POLAR: EXPLORING EARTHSPACE IN THE REALMS OF THE AURORAS FROM THE SUN TO EARTH

Despite the relative vacuum of space, the Sun emits a highspeed solar wind of
electrified particles.  This thin, hot, ionized gas-a plasma called the solar
wind- carries particles and magnetic fields outward from the Sun to the far
reaches of the solar system.  Earth is shielded from the full blast by its
magnetosphere, the region around our planet dominated by its magnetic field.
Some solar wind plasma penetrates the magnetosphere's shield, mixing
turbulently with plasmas there, and is then stored in the two Van Allen
radiation belts around the Earth or in the plasma sheet.  Some particles
descend into Earth's upper atmosphere through the polar cusps, funnel-like
openings in the magnetosphere at the poles.  These energetic particles excite
atoms and molecules in the upper atmosphere to create the Northern and Southern
Lights (the auroras).  The study of the auroras offers a unique opportunity
actually to observe effects of the transfer of energy from the Sun to the
Earth.

By imaging the northern aurora, NASA's POLAR mission will measure the entry of
solar plasma into the magnetosphere over the Earth's magnetic poles and will
study the ways in which this plasma impacts the Earth's uppermost atmosphere,
the ionosphere.  POLAR will measure the flow of plasma to and from the
ionosphere along auroral magnetic field lines and observe particle energy
deposited into the ionosphere and upper atmosphere.  With time, the orbit will
be adjusted to allow measurements to be taken at lower altitudes over the
northern pole and higher altitudes at the southern pole.  From this orbit,
images of the southern aurora will also be taken.

THE MISSION

The third in a series of coordinated solar-terrestrial missions, the POLAR
spacecraft, carrying eleven instruments developed by an international team of
scientists, will be launched in late 1995 by a Delta II rocket from the Western
Space and Missile Center. Its orbit around the Earth will be inclined 90
degrees from the equator.  The furthest point from the Earth's center on this
orbit (the apogee) will be nine Earth radii (57,000 km or 36,000 miles), and
the closest point (the perigee) will be almost two Earth radii (11,000 km or
7,100 miles).

The Geotail satellite, a joint mission of the Japanese Institute of Space and
Astronautical Science (ISAS) and NASA, was launched in July 1992 to study the
behavior of plasma in the tail of Earth's magnetosphere.  A second NASA
satellite, Wind, to be launched in late 1994, will measure the solar wind as it
approaches Earth. These missions will perform simultaneous and closely
coordinated measurements throughout the Earth's space environment, known as
geospace.  In addition, data will be provided from existing spacecraft in
orbits around the equator.  Groundbased and theoretical investigations will
also be conducted.

NASA is also collaborating with the European Space Agency (ESA) in two
additional solar-terrestrial missions, Cluster and the Solar and Heliospheric
Observatory (SOHO).  The Wind and POLAR missions and NASA's contributions to
SOHO, Cluster, and Geotail make up the International Solar- Terrestrial Physics
(ISTP) Science Program. The aim of ISTP is to understand the behavior of the
solarterrestrial system and, therefore, how the Earth's atmosphere responds to
changes in the solar wind.

INVESTIGATIONS

The POLAR instruments can be divided into three categories.  Three instruments
(MFE, EFI, and PWI) measure electromagnetic fields near the POLAR spacecraft.
Five devices (Hydra, TIDE, TIMAS, CAMMICE, and CEPPAD) determine the type of
particles found in the magnetosphere in the region of the Earth's poles.  Three
experiments (VIS, UVI, and PIXIE) image the Earth in visible light, ultraviolet
light, and x-rays.  These last three instruments are on a despun platform that
can be pointed to maintain their viewing field.

MAGNETIC FIELDS EXPERIMENT (MFE).

This investigation will use a pair of fluxgate magnetometers to sample the
magnetic field.  Raw measurements will be processed on board to send to other
instruments.  These measurements will be used to study how the solar wind and
the magnetosphere interact through currents driven in the polar cusp, how
energy and momentum are exchanged with the magnetosphere at the
cuspmagnetosheath interface, and how plasma instabilities occur in the polar
magnetosphere.  Dr. C. Russell, University of California at Los Angeles, is the
Principal Investigator (PI).

ELECTRIC FIELD INVESTIGATION (EFI).

This dual-probe instrument will sample electric fields in the burst mode, which
can be coordinated with the Hydra and TIDE instruments.  The EFI measurements
will be used to determine the electric field structure of the highlatitude
magnetosphere, the cusp, and the plasma mantle over the poles.  It will also
provide direct evidence for fieldaligned electrical potential drops, which may
be responsible for accelerating particles to high energies.  The PI is Dr. F.
Mozer, University of California at Berkeley.

PLASMA WAVE INVESTIGATION (PWI).

PWI will sample electric field noise at frequencies higher than EFI and
magnetic fluctuations above the frequencies detectable by MFE to help identify
normal patterns of plasma behavior.  Attempts will also be made to determine
how these waves propagate.  The wave-particle processes influenced by
electromagnetic turbulence are thought to be central in the transfer of
momentum in the geospace system, particularly in the boundaries between
regions.  Dr. D. Gurnett, University of Iowa, is the PI.

