POLAR is a NASA/GSFC solar-terrestrial mission within the GGS (Global Geospace Science) program and the ISTP (International Solar Terrestrial Physics) program. The objective is to study the ionospheric role in substorm phenomena and in the overall magnetospheric energy balance (measurements of plasma entry and transport into the northern dayside cusp regions at high altitudes and over the southern polar cap at low altitudes, global imaging of the northern auroral zone). 1) 2) 3)
The ISTP Global Geospace Science (GGS) program consists of three spacecraft missions (WIND, POLAR, and GEOTAIL), coordinated ground observing investigations, and theory and modeling investigations. Within the Sun-Earth Connections fleet, POLAR has the responsibility for multi-wavelength imaging of the aurora, measuring the entry of plasma into the polar magnetosphere and the geomagnetic tail, the flow of plasma to and from the ionosphere, and the deposition of particle energy in the ionosphere and upper atmosphere.
Figure 1: Illustration of the POLAR spacecraft (image credit: NASA)
The spacecraft development was managed by NASA; it was built at Lockheed Martin Astro Space in East Windsor, NJ. The bus is spin-stabilized at 10 rpm (a smaller platform of the S/C is despun and can be pointed to maintain the viewing field of certain instruments). The size of the bus is 2.4 m diameter, 1.8 m height (cylinder with conductive surfaces). Solar arrays provide 440 W, including 186 W for payload. 4)
Spacecraft mass = 1297 kg (269 kg propellant, 264 kg science payload). Nominal design life of 3 years.
Launch: The POLAR launch occurred on February 24, 1996 on a Delta II vehicle from Vandenberg Air Force Base, CA.
Orbit: Polar elliptical orbit with an apogee of 9 RE (56,500 km) and a perigee of 2 RE (11,500 km); inclination = 86º; period of ~17.5 hours.
Figure 2: Schematic of the orbit plane precession over a period of several years of the POLAR mission (image credit: NASA)
Figure 3: Artist's rendition of the deployed POLAR spacecraft (image credit: NASA)
POLAR mission status:
During its lifetime, POLAR has had many accomplishments. Observations of energetic neutral atoms have provided the first-ever global images of substorm injections that are the sequence of events that lead to energetic auroral displays. These neutral atom images clearly show the broad extent in space of these energetic atoms and their instantaneous nature in time.
Polar observations have also revealed that solar storms deposit so much energy into Earth's ionosphere that it expands to fill the magnetosphere all the way out to its boundaries, yielding the first-ever global X-ray images of auroras, and shown how dynamic pressure pulses, or ”gusts” in the solar wind, influence the magnetosphere, ionosphere, and auroral ovals, rings around Earth's magnetic poles where auroras are seen.
• POLAR supported the THEMIS prime science magnetotail campaign which took place from January to April 2008. This represented another mission extension from Sept. 2007 through April 2008. The fuel of POLAR was depleted during its final maneuver in February 2008. But even after the fuel was exhausted, the maneuver was continued on the cold helium gas that was left in the tank.
• During a space weather storm on Oct. 22, 2001 POLAR's VIS (Visible Imaging System) observed the aurora borealis and aurora australis (northern and southern lights) expanding and brightening in parallel at opposite ends of the world. This represents the first-ever movie of auroras dancing simultaneously around both of Earth's polar regions. 7) 8)
Figure 4: Schematic of the orbit plane precession over the entire period of the POLAR mission (image credit: NASA)
Observation of local electromagnetic fields in the low frequency range: (MFI, EFI, PWI, HYDRA, TIDE/PSI)
MFE (Magnetic Field Experiment):
MFE PI: C. Russell, UCLA. Objectives: Study of the coupling of the solar wind and the magnetosphere through currents driven in the polar cusp (energy and momentum exchange with the magnetosphere at the cusp-magnetosheath interface). 11)
Instrument: two fluxgate magnetometers mounted on a 6 m boom with associated electronics inside the spacecraft. The sampling of the magnetic field is within the frequency range from 0 - 50 Hz. Magnetic field strengths between 10-6 and 0.6 Gauss are detectable. The magnetic field is measured in three ranges: ±700 nT, ±5700 nT, and ±47000 nT.
