IMAGE (Imager for Magnetopause-to-Aurora Global Exploration)
IMAGE is the first MIDEX (Medium-class Explorer) mission of NASA/GSFC with the overall objective to study the global response of the Earth's magnetosphere to changes in the solar wind. Observations of high spatial and temporal resolution 3-D imagery of magnetospheric plasma motions. IMAGE uses ENA (Energetic Neutral Atom), ultraviolet, and radio imaging techniques to: 1) 2) 3)
• Identify the dominant mechanisms for injecting plasma into the magnetosphere on substorm and magnetic storm time scales
• Determine the directly driven response of the magnetosphere to solar wind changes
• Discover how and where magnetospheric plasmas are energized, transported, and subsequently lost during substorms and magnetic storms.
SwRI (Southwest Research Institute, PI: James L. Burch) of San Antonio, Texas, is the prime partner of NASA in this project.
The minisatellite, built by LMMS (Lockheed Martin Missiles & Space) of Sunnyvale, CA, employs a spin-stabilized platform. It has the form of a regular octagon and measures 2.25 m in diameter and 1.52 m in height. Surface-mounted solar cells (high-efficiency, dual-junction GaInP2/GaAs/Ge cells) provide an efficiency of 20-21.5% with average power of 250 W (21 Ah NiCd batteries for eclipse operation). S/C mass = 496 kg.
IMAGE has a nominal spin period of 2 minutes (or a spin rate of 0.5 ± 0.01 rpm); its spin axis is perpendicular to the orbital plane. The AD&C (Attitude Determination & Control) subsystem consists of the following actuators/sensors: a magnetic torque rod, a passive nutation damper, a three-axis magnetometer for magnetic aspect information, an enhanced sun sensor, and AST (Autonomous Star Tracker) developed by LMMS. AST is mounted with its boresight 10º from the spin axis. The two-axis sun sensor provides spin rate and sun aspect angle information. MCS (Magnetic Control System) controls both spin axis orientation and spin rate. Attitude knowledge is accurate to within 0.1º.
The C&DH (Command & Data Handling) subsystem design employs the MIL-STD-1553B bus which communicates with the instrument controllers over RS-422 interfaces. The CCSDS protocol is used for all internal and external communication. The 1553/CCSDS protocol design provides substantial advantages in terms of spacecraft/payload decoupling (the S/C serves mainly as a “bent pipe” for science data produced by the payload). All time synchronization between S/C and payload is accomplished exclusively over the 1553 bus. 4) 5) 6)
Figure 1: Isometric drawing of the IMAGE spacecraft (image credit: NASA, SwRI)
Table 1: Overview of S/C design features
Figure 2: Block diagram of the IMAGE electrical power subsystem (image credit: SwRI)
RF communications: Three antennas are used for S-band communication with the ground. A medium-gain helix antenna and two low-gain omni-directional antennas. One of the omni antennas is mounted on the aft end panel of the spacecraft; the other is mounted together with the helix antenna on the forward panel. The helix antenna is used to transmit data from the spacecraft to the ground; the co-mounted omni antenna is used to receive uplinked commands and data. Uplink data rate = 2 kbit/s. Downlink of stored science, engineering, and housekeeping data at a rate of 2.28 Mbit/s. In addition to the playback of stored data, the S/C also continuously transmits real-time data at a nominal rate of 38 kbit/s. The real-time data is mainly for CRL of Tokyo, Japan and for NOAA (space environment weather forecasts).
Launch: A launch on a Delta II 7623-9.5 ELV vehicle took place from VAFB, CA, on March 25, 2000.
Orbit: HEO (Highly-elliptical Earth Orbit), a polar orbit of 90º inclination, perigee = 1000 km, apogee = 7.2 RE (45,922 km). The location of the apogee changes during the course of the two-year mission, both in latitude and, because of the Earth's revolution about the sun, in local time.
At the beginning of the mission, apogee is at approximately 40º north geographic latitude and at dusk local time. As the Earth moves around the sun, the plane of the orbit shifts relative to the Earth-sun line (by 30º of longitude each month). During IMAGE's two-year nominal mission, the line of apsides will precess over the pole and return to 40º north geographic latitude. This type of orbit permits the IMAGE instruments to image the inner magnetosphere on nearly a continuous basis. 7)
Figure 3: Illustration of IMAGE orbit rotation over mission time (image credit: NASA)
Figure 4: Artist's view of the IMAGE spacecraft (image credit: NASA)
Operational status of the mission:
• The extensive archival database generated by IMAGE promises to yield new discoveries and will support investigations by other spacecraft and ground-based observatories for many years.
