1NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
2NASA Marshall Space Flight Center, Huntsville, AL 35812, USA
3Southwest Research Institute, San Antonio, TX 78228, USA
4Auburn University, Physics Department, Auburn, AL 36849, USA
5Rice University, Dept. of Space Physics and Astronomy, Houston, TX 77251, USA
Reprinted with permission, Copyright 1997
The purpose of this paper is to provide background information on the IMAGE mission and what it is expected to observe. This will be accomplished by briefly describing the IMAGE mission and instruments and presenting a variety of computer simulations of the IMAGE instrument observations.
Fig. 1 The initial IMAGE orbit and the precession of its line of apsides.
The IMAGE spacecraft will operate with about a 100% duty cycle with all instruments in their baseline operational modes and will generate nearly 200 gigabytes of Level-0 data per year. All IMAGE instrument and housekeeping data will be stored on-board and dumped to a Deep Space Network (NASA) ground station at a high rate approximately once per orbit. In order to support the National Space Weather program, IMAGE will has a 38 kb/s real-time link that will be monitered, on occasion, by the Space Environment Center of the National Oceanic and Atmospheric Administration in Boulder, Colorado.
The IMAGE Level-0 data will be processed into Level-1 data (in the Common Data Format) from which browse images will be generated within 24 hours after their receipt in the Science and Mission Operations Control Center (SMOC) at the Goddard Space Flight Center. The SMOC will post only the latest IMAGE browse products on the World-Wide-Web (WWW) and transfer housekeeping, Level-0, and Level-1 data products to the National Space Science Data Center (NSSDC) for permanent archiving and long-term access. The NSSDC will ingest all IMAGE data into their NASA Data Archive and Distribution Service (NDADS) system for further distribution to the science community and the public. NDADS provides rapid (within minutes) access to a variety of space physics and astrophysics data and supports WWW and email requests for data. Higher level data products (ie: three dimensional images, movies, etc) will be generated by the IMAGE Science Team and archived at the NSSDC.
The IMAGE Science Team will have the responsibility to generate and validate all the data products and claim no proprietary data rights. The IMAGE mission maintains a series of WWW pages which provides the latest information about all aspects of IMAGE, including the type and accessibility of IMAGE data (covered in the IMAGE Project Data Management Plan). These pages are accessible from the IMAGE homepage (see URL: http://image.gsfc.nasa.gov/).
The minimum time resolution for images from all instruments, except the RPI, is the spacecraft spin period of two minutes. The RPI will have modes which will allow density profile determination on shorter time scales. Images can also be constructed over time scales of multiple spin periods.
|Image||Measurement||Critical Measurement Requirements|
|NAI||Neutral atom composi-
tion and energy-resolved
images over three energy
10-500 eV (LENA)
1-30 keV (MENA)
10-500 keV (HENA)
|FOV: 90°x 90° (image ring current at apogee).
Angular Resolution: 8°x 8° (LENA), 4°x 8° (MENA), 4°x 4° (HENA),
Energy Resolution (delta E/E): 0.8
Composition: distinguish H and O in magnetospheric and ionospheric
sources, interstellar neutrals and solar wind.
Image Time: 4 minutes (resolve substorm development).
Sensitivity: effective area 1 cm2 for each sensor.
|EUV||30.4 nm imaging of
|FOV: 90°x 90° (image plasmasphere from apogee).
Spatial Resolution: 0.1 Earth radius from apogee.
Image Time: several minutes to hours (resolve plasmaspheric
|FUV||Far ultraviolet imaging
of the geocorona at
121.6 nm (GEO) and the
aurora at 140-190 nm
(WIC) and 121.6 and
135.6 nm (SI)
|FOV: 15°x 15° (SI) for aurora (image full Earth from apogee),
1°x 360° for geocorona, and 22.4°x 30° (WIC)
Spatial Resolution: 70 km (WIC),90 km (SI)
Spectral Resolution: separate cold geocorona H from hot proton
precipitation (l~0.2 nm near 121.6 nm); reject 130.4 nm and select
135.6 nm electron aurora emissions.
