R. A. Hoffman
Laboratory for Extraterrestrial Physics, Code 696, Goddard Space Flight Center, Greenbelt, MD 20771, USA, E-mail:HOFFMAN@eldyn2.gsfc.nasa.gov
The Polar spacecraft plays a key role in the Inter Agency Solar Terrestrial Physics Program, whose primary objective is to acquire a global understanding of the flow of energy, momentum and mass through the Sun-Earth connected system. With its high apogee over the northern polar region it is positioned to observe many phenomena associated with coupling between the solar wind, the polar magnetosphere, the geomagnetic tail and the ionosphere and atmosphere below. Its full complement of space plasma instruments and three auroral imagers represent the state-of-the-art in experimental capabilities. Participation in the Polar mission is open to all scientists, both to affect the operation of the spacecraft to collect data for particular science problems, or to lead the analysis of data pertaining to identified geophysical events. Considerable information to aid scientific participation is available on the world wide web.
With the launch of Polar on 24 February, 1996, NASA completed placing into orbit all of its spacecraft contributions to the Inter Agency Solar Terrestrial Physics Program (IASTP). Polar became operational at the beginning of April and continues to provide new and unique data to the program.
Fig.1. Energy flow through the Sun-Earth connection system. The Sun's energy via the solar wind flows through the interplanetary meduim, into the various parts of the magnetosphere, and into the ionosphere and upper atmosphere. The roles of the various spacecraft of the IASTP program and ground-based facilities in measuring the energy flow are indicated in the diagram.
The primary objective of the IASTP is to acquire a global understanding of the flow of energy, momentum and mass through the Sun-Earth connected system (Figure 1). The energy and momentum of interest leaves the Sun via the solar wind. The major components of the system --—interplanetary space, the parts of the magnetosphere, the ionosphere and the atmosphere— can be compared to cellular structures separated by thin boundary layers. The bulk of the mass, momentum and energy resides within the large volume cells that play different roles in the transport, storage and evolution of these quantities within the system. However, the physics of the boundary layers determines the coupling of these quantities between the cells. The accomplishment of the primary objective of the IASTP program requires an understanding of the cause and effect relationships within and between the cells of the system.
Because the coupling of energy and momentum between the cells of the Sun-Earth connected system is highly dependent on the temporal and spatial variations of plasma parameters within the cells, the IASTP program is especially interested in the study of transient flows. Transients are related to the discontinuities in the properties of the solar wind whose effects propagate through the system, sometimes culminating in substorms or magnetic storms. Emphasis is being given to coordinating operations among the elements of the IASTP program when the configuration of the satellites is favorable for such studies.
The Polar spacecraft plays a key role in this program and its role has become more critical with the tragic loss of the Cluster mission. Polar is measuring the entry of plasma from the solar wind and energetic charged particles into the polar magnetosphere and the geomagnetic tail and the injection of energy from the geomagnetic tail through the polar auroral magnetosphere into the ionosphere and atmosphere, especially during periods of geomagnetic activity (Figure 1). It measures the flow of plasma out of the ionosphere through the polar magnetosphere to populate the geomagnetic tail and eventually the equatorial magnetosphere. Through detailed observations of the optical and X-ray emissions from the polar atmosphere, Polar also is determining the control exercised by plasma processes in the polar magnetosphere on this energy deposition by charged particles into the Earth's atmosphere.
The roles of the other spacecraft and the extensive facilities making ground-based observations are illustrated schematically in Figure 1. The successful launch of the Fast Auroral SnapshoT (FAST) satellite on August 21, 1996, within NASA's Small Explorer Program adds very high resolution measurements of coupling phenomena between the polar auroral magnetosphere and the ionosphere.
A detailed description of the Polar spacecraft, instruments, collaborations and ground data processing system appears in Russell (1995), as well as in many world wide web sites that will be discussed later.
The Polar spacecraft was placed into an eccentric orbit with an 86° inclination with apogee at 9.0 Re and perigee at 1.8 Re geocentric distances. Apogee is located and will stay for several years over the northern polar region. The changes in latitude and local time of apogee as a function of time are plotted in Figure 2. This figure shows that the orbit plane rotates through all local times in about one year, or about one degree per day, but the latitude changes only about 18 degrees per year. This orbit was selected for excellent long duration imaging of the northern auroral oval and for sampling at high altitudes the characteristics and flows of the ion distributions emanating from the auroral and polar regions of the ionosphere. In the southern hemisphere the high perigee will allow the spacecraft to cut across auroral field lines at altitudes where auroral particle acceleration and precipitation processes are thought to occur.