• In December 2005, the IMAGE spacecraft ended its operations, bringing to a close a successful mission of 5.8 years. IMAGE was the premier producer of new discoveries on the structure and dynamics of the Earth's external magnetic field (magnetosphere) and its contents.
- IMAGE was launched on March 25, 2000. It successfully completed its two-year primary mission and continued providing data into December 2005, when it stopped responding to commands from ground controllers. Analysis indicated the craft's power supply subsystems failed, rendering it lifeless. 8) 9)
Sensor complement: (LENA, MENA,HENA, EUV, FUV, WIC, SI, GEO, RPI)
Table 2: Overview of instrument developers
The IMAGE S/C carries three ENA (Energetic Neutral Atom) imagers whose combined energy coverage permits the detection of ENAs with energies ranging from 1 eV to 500 keV per atomic mass unit (amu). Each neutral atom instrument generates images showing the intensity and spatial distribution of ENA emissions produced in the inner magnetosphere through charge-exchange reactions between geocoronal neutral hydrogen and various magnetospheric ion populations. Neutral atom imaging of the ionosphere and magnetosphere is possible because the Earth's geocorona acts like an imaging screen for magnetospheric and ionospheric ions. The interpretation and quantification of observed ENA signals depend upon the knowledge of the energy dependence and magnitude of the appropriate charge-exchange cross sections. 10)
Figure 5: Schematic view of the instrument layout in the spacecraft spin axis direction (image credit: SwRI)
LENA (Low-Energy Neutral-Atom Imager):
PI: T. E. Moore of GSFC (collaboration of GSFC, University of Maryland, University of New Hampshire, University of Denver, University of Bern, Lockheed Martin Co.). The objective is to detect ENAs in the energy range from 10-500 eV. LENA's primary role is to image the outflow of low-energy ions from the polar ionosphere. The specific objectives are to: 11) 12)
• Measure neutrals without interference from electrons, ions, or UV
• Distinguish neutral protons from oxygen
• Determine ion outflow on five minute time scales over broad range of local times
• Measure energies as low as 10 eV with high counting statistics
The LENA instrument consists of a collimator, a conversion unit, an extraction lens, a dispersive energy analyzer, and time-of-flight mass analyzer with position-sensitive particle detection. LENA is specifically designed of looking at and in the direction of the sun. Principle of operation: Neutral particles enter the instrument through a collimator which filters charged particles. LENA converts neutrals to negative ions through a near specular glancing reflection from a tungsten surface. Negative ions from the surface are then collected by the extraction lens which focuses all negative ions with the same energy to a fixed location. In the extraction lens, the ions are accelerated by 20 kV prior to entering the electrostatic analyzer (they are detected as they pass through a thin foil and strike an image plane detector). Finally, the ions pass into a time-of-flight/position sensing section where ion mass, energy, and angle are determined.
LENA uses electrostatic optics techniques for energy (per charge) discrimination and carbon foil time-of-flight techniques for mass discrimination. The instrument has a 90º x 8º FOV in 12 pixels, each nominally 8º x 8º. The S/C spin provides a TFOV of 90º x 360º, comprised of 12 x 45 pixels.
Table 3: Overview of LENA instrument parameters
Figure 6: Exterior assembly view of the LENA sensor and its Control and Data Handling System (image credit: NASA)
Figure 7: Functional block diagram of LENA (image credit: NASA)
Figure 8: The LENA instrument (image credit: NASA/GSFC)
MENA (Medium-Energy Neutral-Atom Imager):
PI: C. J. Pollock of Southwest Research Institute (collaboration of SwRI, LANL, RAL, USC, UCB, University of West Virginia). Objective: Detection of ENAs in the energy range 1-30 keV. Provision of images of the ring current, near-Earth plasma sheet, and the nightside injection boundary. In addition, MENA images the ion populations of the cusp. The instrument determines the time of flight and incidence angle of the incoming ENAs; from these raw data it calculates their trajectory and velocity and generates images of the magnetospheric regions from which they are emitted.
