The Mission

The Polar spacecraft is the second mission of NASA's Global Geospace Science (GGS) program. It will perform simultaneous, coordinated measurements of the key regions of Earth's geospace, or space environment, with WIND, which was launched last November to measure the solar wind properties. A large array of ground-based scientific observatories and mission related theoretical investigations will also be involved.

The Polar spacecraft, carrying 11 instruments, is scheduled for launch in December on a Delta II rocket from the Western Space and Missile Center, Lompoc, Calif.. Its orbit around the Earth will be inclined 86 degrees from the equator. The furthest point from the Earth on the orbit–the apogee–will be eight Earth radii altitude (32,000 miles or 51,000 kilometers), and the closest point–the perigee–will be 0.8 Earth radius (3,200 miles or 5,100 kilometers).

NASA is collaborating with the European Space Agency (ESA) and the Japanese Institute of Space and Astronautical Science (ISAS) in three additional solar-terrestrial missions, Geotail, SOHO and Cluster. Geotail was launched in July 1992 to explore the tail of the magnetosphere, which is the region around the planet dominated by the Earth's magnetic field. These missions, together with GGS, make up the International Solar-Terrestrial Physics (ISTP) science initiative. The aim of ISTP is to understand the physical effects of solar activity on interplanetary space and the Earth's space environment. This will lead to the capability of predicting the responses of each part of the Sun-Earth connected system to solar activity.

The Science

During periods of magnetic activity, called substorms, some of these particles escape from their regions of storage and precipitate into the upper atmosphere to excite the atoms and molecules of the atmosphere, which then emit light known as the aurora, or the Northern and Southern Lights. These auroras give a visible signature of energy transfer from the Sun to the Earth.

The Polar mission will observe the entry and trans-port of the solar plasma over the Earth's magnetic poles and measure the energy lost into the atmo-sphere by the precipitation of particles by imaging the northern aurora. Polar will also measure the flow of plasma out of the atmosphere on auroral magnetic field lines to add to the population of the magnetosphere. Thus Polar will provide the information on the last phases of the energy transfer processes from the Sun to the Earth by means of the corpuscular radiations from the Sun.

Investigations

• Three instruments (MFE, EFI and PWI) to observe electromagnetic and electrostatic fields in the vicinity of the Polar spacecraft.

• Five instruments (Hydra, TIDE, TIMAS, CAMMICE AND CEPPAD) to characterize the particle populations found in the high-latitude magnetosphere.

• Three instruments (UVI, VIS, and Pixie) to image the auroral emissions in ultraviolet light, visible light and X-rays. These imaging instruments are located on a despun platform which can be pointed to continuously view the Earth.

Electric Fields Investigation (EFI), Dr. F. Mozer, University of California at Berkeley—The three axis, dual probe electric field instrument will sample electric fields at a rate of 40 samples per second in the normal mode and more than 1,000 samples per second in the burst mode. It will provide data for determining the modes, phase velocities and wavelengths of propagating waves and time domain spatial structures. It will also be used to infer the electric-field structure of the high-latitude magnetosphere, the cusp and the plasma mantle, as well as provide direct evidence for field-aligned electric potential drops, which may be responsible for accelerating particles to high energies.

Plasma Wave Investigation (PWI), Dr. D. Gurnett, University of Iowa—PWI will investigate wave-particle interactions in the Earth's magneto-sphere by measuring the spectral and wave vector characteristics of electromagnetic and electrostatic plasma waves. Wave-particle processes are thought to play a central role in the transfer of energy and momentum in the magnetospheric plasma, particularly at the boundaries between magnetospheric regions. The PWI will sample the electric and magnetic components of plasma waves from the ion cyclotron frequencies to frequencies well above the local plasma frequency.

Hot Plasma Analyzer (Hydra), Dr. J. Scudder, University of Iowa—The array of Hydra electro-static analyzers will measure the energy and angular distributions of low energy electrons and ions to identify (in connection with the WIND measurements of GGS) what parts of the near Earth environment are interconnected more or less directly with the Sun. Thus Hydra is focused on the physics of low energy particle access to the near Earth environment and the role of these particles in transient changes to this access. Detailed knowledge of these particles is especially important because they produce the aurora when precipitated into the atmosphere, are charge carriers of field aligned currents, and are a major carrier of energy from the magnetosphere to the atmosphere.