Figure 5: View of the MFE instrument (image credit: UCLA)
The fluxgate sensors are ring core-types, each composed of a 'driver' coil and a 'feedback' coil surrounding a ring-shaped magnetically permeable core. The drive coil is used to periodically drive the the core into saturation, while the feedback coil zeros out any DC field in the core. When the presence of external 'DC' fields in the core are sensed by the appearance of second harmonics of the drive frequency, the current in the feedback coil is changed to keep the core field near zero. Three orthogonal sensors make each of the two fluxgate units. 'Flippers' can mechanically change a sensor in the spacecraft spin plane with the one along the spin axis in order to determine any zero-level offsets in the measured fields.
EFI (Electric Fields Instrument):
EFI PI: F. Mozer, U. of California, Berkeley. Objectives: Study of the ambient vector electric field structure of the high-latitude magnetosphere, the cusp, and the plasma mantle. 12)
EFI is a boom-mounted dual-probe instrument. Sampling of the electric field is performed between 0 and 20 kHz. Electric field strengths between 0.1 and > 1000 mV per meter at a rate of 40 samples per second in the normal mode, more than 1000 samples/s in the burst mode. The burst will be coordinated with HYDRA and TIDE instruments.
PWI (Plasma Wave Instrument):
PWI PI: D. Gurnett, U. of Iowa. Objectives: Study of wave/particle processes mediated by electromagnetic turbulence (momentum transfer in the geospace system, particularly in the boundaries).
The PWI employs seven distinct sensors for detecting the electric and magnetic fields of plasma waves. These sensors consist of: a pair of orthogonal two-sphere electric antennas in the spin plane of the S/C with sphere-to-sphere separations of 100 m and 130 m respectively, shared with the EFI; a short two-sphere electric antenna aligned along the spacecraft spin axis with a sphere-to-sphere separation of 14 m, also shared with EFI; a triaxial magnetic search coil mounted on the end of a rigid stacer boom, shared with EFI; and a magnetic loop antenna mounted on the same boom and oriented parallel to the 100 m electric antenna in the spin plane. The spheres on the electric antennas are 9 cm in diameter and each contains a high-impedance preamplifier that provides signals to the boom deployment mechanisms. Amplifiers in the deployment mechanisms buffer signals to EFI and PWI independently. Each of the three magnetic search coils consists of two bobbins mounted on high permeability micro-metal cores 40 cm long. The loop sensitivity constant is 385 micro V/nT Hz and the resonance frequency is 45 kHz.
Status: PWI exhibited an undervoltage condition on the digital power monitor (DPM) and lost all digital telemetry on Sept. 16, 1997. The probable source of the problem was an ”open” (~10 ohm) circuit occurring in the power supply cutting current to the digital logic. On Oct. 17, 1997, the instrument power will remain on even though it is still in an undervoltage condition and can provide PWI with no data. Leaving the power on has two benefits: 1) EFI has access to the PWI magnetic search coil antennas, and 2) the search coils are prevented from becoming too cold during long eclipse period. 13)
HYDRA (Hot Plasma Analyzer Experiment):
HYDRA PI: J. Scudder, GSFC. Objectives: Study of low-energy electrons in the global magnetic topology. Study of electron and ion signatures that accompany geomagnetic substorms, auroral arcs, field-aligned currents, and particle precipitation.
The HYDRA ensemble is comprised of three subsystems: DDEIS (Duo-Deca-Electron-Ion-Spectrometer) or an ensemble of electrostatic analyzers measuring electrons and ions in energy per unit charge between 1 eV and 30 keV and observes them in 12 directions simultaneously. In addition, a device with a position-sensitive array, called PPA (Parallel Plate Analyzer), images electrons within and near the magnetic field with a 1.5º angular resolution, energy ranges between 10 eV and 10 keV. Sampling rates: 2/second. The third subsystem is the DPU (Data Processing Unit) and the UV calibration system.