The MENA instrument (a slit-type imager) is composed of three identical sensors, mounted to a common DPU (Data Processing Unit) assembly. Each sensor is supported by two dedicated FFE (Front End Electronics) cards, time of flight (FEETOF), a pulse height analyzer (FEEPHA), and a triple-function (+10 kV, +4 kV, -1kV) high-voltage power supply. Each of the three sensors provides 1-D imaging of incident ENAs in the polar angle direction based on a spherical coordinate system. The second imaging dimension (azimuth) is obtained using collimation and S/C spin. The sensors are mounted to provide look directions that view a common azimuth (spin) angle, but are offset from one another in their polar angle field of view. The center of the sensor 2 look direction is perpendicular to the spin axis, at a polar angle of 90º. 13)
The MENA instrument has a mass of 13.9 kg, power consumption = 22.5 W, data rate = 4.3 kbit/s, size: 42.5 cm x 22.3 cm x 29.3 cm.
Figure 9: Block diagram of the MENA instrument (image credit: SwRI)
Figure 10: Isometric drawing of the MENA imager (image credit: SwRI)
Figure 11: The MENA flight instrument prepared for integration (image credit: SwRI)
ENAs, charged particles and photons incident from within the sensor's FOV enter through the collimator, where charged particles with energies up to 13 times the adjustable applied voltage are removed by electrostatic deflection. The remaining particles and photons must pass through a free-standing UV blocking grating, where the UV photons are removed around a very wide stop band by the optical properties of the grating. The grating structure eliminates the solar hydrogen Lyman-α (121.6 nm) light reflected from the geocorona. The detector system uses MCP detector arrays (The detector assembly consists of a Hamamatsu MCP and an anode that employs a novel ”capacitive charge division” technique to determine the position of the ENA impacts on the MCP).
The TOF for each ENA detected is determined by the FEE from the start and stop pulses triggered in the MCP. This value, together with the positions of the start and stop pulses on the MCP, is processed by the look-up tables to compute the incidence angle of an incoming ENA, the length of its path through the detector, and its velocity. The amplitude of the start and stop pulses i.e. their “pulse height,” is also measured. Knowledge of both the spin phase and the incidence angle is needed to determine the position in the sky from which the detected ENAs are emitted and to produce the image of the ENA emission region.
HENA (High-Energy Neutral-Atom Imager):
PI: D. G. Mitchell of JHU/APL [collaboration of APL, University of Maryland, University of Arizona, MPAe (Katlenburg-Lindau, Germany)]. Detection of ENAs in the 10-500 keV energy range. HENA imaging is focused principally on the ring current, inner plasma sheet, and substorm injection boundary. HENA is a modified version of the Cassini INCA instrument, it provides global images of ENA emissions from Saturn's magnetospheric ion populations. The two main HENA components are the sensor and the main electronics unit (MEU)
The HENA sensor consists of alternately charged deflection plates mounted in a fan configuration in front of the entrance slit; three microchannel plate (MCP) detectors; a solid-state detector (SSD); two carbon-silicon-polyimide foils, one at the entrance slit, the other placed just in front of the back MCP; and a series of wires and electrodes to steer secondary electrons ejected from the foils (or the SSD) to the MCPs. 14) 15) 16)
Figure 12: Schematic view of the HENA head (image credit: JHU/APL)
Figure 13: Block diagram of the HENA instrument (image credit: JHU/APL)
Figure 14: Illustration of the HENA instrument (image credit: SwRI)
HENA determines the velocity of the ENAs that it detects by measuring their time of flight (TOF) and trajectory through the sensor, i.e., from the entrance slit either to the back foil and 2-D imaging MCP detector or to the SSD.
Table 4: EUV instrument parameters
EUV (Extreme Ultraviolet Imager):
PI: B. R. Sandel, University of Arizona. The objective is to detect solar EUV photons with a wavelength of 30.4 nm that are resonantly scattered by singly ionized helium in the plasmasphere, the torus of cold dense plasma surrounding the Earth in the inner magnetosphere. A sophisticated computer deconvolution technique is used to translate the EUV photon counts registered by the instrument into images of the plasmasphere. detect solar photons. 17) 18) 19)
The imager consists of three identical sensor heads mounted one above the other in a common bracket; a common electronics module. Each sensor head has a FOV of 30º, the three sensors are tilted so that their FOVs overlap, giving the imager a fan-shaped IFOV of 30º by 84º. With each rotation of the S/C, the imager completes a 360º sweep of the sky, resulting in a TFOV of 84º by 360º. Spatial resolution is about 0.6º at apogee (EUV can distinguish plasma-spheric features with scale sizes down to about 640 km). This resolving power enables the study of fine-scale density structures.
