High-Latitude Electrodynamics - The Earth's Ionosphere: Plasma Physics and Electrodynamics - page 386

directed from dusk to dawn, which is opposite from the polar cap electric field.
The resulting ionospheric plasma flow and electric field patterns are shown in
Fig. 8.3a. The flow is made up of antisunward flow at the highest latitudes result-
ing from the connection of the open magnetic field lines to the solar wind electric
field discussed earlier. The return sunward flow in the auroral zones results from
the electric field E a , which in turn is determined by the potential difference across
the closed field line portion of the magnetosphere.
Figure 8.3b shows the electric and magnetic fields measured in a polar pass of
the S3-2 satellite field. The electric field detector had a higher spatial resolution
but otherwise the measured electric field looks remarkably similar to the mag-
netic field. It is easy to show that the Poynting flux is almost always downward
and consistent with

P values for the polar cap and auroral oval (see the next

section).
The electric field reversal at the polar cap boundary is associated with the
so-called Region 1 field-aligned or Birkeland currents ( R 1 in Fig. 8.2a) after the
scientist who first postulated their existence. They are closed at one end by cur-
rents in the magnetosheath. In the summer polar cap, a large fraction of the
Region 1 currents close across the conducting polar cap. Note from examina-
tion of Fig. 8.2 that the ionospheric and internal magnetospheric currents and
associated electric fields all have the same direction at any one place, designating
a load, whereas in the magnetosheath the electric field and current have opposite
signs, as required for a generator. In fact, the entire system is analogous to a
magnetohydrodynamic (MHD) generator, where the solar wind in the magne-
tosheath is the flowing conductor connected by the Region 1 Birkeland currents
to the ionosphere-magnetosphere system, which is the load. Region 2 currents
are connected to Region 1 currents by another B 2

dt current, since
the magnetic field increases as the plasma from the tail nears the earth and E

(ρ/ ) B × d V /

/ B

decreases. Here J × B is antisunward (observe that J flows counter to E in the

⊥ ⊥

equatorial plan of Fig. 8.2b).

8.1.3 Energy Transfer
Further insight into the methods by which energy is transmitted and converted
in this system can be obtained from the idealized model of Region 1 currents
and the closure current across the summer polar cap, shown in Fig. 8.4. Here,
two parallel current sheets of thickness dx are oriented in the y - z plane and
carry equal and opposite current densities in the z direction, which is along the
magnetic field. These current sheets represent the Region 1 currents connecting
theMHD generator in the solar wind or magnetosheath plasma to the ionosphere
on the dawn and dusk sides of the polar cap, respectively. The current sheets are
closed in the polar cap ionosphere, represented here as a resistive medium of
vertical extent h having a uniform conductivity, σ p , perpendicular to the current

sheets. Associated with the current sheets are a magnetic field (δ B ) and an electric

field (δ E ) . Now consider a surface S 1 that is bounded by a rectangular loop 1,