**Polar Campaign**

**Auroral Acceleration Region**

**Scientific Rationale**

We do not understand well how the magnetosphere and the ionosphere are
coupled. We see features in the aurora but do not know to what features
or processes in the magnetosphere these auroral features correspond.

**Approach**

We need conjugate studies with measurements in the range 2-4 R~E~ over
the aurora. Satellite conjunctions are difficult. THEMIS has addressed
this problem by establishing an all-sky-imager network covering all of
Canada and Alaska. This network will be completely installed in the fall
of 2005 and winter of 2006 before THEMIS is launched. It is available
for conjugate studies with Polar. An associated network of magnetometers
is also available. THEMIS benefits by having a pilot science campaign to
hone its end-to-end data and analysis systems prior to the launch of
THEMIS in the fall of 2006.

**The Campaign**

**a) Polar observations**

In mid-September the Polar orbit plane lies in the non-midnight meridian
with perigee at mid northern latitudes and apogee at mid southern
latitudes over the nightside. The inbound pass to perigee cuts through
auroral field lines in the critical 2 to 4 R~E~ geocentric distance
range. Over the next three months the orbit plane sweeps over the
midnight or dusk sector, a critical region for substorm onset. Then from
January through March the perigee region passes over the dawn to
midnight sector. Polar would take in situ particles and fields data in
Science Mode 2 while magnetically connected to the THEMIS network.

**b) THEMIS observations**

The THEMIS all-sky-imagers require a dark sky although the magnetometer
network does not. Thus their prime operating period is from
mid-September through mid-March. The THEMIS network would provide a
composite all sky image from Labrador to Alaska and the northern 48
states to the arctic ocean. These images will be available in thumbnail
versions in near real time at a rate of up to 0.1 Hz. The magnetometer
data will be available at 2 Hz. Images will be taken whenever the sky is
dark enough to detect the auroras.

**c) Other observations**

FAST may be operational at this time. If so, it can fill in below
Polar's perigee.

**What can FAST add to the campaign?**

**Auroral Acceleration Region Physics**

During fall of 2005, FAST will be acquiring data in the northern
hemisphere, while Polar perigee will also be in the north. Thus, both
spacecraft are ideally suited to provide complementary measurements of
particles and fields in and above the auroral acceleration region. The
THEMIS ground system facilities will provide measurements of magnetic
fields and auroral emissions at the foot print of the auroral flux
tubes.

The auroral acceleration region can be divided into three types:

1)  Inverted-V aurora

2)  Return current aurora

3)  Alfvèn aurora

Taking these in order:

The inverted-V aurora are the classical aurora, generated by electrons
that have fallen through a large scale potential drop. These electrons
carry upward current. The currents are usually relatively unstructured.

The return current aurora are associated with downward currents. The
electrons in this region are usually bidirectional (albeit with a net
upward flux), and the distribution function has been heavily processes
by wave-particle interactions. The currents can be quite filamentary.

The Alfvèn aurora are often associated with the plasma sheet boundary
layer. Much like the return current aurora, the electron fluxes are
bidirectional and modified by wave-particle interactions. The
field-aligned currents are highly structured, and the net current can be
near zero. Sometimes a narrow flow channel is observed in the region of
Alfvèn aurora.

The conjunctions of Polar, FAST and THEMIS ground-stations provide a
unique opportunity to further our understanding of the auroral
acceleration region.

Observations:

Polar observations: higher altitude field-aligned currents, source
plasmas, structure of the magnetospheric flows. FAST observations:
field-aligned currents, accelerated particles, possible electrostatic
shocks. THEMIS ground observations: magnetic fields from the ionospheric
currents, inferred flows, precipitating particle spectra as inferred
from auroral signatures.

Physics addressed:

Mapping of electric fields and currents from Polar to FAST altitudes: Is
the ionosphere shielded from the magnetospheric convection by the
parallel potential drops? Are field-aligned currents more or less
filamented as a function of altitude? If degree of filamentation
changes, is it a function of current polarity?

Conversion of electromagnetic energy carried by Alfvèn waves to
energetic particle fluxes: Provide an energy budget as a function of
altitude, testing energy conservation

Mapping of ionospheric closure currents: Where do the inverted-V
currents close? Are closure currents longitudinal (zonal) or latitudinal
(meridional)? What about conductivity enhancements as indicated bu
enhanced precipitating particle fluxes? Is there a region of zonal flow
at high latitudes that is associated with the boundary layer Alfvèn wave
aurora?