Background notes about FAST auroral cavity nugget:

1. Auroral arcs are produced by energetic electrons that are accelerated
along magnetic field lines by electric potential drops.  The current
carried by auroral electrons is part of a global electrical circuit that
results of plasma convection throughout earth's magnetosphere and is ultimately
powered by the solar wind.

2. The auroral acceleration region is the interface between the "collisionless"
magnetospheric plasma and the "resistive" ionosphere. The role of electric
fields is very different in the two regions. In the collisionless plasma, the
plasma flows are perpendicular to electric and magnetic fields (ExB flow).
When the magnetospheric electric field maps along the B field to the 
ionosphere,
collisions between the ions, electrons, and neutral gas allow the electrons to
drift in the direction of the perpendicular E field and carry current. The
divergence of this horizontal current at boundaries (such as the polar cap
boundary) must be carried by magnetic field-aligned currents connecting to the
magnetosphere. The simple description is that collisional drag on the
ionospheric plasma is tranferred to the magnetospheric plasma by the
field-aligned current.

3. The parallel electric fields that produce auroral acceleration are created
in order to maintain current continuity for the field-aligned 
currents described
above. The primary requirements that lead to parallel electric fields are
quasi-neutrality and current continuity.  The hot ionospheric electrons are
the primary current carriers. Earth's dipole magnetic field forms a magnetic
mirror, so a parallel electric field is needed to balance the mirror force
acting on the electron current carriers. This electric field forms the high
altitude portion of the auroral acceleration potential. This explaination for
parallel electric fields was proposed in the 1960's, but it is 
incomplete, since
it doesn't explain the formation of the auroal density cavity.

4.  The auroral density cavity results from a double potential drop.  The lower
potential structure originates primarily because of secondary electrons that
are created when energetic auroral electrons collide with the upper atmosphere.
Each primary electron creates many secondary electrons and ions. The hot
electrons expand upward, but the more massive ions are gravitationally bound.
An ambipolar electric field is formed to restrain these electrons. It also
can accelerate some of the ions which escape upward as an ion beam. The
resulting region between the upper and lower potential structures thus becomes
a low density cavity that contains mainly upward accelerated ions and downward
accelerated magnetospheric electrons. FAST measurements have allowed us to
understand the structure and orgin of this region in detail.

5.  The density cavity has further consequences that aren't described in this
nugget.  One extremely important consequence of the cavity is the generation
of Auroral Kilometric Radiation by the electron cyclotron maser 
mechanism, which
we presented in an earlier nugget. This mechanism requires both the low density
cavity for the maser mechanism to work, and the energetic electron beam which
supplies the energy source.

6. The unique capabilities of FAST instruments to obtain high resolution plasma
and field measurements were essential for discovering the properties of the
auroral density cavity.