currents that enter the ionosphere.
The portion of the tail and ring current that is closed through the ionosphere is
called the partial ring current, J PR . These currents, the so-called Region 2 cur-
rents, are labeled R 2 in the figure and link the inner magnetosphere with the
auroral oval near its equatorward edge. Region 1 currents, labeled R 1 , link the
poleward portion of the auroral oval and the polar cap to the magnetosheath,
solar wind, or the boundary layer plasma near the magnetopause. Figure 8.2b
shows how the magnetospheric electric field, E m , maps to the ionosphere on
closed field lines.
The ionosphere is not a passive element in this circuit because some of the
hot plasma in the magnetosphere can move along the magnetic field and strike
the atmosphere, producing significant ionization. This particle precipitation
produces the discrete and diffuse auroral airglow and can play a dominant role
in determining the ionospheric conductivity in the eclipsed ionosphere. Notice
that the electric field mapping causes the sign of the electric field to be reversed
(e.g., Fig. 8.2b) so that in the ionospheric auroral zone the electric field E a is

( R e )