Fig. 2. The geographic latitude and local time of apogee as a function of day of year in 1996 for POLAR.
The Polar spacecraft contains a full complement of instruments that represent the state-of-the-art in experimental space plasma physics. Both new technology detectors and advanced data processing techniques have been incorporated within the instruments. A summary of the instruments and measurement ranges is given in Table 1. Detailed descriptions of the instruments appear in Russell (1995). A sketch of the spacecraft showing locations of the instruments appears as Figure 3. The spacecraft is spin stabilized with the spin axis normal to the orbit plane.
Table 1. Summary of the Polar Instruments
|Magnetic field instrument
Electric field instrument
Plasma wave instrument
First plasma analyzer
Thermal ion dynamics experiment
Torodial imaging mass-angle
Comprehensive energetic particle
pitch angle distrubution
Charge and mass magnetospheric
ion compositon experiment
Polar ionospheric X-ray imaging
Visible imaging system
|dc magnetic fields; 0 - 10 Hz
electric fields and plasma density
0 to above 20 kHz
ac electric and magnetic field
0.1 Hz - 800 kHz
energy, high resolution distributions of ions
and electrons 2 eV to 35 keV
mass, energy, distributions of low energy ions
and electrons; 0 eV to 300 eV
mass, energy, distributions of medium energy
ions 15 eV to 32 keV
mass, energy, distributions of high energy ions
10keV to 1 MeV
high energy ions
6keV to 60 MeV
imaging of auroral X-rays
3 to 60 keV
imaging of auroral at visible wavelengths
earth camera at ultraviolet wavelengths
imaging of aurora at ultraviolet wavelengths
The three auroral imaging instruments are mounted on a despun platform on one end of the spacecraft which is programmed to point at various portions of the auroral oval. For thermal and power reasons, this end of the spacecraft cannot be illuminated, so the spacecraft must be inverted every six months. The Plasma Wave Instrument includes a wideband receiver that provides essentially continuous waveforms over a broad bandwidth, up to 90 kHz. A Xenon ion gun associated with the TIDE instrument is used to control the potential of the spacecraft. As part of the CEPPAD investigation, a source loss cone energetic particle spectrometer (SEPS) is mounted on the despun platform. During the operations of SEPS the platform can be pointed along the magnetic field to obtain the distribution and energy of particles in the loss cone. Due to the importance of the auroral imagers to the IASTP program their characteristics are given in Table 2. _The images provide an instantaneous reference system for the in-situ measurements from the variety of IASTP spacecraft with respect to the spatially and temporally varying phenomena in near-Earth space. From the image data some knowledge of the highly variable ionospheric conductivity patterns over the dark auroral and polar regions can be obtained.
Table 2. Characteristics of the Auroral Imagers
|Instrument||FOV (deg)||Wavelength||Apogee Spatial
|20 x 20
(5.4 x 6.3)
20 x 20
(2.8 x 3.3)
20 x 20
40 x 40
308 to 732 nm
308 to 732 nm
124 to 149 nm
* Multiwire position-sensitive gas proportional counter
Fig. 3. Drawing of the Polar spacecraft showing the locations of the scientific instruments.
Coordination of the IASTP missions must be maintained through their flight phases to optimize the opportunities for major advances in our understanding of the Sun-Earth system. An international body, the Inter-Agency Consultative Group (IACG), promotes such coordination through scientific campaigns that address questions that can only be answered by observations from the multiple spacecraft. Four campaigns have been identified to date, including magnetotail energy flow and the role of nonlinear dynamics, boundaries in collisionless plasmas, and solar events and their manifestations in interplanetary space and geospace. Workshops have been held to define the strategy for each campaign. Due to optimum spacecraft orbit characteristics, the Working Group for joint mission planning of the IACG has endorsed a five month period including October 1996 through February 1997 as phase II of the first two topics. In addition the months of December 1996 through February 1997 have been declared the International Auroral Study period.