The EUV instrument has a mass of 15.5 kg, power consumption of 9.0 W, size of 49.7 cm x 23.3 cm x 49.5 cm.
Figure 15: Block diagram of the EUV Imager (image credit: University of Arizona)
Figure 16: Front view of the EUV Imager with sensor heads (image credit: University of Arizona)
Figure 17: Photograph of the EUV curved surface flight MCP sensor (image credit: University of Arizona)
The detector design uses a novel photon counting detector scheme with a spherically curved MCP (Microchannel Plate) stack and wedge and strip readout for the EUV instrument. The system includes two main elements, the detector and the detector electronics. The detector consists of a Kovar-alumina brazed body assembly containing the MCPs and readout anode. The detector electronics consists of three boards, amplifier, ADC and interface electronics, necessary to encode photon event data. Incoming photons strike the bare surface of the MCP, where they may eject photo-electrons, which are multiplied and deposited onto a wedge and strip anode. The charge is proportionately divided among the wedge, strip and zigzag electrodes, and then the signals are transferred to the detector electronics. Each of the three signals is amplified and converted to digital form before being sent to the EUV controller and subsequently converted to X,Y photon positions.
FUV (Far Ultraviolet Imager):
• The Wideband Imaging Camera (WIC) to image the aurora in broad band for maximum spatial resolution day and night
• The Spectrographic Imager (SI) to measure different types of aurora and separating them by wavelength and to measure proton induced aurora by removing the bright geocorona emissions
• Geocorona photometers (GEO) to observe the distribution of the geocorona emissions to derive the magnetospheric hydrogen content responsible for neutral atom generation in the magnetosphere.
1) WIC (Wideband Imaging Camera). The WIC instrument is designed to image the whole Earth and the auroral oval from satellite distances greater than 4 Earth radii to the center of the Earth. A curved image intensifier is optically coupled to a CCD and the optics provides a FOV of 17º x 17º. Spectral range: 140-160 nm; resolution elements of < 0.1º; temporal resolution of 120 s; the size of the final images is 256 x 256 pixel elements, corresponding to spatial resolutions of < 100 km at apogee distances. 23)
Figure 18: Schematic view of the WIC instrument (image credit: UCB/SSL)
Figure 19: Illustration of the WIC flight unit (image credit: UCB/SSL)
2) SI (Spectrographic Imager). SI was largely designed, tested and calibrated at CSL (Centre Spatial de Liège) and LPAP (Laboratoire de Physique Atmosphérique et Planétaire) of Liege, Belgium in the frame of the PRODEX program of ESA. The objective is to image the whole Earth proton aurora from satellite distances greater than 4 Earth radii to the center of the Earth. It uses a reverse Wadsworth design to select the Doppler shifted Lyman H-alpha line at 121.82 nm in the ultraviolet part of the optical spectrum and to reject the non-Doppler shifted Lyman H-alpha from the geocorona at 121.567 nm. The FOV is 15º x 15º. The temporal resolution between two images is 120 s and the size of the final images is 128 x 128 pixel elements, corresponding to spatial resolutions of < 100 km at apogee distances. 24)
SI detector system: A set of MCP (Microchannel Plate) detectors was developed utilizing a cross delay-line readout system for the FUV Spectrographic Imager. Two detectors are required for the two pass bands. Both detectors are nearly identical, the only difference being the position of the input window on the detector cover plate. Each detector, optimized for operation in the far ultraviolet with a KBr photocathode, provides high spatial resolution and good linearity over a 20 mm2 format.