Image Time: 2 minutes (resolve auroral activity).
|RPI||Remote sensing of
electron densities and
boundary locations using
|Density range: 0.1-105 cm-3 (determine electron density from inner
plasmasphere to magnetopause).
Spatial resolution: 500 km (resolve density structures at the
magnetopause and plasmapause).
Image Time: 1 minute (resolve changes in boundary locations).
The science requirements driving the NAI instrumentation for IMAGE are to image the inner magnetosphere and to resolve the major species contributing to neutral atom fluxes. To meet these requirements a suite of three NAI instruments will provide angle, energy, and composition-resolved images at energies from 10 eV to 500 keV. The three NAI instruments are necessary because of the different techniques that apply to observing low (0.01 to 0.3 keV), medium (1 to 30 keV), and high (10 to 500 keV) energy neutral atoms. Angular information is obtained over 90° fans with angular resolution between 4° x 4° and 8° x 8° depending on species and energy. Spacecraft spin is used to obtain angular information in the orthogonal (azimuthal) direction. All three instruments have collimators that consist of serrated, blackened surfaces to reduce internal scattering. The collimators contain deflection potentials of 10 kV that deflect and absorb charged particles below 100 keV/e. Small broom magnets remove electrons with energies <200 keV.
LENA. The LENA instrument consists of a collimator, conversion unit, extraction lens and acceleration region, dispersive electrostatic energy analyzer, and time-of-flight (TOF) mass analyzer with position-sensitive particle detection. Particles enter the instrument through a collimator with elevation acceptance defined by the height of the collimator. Neutrals are converted to negative ions through near specular reflection from a low-work-function cesiated tungsten (Cs-W) conversion surface (Wurz et al., 1995). The surface is segmented to cover a 90° azimuthal acceptance. The Cs-W is resurfaced approximately once or twice per week. LENA is also an ion mass spectrograph. Neutrals converted to negative ions at the conversion surface are accelerated and analyzed for energy, mass/charge and elevation angle by a spherical electrostatic analyzer.
LENA Simulations. Simulations of the LENA observations are shown in Figure 2. The first and third panels of Figure 2 show the expected differential fluxes of neutral O and H produced from charge exchange of the upflowing ions in the auroral zone, polar cap, and cleft ion fountain. The images in the second and fourth panels (labeled instrument counts) were produced by integrating along the line of sight from a observing point just beyond perigee in the nominal IMAGE orbit. The instrument counts were constructed from O and H neutral atom fluxes by folding in the instrument response, which included the appropriate angular resolution and per pixel sensitivity, and adding random Poisson noise. The assumed geophysical conditions at the time of these simulated images were high solar activity (F10.7=200) and high magnetic activity (Kp=7).
Fig. 2. The first and third panels show line-of-sight integrated images of neutral atoms (23-55 eV) produced by modeled cleft ion fountain, polar cap and auroral zone ion upflows as well as backsplash oxygen neutral atoms produced by precipitating ring current O+. The second and fourth panels show the expected LENA results.
The model used to produce these images assumes that the ionospheric ion outflow flux, from any location, is proportional to the local energy flux carried by precipitating electrons as given by the Hardy et al.  empirical model, normalized so that the total outflow flux is that given by Yau et al. . The model finds the steady state ion flux as a function of energy, pitch angle, location and species by tracking particles in dipole magnetic and convection electric fields, launched from an altitude in the ionosphere where they are heated to a 10-30 eV initial energy. The ion fluxes are converted to neutral atom fluxes through charge exchange with thermospheric hydrogen, oxygen, helium and ionospheric O+. Results are dependent on Kp and F10.7 through the dependence of the model's inputs (thermospheric composition, ion outflow fluxes, auroral zone location, convection electric field) to these parameters.