The Science Planning and Operations Facility (SPOF) at Goddard Space Flight Center develops the detailed science operations plans for the Wind and Polar spacecraft, thus assisting the implementation of the campaigns defined by the IACG. It also identifies periods when the ISTP satellite configuration is optimal for special studies, such as when the Wind and Geotail spacecraft cross the magnetic tail simultaneously while IMP-8 is monitoring the solar wind. The Geospace Energy Model (GEM) program of the National Science Foundation and the Solar Terrestrial Energy Program (STEP) administered by the Scientific Committee on Solar Terrestrial Physics (SCOSTEP) are coordinating the involvement of the broader scientific community and especially the correlative ground observations.
SCIENCE COMMUNITY PARTICIPATION
Participation in the Polar mission is open to all scientists. Access to information needed to participate is available primarily on the world wide web and will be described within the context of NASA's ISTP program (NASA's part of IASTP). Just one URL will introduce the interested scientist to a table of contents for all ISTP home pages: http://www-istp.gsfc.nasa.gov. A "cover page" provides direct access to each major entity of the ISTP program, and this is followed by a table of contents for each major entity.
Three types of aids for science operations planning are available for the scientist interested in affecting the operations schedules to acquire data for a particular problem. For macro-scheduling of operations, the IACG campaign periods are defined, and the International Geophysical Calendar provides World Days for coordinated operations especially with radars and for which Polar operations will be given high priority. A long-term Polar science operations plan with appropriate updates is also on the web. For detailed scheduling of ISTP spacecraft the SPOF has generated a variety of spacecraft trajectory plots with a GIF_WALK web interface (see http://www-spof.gsfc.nasa.gov). One plot-type maps the magnetic footprint of the Polar satellite with respect to ground-based observatories. The Satellite Situation Center of the National Space Science Data Center at Goddard produces region occupancy reports, i.e., times when the spacecraft are within statistically defined regions of the magnetosphere. For optimal configurations of the ISTP spacecraft, or any special types of operations, especially for the Polar spacecraft, GGS Special Operations Periods (GSOPs) are defined. To obtain information on the pointing directions of the Polar auroral imagers, the despun platform pointing plan is also available. All this information can assist any scientist in the submission of a Science Planning Operations Topic (SPOT) to the SPOF by using a template that appears on the web (http://www-spof.gsfc.nasa.gov/scripts/spot.html). A SPOT specifies special times and operational configurations of spacecraft and instrument modes for the purpose of collecting data for a particular science problem. The acceptance of such a plan commits the Polar program to acquire the requested data and to coordinate its operations with other ISTP facilities. A master schedule of operations is being compiled and will be available on the web. The plan will be archived so that searches can be performed for particular operational conditions.
A number of data aids are available, the most important being Key Parameter (KP) Survey Plots, also with a GIF_WALK web interface (http://cdaweb.gsfc.nasa.gov/cdaweb/istp_public/). These low-resolution time series data plots are designed for easy survey of the ISTP scientific data for scientifically interesting events worthy of detailed analysis. Plots of Key Parameters from Wind, Geotail, IMP-8, various geosynchronous satellite data and ground-based observatories are regularly available, and the Polar KP Survey Plot is under construction and will appear also on the web. The Key Parameter data set itself can be obtained on CD-ROMs that are produced on a routine basis as the data become available. Currently over 50,000 instrument data days have appeared in this form. These CD-ROMs can be obtained by interested researchers by contacting WMISH@ISTP1.gsfc.nasa.gov. The Space Physics Data Facility at Goddard has developed two visualization tools, the Space Physics Catalog (SPyCAT), which is a web-based interface to request publicly available KPs, and the Coordinated Data Analysis Web (CDAWeb), a web-based system for browse, plotting and listing. Most instruments in the ISTP program have their own home pages, many of which provide additional data plots.
Any scientist can suggest an analysis project by the submission of a GGS Event, for which a template appears on the web. If accepted by the GGS science team (Wind and Polar scientists) this scientist must be willing to maintain a web page about the event and lead the analysis effort. Data from the required instruments will be submitted to the Central Data Handling Facility at Goddard in CDF format for compilation and distribution on a CD-ROM. Opening of the data sets to the entire community at the earliest practical time is a goal of NASA's ISTP, and therefore, Polar program.
The assistance of W. H. Mish, M. Peredo and M. Hesse in the preparation of this paper is appreciated.
Russell, C. T., ed., The Global Geospace Mission, Kluwer Academic Publishers, Dordrect, The Netherlands, (1995).