Figure 6: Illustration of the HYDRA subsystems (image credit: University of Iowa)
TIDE/PSI (Thermal Ion Dynamics Experiment / Plasma Source Investigation):
TIDE/PSI PI: T. Moore, NASA/MSFC. Objectives: Study of low-energy ions (transport mechanisms) to evaluate the ionosphere as a source of plasma for the magnetosphere. TIDE samples ions extracted from the ionosphere (whose mass is determined by a time-of-flight scheme). Energy range: 0.1 eV per charge to 100 eV per charge. 14)
TIDE views the plasma environment through seven independent energy analyzers that combine large collection area electrostatic mirrors with conventional retarding potential analyzers. The seven apertures are arranged in a fan to resolve polar angle relative to the spacecraft spin axis, while the spin of the spacecraft sweeps them through azimuth angle, providing nearly a full view of the sky. This permits the measurement of winds up to 300 km/s in arbitrary directions and temperature up to several million K (300 eV).
PSI is a source of low energy xenon ions and electrons that serves to ”ground” the POLAR spacecraft electrically to the space plasma environment. Without PSI, the spacecraft would become positively charged in low density plasmas owing to emission of electrons by the photoelectric effect of sunlight on the spacecraft surfaces. This charge would prevent low energy ions from ever reaching the spacecraft or the TIDE apertures. Conversely, in the very hot plasmas that produce auroras, the spacecraft would become negatively charged by as much as several kV, potentially leading to harmful discharges.
Figure 7: The TIDE instrument with TOF analyzer cover removed to expose the detector systems (image credit: NASA/MSFC)
Figure 8: View of the PSI module (image credit: NASA/MSFC)
Observation of particle populations associated with electromagnetic fields (TIMAS, CAMMICE, CEPPAD)
TIMAS (Toroidal Imaging Mass-Angle Spectrograph):
TIMAS PI: E. Shelley, Lockheed Palo Alto Research Lab and W. K. Perterson, Laboratory for Atmospheric and Space Physics at the University of Colorado, Boulder, CO. Objectives: Study of the properties, location, and morphology of the polar cusp, which is the principal source region for entry of solar wind plasma and hot ionospheric plasma into the magnetosphere. The TIMAS instrument samples ions of resolved mass that are either energized ions of ionospheric origin or stored particles of solar wind origin. Energy range: 50 eV - 30 keV. Sampling rate is 10 times per minute (one per satellite spin). TIMAS data is used in combination with data from TIDE and from SWICS on the WIND satellite. 15) 16)
Table 1: Key TIMAS instrument parameters
Figure 9: Illustration of the TIMAS instrument (image credit: LASP)
TIMAS status: On Oct 29, 1999, all telemetry reported from TIMAS became invalid (zeroes). Thereafter, collection of valid values was sporadic, ending on July 15, 2000. The fault appeared to be located at the interface between TIMAS and the GGS Telemetry Module 1 (GTM1) and was loosely temperature dependent. On March 27, 2001 the POLAR spacecraft switched to its backup telemetry module and restored telemetry capture of the TIMAS mid-energy mass spectrometer. TIMAS immediately detected new terrestrial source ion signature at the dayside magnetosphere during magnetic storm period. 17)
CAMMICE (Charge and Mass Magnetospheric Ion Composition Experiment):
CAMMICE PI: T. Fritz, Boston University (formerly of Los Alamos National Laboratory). Objectives: Study the mechanisms that control the energization, storage, and precipitation of particles in the high-latitude magnetosphere. CAMMICE permits determination of the composition of major ion constituents in the near-Earth plasma sheet and in the ring current. For full angular distribution, CAMMICE measures at a rate of 10 samples per minute, or once per spin of the S/C. The angular resolution approaches 0.2º. 18) 19)
CAMMICE) consists of two sensor systems HIT (Heavy Ion Telescope) and MICS (Magnetospheric Ion Composition Sensor) designed to measure the charge and mass composition within the Earth's magnetosphere over the energy range of 6 keV/q to 60 MeV/ion.
• HIT uses a three-element solid-state detector telescope to measure the rate of energy loss and the ion incident energy. These parameters permit a unique determination of the ion mass, elemental identification, and incident energy over the energy range from 100 keV per ion to 60 MeV per ion.