MENA. The MENA imager is a slit camera with straight-through optics, which samples and resolves velocities, masses, and polar angles within a 107° fan. The MENA analyzer consists of a collimator, UV rejection grating, start foil, position-sensitive anode, TOF analyzer, and pulse-height analyzer. The collimator plates use electrostatic deflection to reduce charged particle background. The UV grating acts as a wave guide to reduce the Lyman alpha fluxes by a factor of 104 from the sun and geocorona. Velocity analysis, through TOF measurements made with the start/stop MCP, combined with pulse-height analysis, yields mass resolution sufficient to separate H+ and O+.
HENA. The HENA is a slit camera with a 90° x 120° field of view and a segmented focal plane incorporating an imaging solid-state detector (SSD) array in one portion and an MCP with position-sensitive anode in the other. Pulse height analysis of the SSD pulses provides total energy, which, combined with the TOF velocity determination, yields neutral atom mass.The MCP pulses are also pulse-height analyzed, separating of H and O. Each pixel is viewed both by the SSD array and the MCP as the scene is scanned. HENA acquires angular images by locating the start pulses on the entrance slit and the stop pulses in the image plane. The collimator serves to suppress charged particle entry by biasing adjacent collimating plates at ±10 kV.
Fig. 3. Left hand panels: Simulated ring current flux of 1.5 keV H+ and 31 keV H+ at the equator during a geomagnetic storm on October 19, 1995 as predicted by the ring current model of Fok et al. (1995). Middle panels: The corresponding neutral atom fluxes projected along polar and spin azimuth angle. Right hand panels: instrument count of a 10 minute exposure as would be obtained by the MENA and HENA instruments respectively from IMAGE near apogee.
MENA and HENA Simulations. The neutral atom images that will be acquired by the IMAGE LENA, MENA, and HENA instruments are the result of line-of-sight integrations for which the number of counts in an individual pixel reveals how many particles came from that direction but not how far along that line-of-sight direction they originated. In order to extract the maximum amount of scientific information from these images, it is important to deconvolve the unknown source from the data. An arsenal of techniques are being developed to accomplish this task. All are based upon using the principles of Bayesian statistics to impose objective, clearly-defined criteria to select among the set of solutions that provide an acceptable fit to the data.
To analyze simulated data (Moore, et al., 1995), such as that shown in Figure 3, it is assumed that the ions are trapped along magnetic dipole field lines while conserving both energy and the first adiabatic invariant. The equatorial pitch angle distribution of the ions is expanded in terms of bicubic splines for the two space dimensions and in terms of Legendre polynomials for the pitch angle dependence. Two conditions are imposed upon the solution, (1) fit the data, and (2) be as smooth as possible by minimizing the second derivative. The relative balance between these two criteria is determined using the technique of generalized cross validation which is based on the idea of repeating the solution with each of the pixels in turn removed from the computation and then choosing the smoothest solution. This technique yields a deconvolved 2-dimensional pitch angle distribution from a single image (Perez, et al., 1996) or from a stereoscopic set of images of the same source that is consistent with the observations and that introduces no spurious structure. A method for analyzing a time-sequence of images has also be implemented. Similar methods that yield deconvolved 3-dimensional sources are being developed.
There are four photon imagers on IMAGE. The EUV instrument images resonantly scattered solar emissions from cold plasmaspheric He+ at 30.4 nm. Two FUV instruments, SI (Spectrographic Imager) and WIC (Wideband Imaging Camera) image the Earth's electron and proton auroral emissions, and a Geocorona Oxygen Cell (GEO) images the hydrogen geocorona at 121.6 nm.
EUV. Effective imaging of plasmaspheric He+ requires global "snapshots" in which the high apogee of the IMAGE mission and the wide FOV of the EUV imager provide, in a single exposure, a map of the entire plasmasphere from the outside with a sensitivity of 0.2 count/s-pixel-Rayleigh (R), a spatial resolution of 0.1 RE, and a time resolution of several minutes. The 30.4-nm feature is easy to measure because it is the brightest ion emission from the plasmasphere, it is spectrally isolated, and the background is negligible. Measurements are easy to interpret because the plasmaspheric He+ emission is optically thin, so its brightness is directly proportional to the He+ column abundance.