Table 5: Performance requirements of the SI detector
Figure 20: View of the SI instrument with its cover off (image credit: UCB/SSL)
3) GEO (Geocorona Photometer) is designed to measure the hydrogen Lyman-alpha emission of the neutral atmosphere. The three photometers have a field of view of 1º x 1º and look into three different directions perpendicular to the spin axis of the satellite and tilted by ±28º. They are ultraviolet photon counters with filters giving the desired bandpass (121.6 nm). An additional O2 gas cell provides an excellent throughput of the hydrogen radiation and simultaneously rejects the photons at 130.4 nm from excited neutral oxygen. The temporal resolution between two complete measurements around 360º is 120 s.
Figure 21: View of the GEO instrument with its side cover removed to show the proton tube (image credit: UCB/SSL)
Figure 22: Single GEO detector tube with built in MgF2 lens and aperture to limit the FOV to 1º (image credit: UCB/SSL)
RPI (Radio Plasma Imaging):
PI: B. W. Reinisch, University of Massachusetts at Lowell. The objective is to characterize plasma in the Earth's magnetosphere utilizing imaging in the radio frequency range. The RPI instrument is a low-power radar which operates in the radio frequency bands which contain the plasma resonance frequencies characteristic of the Earth's magnetosphere (3 kHz to 3 MHz). RPI can locate regions of various plasma densities by observing radar echoes from the plasma that are reflected where the radio frequency is equal to the plasma frequency. By stepping through various frequencies for the transmitted signal, features of various plasma densities can be observed and, by fitting contours and/or magnetospheric models to the features, a 3-D specification of the shape of the magnetosphere can be created. 25) 26)
Figure 23: Block diagram of the RPI (image credit: SwRI)
The RPI instrument consists of an electronics enclosure, four 250 m wire antennae with deployers (including switches and couplers), and a z-axis boom canister containing two 10 m lattice boom antennae and two preamplifiers. Instrument mass = 49.8 kg, power = 133.98 W (peak) and 30.8 W (average). 27) 28)
Table 6: Representative RPI targets
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19) “IMAGE Instrument Specification for the Extreme Ultraviolet Imager (EUV),” Oct. 10, 1998, SwRI, Document No. 8089-ISEUV-01
20) S. B. Mende, H. Heetderks, H. U. Frey, M. Lampton, S. P. Geller, S. Habraken, E. Renotte, C. Jamar, P. Rochus, J. Spann, S. A Fuselier, J.-C. Gerard, R. Gladstone, S. Murphree, L. Cogger, “Far ultraviolet imaging from the IMAGE spacecraft: 1. System Design,” Space Science Reviews, Vol. 91, No 1-2, January 2000, pp. 243-270, URL: http://sprg.ssl.berkeley.edu/sprite/ago96/publication/IMAGE_paper2.pdf
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24) S. B. Mende, H. Heetderks, H. U. Frey, J. M. Stock, M. Lampton, S. P. Geller, R. Abiad, O. H. W. Siegmund, S. Habraken, E. Renotte, C. Jamar, P. Rochus, J.-C. Gerard, R. Sigler, H. Lauche, “Far Ultraviolet Imaging from the IMAGE Spacecraft: 3. Spectral Imaging of Lyman-Alpha and OI 135.6 NM,” Aug. 27, 2003, URL: http://sprg.ssl.berkeley.edu/sprite/ago96/publication/IMAGE_paper3.pdf
25) J. L. Green, R. F. Benson, S. F. Fung, W. W. L. Taylor, S. A. Boardsen, B. W. Reinisch, D. M. Haines, K. Bibl, G. Cheney, I. A. Galkin , X. Huang, S. H. Myers, G. S. Sales, J.-L. Bougeret, R. Manning, N. Meyer-Vernet, M. Moncuquet, D. L. Carpenter, D. L. Gallagher, P. Reiff, “Radio Plasma Imager measurements,” Space Science Reviews, IMAGE special issue, Vol. 91, No 1-2, 2000, pp. 361-389
26) S. F. Fung, R. F. Benson, D. L. Carpenter, B. W. Reinisch, D. L. Gallagher, “Investigations of irregularities in remote plasma regions by radio sounding: applications of the Radio Plasma Imager on IMAGE,” Space Science Reviews, Special issue on the IMAGE mission, Vol. 91, No 1-2, 2000, pp. 391-419
28) “Instrument specification for the Radio Plasma Imager (RPI) Investigation,” http://image.gsfc.nasa.gov/rpi/publication/rpispe2a.doc
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.