The polar cap is a region in which the terrestrial field lines are magnetically connected to the
solar wind. Accordingly the solar wind's convection electric field is mapped across the polar cap,
causing there to be a potential induced across the polar cap. Since the solar wind electric field is
typically in the dawn-dusk directions, the potential is also in the dawn-dusk directions. The size of the
polar cap, the extent of the region with open field lines, and so the size of polar cap potential
depends on the
amount of magnetic reconnection occurring at the magnetopause. Accordingly the polar cap potential
is larger when the IMF 
component is southwards. Measured values range from about 20 kV in
quiet times to about 150 kV in active times.
Figure 17.7 [Hughes, 1995] shows how plasma and the frozen-in field lines move in the magnetosphere and across and around the polar cap during times of southward IMF and enhanced magnetic reconnection near the nose of the magnetopause.
Figure 17.7: Flow of plasma and associated frozen-in field lines in the magnetosphere and across the
polar cap as a result of magnetic reconnection at the magnetopause for southwards IMF [Hughes, 1995].
The return paths of the anti-sunwards plasma flow across the polar cap lead to the auroral electrojets.
Note that the plasma flows anti-sunward across the polar cap due to the 
drift, as required
for the field lines to convect from the dayside to the nightside, but then forms return paths on the
dusk and dawn sides at lower latitudes. The flow pattern thus has two cells. These return paths give rise
to the so-called ``auroral (or convection) electrojets'' responsible for the magnetic perturbations caused
by auroral activity.
It should be questioned why these convection patterns give rise to any currents. The reason for currents
arising is due to the weakly collisional nature of the ionospheric plasma: both electrons and ions experience
the magnetic and electric forces, leading to drift but the more frequent collisions of
ions rather than electrons with atmospheric neutrals and the smaller electron mass lead to the electrons
undergoing the drift while the ions have a much smaller net drift, thereby leading to
a current flowing in the drift direction. The currents are concentrated in the auroral
oval due to the much higher conductivity there (due to higher plasma densities), thereby leading to
currents flowing in the auroral electrojets.
Finally, note that the directions of the convection velocities and currents reverse when the IMF reverses directions. This implies that substantial variations should exist in the magnetic fields observed at auroral, polar cap, and mid latitudes during space weather events, as indeed described in Lecture 15 and associated references. Figure 17.8 illustrates these differences schematically [Cravens, 1997] and it is noted that the current in the electrojets during substorms is considerably larger and more concentrated near midnight than during quiet times.
Figure 17.8: The different directions of plasma and current flow in the auroral electrojets during
quiet times (``convection electrojets'') and substorms [Cravens, 1997].