As described in Lectures 15 and 16, Earth's auroral regions are coupled to the solar wind,
magnetosphere, and ionosphere and are therefore the site of a number of space
weather phenomena. These include the electromagnetic radiation of the ``aurora borealis''and
``aurora australis'' (the ``northern'' and ``southern lights'', respectively), enhanced ionization and
conductivity in the auroral ionosphere due to impact ionization processes, enhanced particle
precipitation, and the rapidly varying magnetic fields associated with currents flowing
in the auroral electrojet regions of the ionosphere. This Lecture contains a brief discussion of the
physical processes involved in the production of the auroral lights, the different classes of
auroral radiation associated with various precipitating particle populations, the existence of
field-aligned currents and potential drops in the auroral regions, auroral substorms,
and the auroral electrojets. The environment of the polar cap will also be discussed, including the
characteristic size of the potential across the polar cap, the 
motion of plasma across
the polar cap, and the auroral electrojets.