EUV consists of three identical sensor heads serviced by a common electronics module. It employs elements of new technology, multi-layer mirrors. Because it is simple and lacks moving parts, the EUV is rugged and reliable. Each sensor head has a field of view of 30° x 30°. The three sensors are tilted relative to one another to cover a fan-shaped instantaneous FOV of 90° x 30°. As the satellite spins, the fan sweeps a 90° x 360° swath across the sky.
Each EUV sensor achieves high throughput and a wide field of view by using a large entrance aperture and a single spherical mirror. A multi-layer reflective coating on the mirror selects a narrow 5-nm passband around the 30.4 nm line. To circumvent the red leak in the multi-layer mirror, a metal foil filter blocks H Lyman alpha, from the geocorona. The detector consists of two curved, tandem MCPs with an alkali halide front surface photocathode. The detector's spherical input surface minimizes the effects of spherical aberration. Readout from the detector is from a 128 x 128 wedge and strip anode. The sensitivity (accounting for the duty cycle inherent in a spinning spacecraft) is 0.2 count/(sec pixel) per R, where the pixel size is taken to be 0.1 RE. By summing pixels to make a spatial resolution element (or resel) of 0.5 RE, the count rate is 5 counts/(sec resel) per R.
Fig 4. A simulated thermal plasma distribution during a storm time recovery is shown in the left hand panel. A modeled EUV image from near apogee of the IMAGE orbit is shown in the right hand panel. The simulated image reflects what the EUV instrument would see for the distribution of plasma shown in the left panel (note: model oriented slightly differently from the EUV image).
EUV Simulations. A simulation of convecting thermal plasmas in the magnetic equatorial plane is used together with a global empirical model of thermal plasma in the magnetosphere to predict the distribution of cold helium ions using (once again) the October 1995 magnetic cloud event. This simulation includes ionospheric filling, saturation, and solar wind driven convection. The left hand panel in Figure 4 shows the model plasma distribution during the recovery phase of the substorm that was observed during the event. The sun is to the left of this Figure. An important feature in the thermal plasma model is an extended plasmaspheric tail which results from the recapture and corotation of previously sunward convected plasmaspheric plasma. The right hand panel of Figure 4 is the corresponding simulated EUV instrument image that would be observed near apogee of the IMAGE orbit. The appearance of the Earth's shadow in the simulated EUV image provides an immediate orientation for the observer. The simulated image is a composite from all three EUV cameras. Poisson and dark counting noise has been added, along with instrument sensitivity and variation in the relative integration time across the conical instrument apertures. The relative sensitivity across the field of view has been deconvolved in this image. Enhanced noise at the left and right image edges and in two vertical bands results from that deconvolution. The extended plasmaspheric tail that has been swept up by corotation is near 10 hours MLT.
FUV. Science requirements driving FUV imager designs are (1) to image the entire auroral oval from a spinning spacecraft at 7 RE apogee altitude, (2) to separate spectrally the hot proton precipitation from the statistical noise of the intense, cold geocorona, and (3) to separate spectrally the electron and proton auroras. The FUV consists of two imagers that combine high spectral discrimination, high spatial resolution, and the greatest possible sensitivity to meet these requirements.
In the FUV range up to ~160 nm, there are several bright auroral emission features that compete with the dayglow emissions. For the electron aurora, the brightest is 130.4 nm OI, which is multiply scattered in the atmosphere and thus is not optimal for auroral morphology studies. The next brightest is the 135.6 nm OI emission. Separation of the 130.4 and 135.6 nm lines necessitates the use of a spectrometer because even reflective narrow-band filter technology cannot satisfy the ~3 nm wavelength resolution requirement. Above 135.6 nm, weak LBH lines can be detected using narrow-band filter technology. Separate imaging of the intense, cold geocorona (Lyman alpha emissions at 121.6 nm) and the less intense, Doppler-shifted Lyman alpha auroral emissions requires significantly higher spectral resolution (0.2 nm).