Thermal Ion Dynamics Experiment and Plasma Source Instrument (TIDE-PSI), Dr. T. Moore, NASA/Marshall Space Flight Center—TIDE will measure the composition, winds, temperature and heat flows of low-energy ions from the ionosphere and solar wind. These measurements will be used to identify the mass-dependent energization mechanisms that drive low-energy ion flows, to investigate the transport paths along which plasma is circulated in the polar regions, and to evaluate the ionosphere and solar wind as sources of the storm-time hot plasmas in the magnetosphere. The PSI emits a low temperature plasma from the spacecraft that will keep POLAR electrically "grounded" to the plasma environment so that the lowest energy particles can reach the TIDE instrument.

Toroidal Imaging Mass-Angle Spectrograph (TIMAS), Dr. E. Shelley, Lockheed Martin Palo Alto Research Laboratory—TIMAS will sample the higher-energy ions of ionospheric origin and stored particles of solar wind origin than TIDE. Routine data are taken 10 times a minute—once per satellite spin—but this rate is reprogrammable by ground command. TIMAS will study the location, properties and morphology of the polar cusp, a key source region for entry of solar wind plasma and the hot ionospheric plasma into the magnetosphere. The data will be used in combination with data from TIDE and data from the WIND spacecraft to determine the ultimate origin of the ion plasma in the high-latitude magnetosphere. Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE), Dr. T. Fritz, Boston University—CAMMICE will determine the composition of major ion constituents in the ring current and near-Earth plasma sheet. For full angular distributions, CAMMICE measures at a rate of 10 samples per minute, or once per spin period. This investigation will lead to an understanding of the mechanisms that control the energization, storage and precipitation of particles in the high-latitude magnetosphere.

Comprehensive Energetic-Particle Pitch Angle Distribution (CEPPAD), Dr. B. Blake, Aerospace Corp.—The CEPPAD investigation provides detailed energy spectra and angular distributions of energetic particles. The sensors separate protons from electrons and provide a complete three-dimensional particle distribution measurement every six seconds. It will provide quantitative information on the sources, energization, transport and losses of energetic particles in the magnetosphere. It also will measure the rate of particle precipitation into the Earth's upper atmosphere, part of the energy input that causes auroras and other emissions.

Ultraviolet Imager (UVI), Dr. G. Parks, University of Washington—UVI will image the dayside and nightside auroras in the vacuum ultraviolet range using five specially designed filters. The detector is an intensified charge-coupled device used in conjunction with a fast reflective optical system to image an eight-degree field of view at a nominal rate of two frames per minute. UVI will provide spatial and temporal descriptions of the auroras. Data from the images will be used as input to models to calculate the total particle energy flux deposited into the atmosphere, the characteristic energies, ionospheric conductances and ultimately the spatial evolution of electric fields.

Visible Imaging System (VIS), Dr. L. Frank, University of Iowa—VIS will use an image intensifier system and 12 visible narrowband filters and produce five separate auroral images per minute. Data from VIS and theory investigations will be used to assess the dissipation of magneto-spheric energy into the auroral ionosphere. A model of energy flow within the magnetosphere will be developed using VIS data in three ways: to illustrate the topology of the magnetosphere, to delineate the response of the magnetosphere and the magnetotail to substorms and variable solar-wind conditions and to identify the locations of suprathermal charged-particle acceleration. An Earth camera will provide full Earth images in ultraviolet wavelengths at a lower repetition rate.

Polar Ionospheric X-Ray Imaging Experiment (PIXIE), Dr. D. Chenette, Lockheed Martin Palo Alto Research Laboratory—PIXIE will measure the spatial distribution and temporal variation of X-ray emissions from the Earth's atmosphere due to the precipitation of electrons from the magneto-sphere at energies generally higher than those producing the UV and visible emissions. PIXIE will use a pin-hole camera concept to obtain global images at intervals of 60 seconds or better. The morphology of the X-ray emission patterns, the energy distribution of the precipitating electrons producing the X-rays, and their effects upon the atmosphere will be calculated from the observa-tions. These data will be used to further calculate the total electron energy deposition rate and the altitude profile of electrical conductivity.

Spacecraft and Mission Operations

Several NASA and Goddard Space Flight Center (GSFC) facilities will play key roles in the collection and dissemination of Polar science data. The NASA Deep Space Network will be used to command the spacecraft and to collect Polar science data via radio link. At GSFC raw data will be processed, organized and stored. The Project's Central Data Handling Facility will produce "key parameter data" for surveying the much larger volume of raw data. Detailed analysis of the data will be performed by investigators at their own sites and the data will be shared through the NASA Science Internet connections throughout the United States, Japan and Europe.

Management

NASA/Goddard Space Flight Center, Greenbelt, Md.
Joseph A. Dezio, Project Manager, Flight Projects Directorate
Dr. Mario H. Acuņa, ISTP Project Scientist, Space Sciences Directorate
Dr. Robert A. Hoffman, Polar Deputy Project Scientist, Space Sciences Directorate