FAST PLASMA ANALYZER (Hydra).

Hydra's electrostatic analyzers will filter electrons and ions and study them
in twelve directions simultaneously.  Hydra will image electrons traveling near
the magnetic field direction with 1.5-degree angular resolution.  The
low-energy electrons sampled by Hydra will be especially good indicators of the
global magnetic shape when compared with simultaneous measurements by SWE in
the solar wind.  Hydra will also capture the patterns of electrons and ions
that accompany geomagnetic substorms, auroral arcs, field-aligned currents, and
particle precipitation.  The PI is Dr. J. Scudder, University of Iowa.

THERMAL ION DYNAMICS EXPERIMENT AND PLASMA SOURCE INSTRUMENT (TIDE/PSI).

TIDE will sample and measure the mass of lowenergy ions that come from the
ionosphere, identifiable by their relatively low state of ionization compared
with ions of solar wind origin.  These measurements will be used to evaluate
the ionosphere as a source of plasma for the magnetosphere, to identify how the
low- energy ionospheric ions are energized and transported, and to study their
storage and loss.  Dr. T. Moore, NASA/Marshall Space Flight Center, is the PI.

TOROIDAL IMAGIN MASS-ANGLE SPECTROGRAPH (TIMAS).

TIMAS will sample higher-energy ions that originate either in the ionosphere or
the solar wind.  The instrument will study the polar cusp, a principal entry
region for solar wind plasma into the magnetosphere.  TIMAS data will be used
with data from TIDE and the SWICS instrument on the Wind satellite to determine
the ultimate origin of mass in the high-latitude magnetosphere.  The PI is Dr.
E. Shelley, Lockheed Palo Alto Research Laboratory.

CHARGE AND MASS MAGNETOSPHERIC ION COMPOSITION EXPERIMENT (CAMMICE).

CAMMICE will determine the composition of major energetic ion constituents of
the ring current and nearEarth plasma sheet.  This investigation will increase
our understanding of the way in which particles in the magnetosphere are
energized, stored, and precipitated.  Dr. T. Fritz, Boston University, is the
PI.

COMPREHENSIVE ENERGETIC-PARTICLE PITCH-ANGLE DISTRIBUTION (CEPPAD).

The CEPPAD investigation will identify energetic particles and provide
quantitative data on the sources, energization, transport, and losses of these
particles in the magnetosphere.  It will also measure the rate of particle
precipitation into the Earth's upper atmosphere that is partly responsible for
auroras and other emissions.  Dr. B. Blake, Aerospace Corporation, is the PI.

ULTRAVIOLET IMAGER (UVI).

UVI will image the auroras using five specially designed filters.  The detector
is an intensified charge- coupled device used with a fast reflective optical
system and will provide simultaneous global imaging at distances of more than
six Earth radii.  UVI will provide descriptions of the auroras in time and
space and images of total particle energy flux, characteristic energy,
thermospheric neutral composition, and ionospheric conductances.  These images
will be used to map the evolution of electric fields needed for global modeling
of the thermosphere at the lower end of the solarterrestrial chain.  The PI is
Dr. M. Torr, NASA/Marshall Space Flight Center.

VISIBLE IMAGING SYSTEM (VIS).

VIS will use an image intensifier readout through twelve visible narrow band
filters and produce five separate auroral images per minute.  Data from VIS and
from theory investigations will be used to assess the way in which
magnetospheric energy is dissipated into the auroral ionosphere, and to model
energy flow within the magnetosphere.  The PI is Dr. L. Frank, University of
Iowa.

POLAR IONOSPHERIC X-RAY IMAGING EXPERIMENT (PIXIE).

Using a multiple-pinhole x-ray camera, PIXIE will measure the distribution and
time variation of x-ray emissions from the Earth's atmosphere.  The morphology
and spectra of energetic electron precipitation and its effect upon the
atmosphere will be determined and used to calculate the total energy deposition
rate of the precipitated electrons, their energy distribution, and the pattern
of ionization and electrical conductivity.  Dr. W. Imhof, Lockheed Palo Alto
Research Laboratory, is the PI.

THE POLAR SPACECRAFT AND MISSION OPERATIONS

The POLAR spacecraft is a spin-stabilized cylinder that measures 2.4 meters
(7.9 ft) in diameter and 2.1 meters (6.9 ft) in height.  The spacecraft weighs
approximately 1005 kg (2220 lbs) and carries an additional 269 kg (590 lbs) of
fuel for orbit maneuvering.  The mission is designed to last for at least three
years.

Several NASA facilities will support the collection and dissemination of POLAR
science data.  The NASA Deep Space Network will be used to command the
spacecraft and collect POLAR data via radio signals.  The Central Data Handling
Facility at NASA's Goddard Space Flight Center will produce "key parameters" to
serve as a guide to the much larger volume of raw data.  Detailed analysis of
the data will be performed by investigators using computers at their own sites
and sharing the data through NASA Science Internet connections throughout the
United States, Japan, and Europe.
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