• MICS uses an ellipse-shaped electrostatic analyzer, a secondary-electron generation/detection system and a solid-state detector to measure the energy, time-of-flight, and the energy per charge of the incident ion flux. These three parameters permit a unique determination of the ion charge state, mass, and incident energy over the energy range from 6 keV/e to 400 keV/e.
Figure 10: Illustration of the HIT device (image credit: Boston University)
Figure 11: Photo of the MICS device (image credit: MPAE Lindau)
CEPPAD (Comprehensive Energetic-Particle Pitch Angle Distribution):
CEPPAD PI: B. Blake, The Aerospace Corporation, El Segundo, CA). Objectives: Investigation of quantitative information on the sources, energization, transport, and losses of energetic particles in the magnetosphere. Measurement of the rate of particle precipitation into the Earth's upper atmosphere. The CEPPAD investigation uses a variety of techniques to provide detailed energy spectra and angular distributions of energetic particles. 20)
The CEPPAD instrument consists of three packages. Two are spacecraft body mounted and the third is located on the despun platform. The first body-mounted package consists of the IPS (Imaging Proton Sensor) and the DPU (Digital Processing Unit). The second consists of the IES (Imaging Electron Sensor) and the HIST (High Sensitivity Telescope). The single despun platform package is the SEPS (Source/Lose-Cone Energetic Particle Spectrometer). The IPS, IES and HIST all use the body-mounted DPU. The SEPS sensor is independent of the body mounted sensor and contains a separate digital processing unit. This approach was taken because of the limitations inherent in communicating between the spacecraft body and despun platform.
• IPS (Imaging Proton Sensor). The IPS measures protons over the energy range from about 14 - 1500 keV in 16 energy bands over nine separate look directions. It uses a spectrometer which incorporates MSSD (Microstrip Solid State Detector) having a planar configuration with six individual elements. The nine detectors are arranged to be at the polar angles: 10º, 30º, 50º, 70º, 90º, 110º, 130º, 150º, and 170º with respect to the S/C spin axis. Each detector has an IFOV of 20º (polar direction) x 11.25º (azimuthal direction). The total IFOV is 180º x 11.25º. A full coverage of the unit sphere is obtained once every 6 second spin. Although designed to measure ions in-situ, the IPS detects also ENAs (Energetic Neutral Atoms) with high efficiency. 21) 22)
Figure 12: Illustration of the IPS instrument (image credit: The Aerospace Corp.)
• IES (Imaging Electron Sensor). The IES measures electrons over the energy range from about 25 - 400 keV using a spectrometer which incorporates a MSSD with a 0.5 cm x 2.1 cm planar configuration and five individual elements. The MSSD forms the image plane for a sensor segment with a ”pin-hole” aperture. The MSSD has a thick “dead layer” and thus does not respond to protons with energies below about 250 keV. The “pin-hole” aperture accepts electrons over a 60º angular segment in a plane containing the satellite spin axis. Each of the five detector elements in the MSSD detects electrons in a 12º angular subinterval of the 60º FOV. The complete IES system has a nominal geometric factor of 6 x 10-3 cm2. The nominal angular resolution of a detector element is 12º x 12º and its elemental geometric factor is approximately 3.8 x 10-4.
The HIST (located under the IES) uses three detector elements to measure electrons from 350 keV to 10 MeV and protons from 2.15 to 80 MeV. Detector A is a 300 micrometer thick, 300 square mm surface-barrier. Detector B is a 2000 micrometer thick, 200 square mm ORTEC surface-barrier. Detector C is a Bicron plastic scintillator with a Hamamatsu R3668 photomultiplier tube. The HIST attempts to provide a ”clean” measurement of very energetic electrons. 23)
Figure 13: The IES and HIST instruments (image credit: Boston University)
• SEPS (Source/Lose-Cone Energetic Particle Spectrometer). SEPS consists of two independent telescopes which measure both the energetic electron and ion fluxes in the vicinity of the magnetic field-aligned loss, and source cone regions with high sensitivity and with fine angular and time resolution. The electron telescope has twice the sensor area of the ion telescope and uses aperture wheels to vary its dynamic range. Particle angular imaging is obtained using pinhole camera apertures in front of the electron XY position-sensitive detectors. The ion telescope is similar to the electron telescope except for the reduced sensor area and the fact that the aperture wheels are replaced by magnets which sweep out electrons. 24)
Table 2: Parameter definition of SEPS
Imaging instruments for spatial observations (UVI, VIS, PIXIE)
High resolution global imaging from Polar is a critical element for determining solar influenced controls of the upper atmosphere: 25)
• The importance of magnetospheric dynamics on the aeronomy of the upper atmosphere is known but it has never been clear how important, how extensive, how often, and how extreme the effect can be.
• POLAR imaging provides a global view of the location and allows the intensity of the energetic processes to be inferred.
• POLAR investigators have shown that catalytic atmospheric species can vary by as much as a factor of ten during extreme storm conditions.
UVI (Ultraviolet Imager):
UVI PI: M. Torr, NASA/MSFC. Objectives: Study of the spatial and temporal descriptions of the aurora and images of total particle flux, characteristic energy, thermospheric neutral composition, and ionospheric conductances. UVI images the dayside and nightside auroras in the vacuum UV range using five specially designed filters. The detector is an intensified CCD used in conjunction with a fast reflective optical system to image an 8º FOV at a nominal rate of two frames per minute. 26)
The UVI is able to detect and provide images of very dim emissions with a wavelength resolution never achievable before. The resulting images permit to quantify the overall effects of solar energy input to the Earth's polar regions. The key wavelengths imaged are: 130.4 nm (OI 1304 filter), 135.6 nm (OI 1256), 140-160 nm (LBH short), 160-175 nm (LBH long), and 175-190 nm (solar). LBH stands for Lyman-Birge-Hopefield (spectral bands in the 140-180 nm range).
Table 3: Instrument parameters of the UVI
Figure 14: Photo of the UVI instrument during assembly (image credit: NASA)
VIS (Visible Imaging System):
VIS PI: L. Frank, University of Iowa. Objectives: Quantitative assessment of the dissipation of magnetospheric energy into the auroral ionosphere. Development of an energy flow model within the magnetosphere 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 solar-wind conditions, and to identify the locations of and mechanisms for suprathermal charged-particle acceleration.
The VIS instrument consists of a set of three low-light-level cameras. Two of these cameras share primary and secondary optics and are designed to provide images of the night-time auroral oval at visible wavelengths (FOV= 20ºx 20º). A third camera is used to monitor the FOV directions of the sensitive auroral cameras with respect to the sunlit Earth. The two auroral cameras have spatial resolution of 10 km (0.011º x 0.013º) and 20 km (0.22º x 0.25º) respectively from an orbital altitude of 8 RE. A CCD image has a size of 256 x 256 pixels and takes 12 s to acquire. VIS uses an image intensifier readout through 12 visible narrow-band filters producing five separate auroral images per minute.27) 28)
Figure 15: Photo of the VIS instrument (image credit: University of Iowa)
PIXIE (Polar Ionospheric X-ray Imaging Experiment):
PIXIE PI: W. Imhof, Lockheed Palo Alto Research Lab. Objectives: Study of the morphology and spectra of energetic electron precipitation and its effect on the atmosphere. Derivation of the total electron energy deposition rate, the energy distribution of the precipitating electrons, and the altitude profile of ionization and electrical conductivity. 29)
PIXIE uses a pinhole camera concept to measure the spatial distribution and temporal variation of X-ray emissions from the Earth's atmosphere. The time resolution of images is 60 seconds or better (the optimal radial distance to obtain the best images is 6 RE).
Table 4: Instrument summary of the Polar S/C payload
Figure 16: Illustration of the PIXIE instrument (image credit: The Aerospace Corp.)
Data: Onboard recording capability of science data (1.3 Gbit digital tape recorder). Science data transmission of 56 kbit/s (RT) and 512 kbit/s (PB) to DSN (four 60 minute nominal contact periods per day). The Polar S/C provides onboard interconnection of instrumentation for data communication. Data sharing among the instruments can be triggered by pattern recognition schemes in onboard computers.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.