Spectrographic Imager (SI). The relatively high wavelength resolution requirement is satisfied by the SI instrument. The 0.2-nm wavelength resolution drives the size of the instrument and consequently the number of mirrors in the optics system. Also the narrowness of the slits in the spectrometer limit the dwell time during which a pixel is in the field of view. The SI is a Wadsworth spectrometer, which uses a diffraction grating to produce separate images of 135.6 nm emissions from the electron aurora and 121.8 - 122.2 nm Doppler-shifted Lyman alpha emissions from the proton aurora. The 130.4 nm oxygen airglow emission and the geocorona 121.6 nm Lyman alpha emission are blocked out. The detectors use a KBr photocathode on a MgF2 window image tube with MCP intensification. The intensified image is detected by a crossed delay-line type detector with two 32 x 128 pixel active areas. The Geocorona Oxygen Cell (GEO) provides three 1° narrow-band photometer channels of geocorona data at 121.6 nm. For some orientations of the spin axis, the Sun may enter the field of view of SI at some spin phases. For these times the control microprocessor will automatically reduce the high voltage to the MCPs to avoid excessive counting rates. The filters will prevent damage to the detector by focused visible sunlight. Excellent observations of the geocorona have been previously accomplished by Rairden et al.,  on the Dynamics Explorer spacecraft.
Wideband imaging camera (WIC). The relatively high sensitivity requirement for auroral imaging is satisfied by the WIC. This imaging camera uses the basic design flown on the Freja and Viking (Anger et. al, 1987) satellites to measure the auroral LBH emissions in a relatively broad band from 140 nm to 190 nm. The large field of view permits a long integration period and increases its sensitivity. The WIC optics design is identical to that of the Freja camera. Incident photons pass through a filter that blocks Lyman alpha emissions and protects the detector from direct, focused sunlight. The primary and secondary mirrors have a coating that is highly reflective (>60%) in the FUV but has minimum (<3%) reflectance out of band. MCPs are used to intensify the image, which is produced on a phosphor and fiber-optically coupled to a diode array. Operation of the WIC is essentially identical to that of SI. Readout occurs once per 0.1° of rotation for a frame rate of 30 frames/s at 0.5 rpm. The camera data are digitized and co-added in memory and the addresses are selected according to the rotational phase of the spacecraft. This technique minimizes distortion.
FUV Simulations. The magnetic cloud event of October 18-19, 1995 was used as a comparative period for simulating IMAGE data. In order to simulate an SI observation during this time period, Hardy statistical models (Hardy et al. 1987; 1991) were used to represent auroral activity. Using the model energy flux and mean energy of precipitating electrons and protons the brightness of HI Lyman alpha, OI 135.6 nm, and N2 LBH emissions (using the yield curves determined by Strickland et al., 1993) can be estimated. Realistic levels of dayglow emissions were added and images simulated using the code of Gladstone (1994). Estimated brightnesses were converted to counts using the estimated SI sensitivity of 5 cps/100R/spin, and appropriate dark and counting noise were added to make the images more realistic. Auroral emissions stronger than the dayglow would allow aurora to be measured on the dayside with FUV.
Figure 5 shows an example OI 135.6 nm image generated for the substorm on October 18, 1995. The IMAGE spacecraft is assumed to be near apogee. This vantage point provides an excellent view of the northern auroral oval when the magnetic cloud event is most intense, and demonstrates that the FUV imagers should obtain excellent data during the IMAGE mission.
Fig. 5. Simulated OI 135.6 nm image as seen from by the SI instrument on IMAGE. The SI selects photons from a 3-nm region centered on the OI 135.6 nm doublet, and has a field-of-view of 16° x 16° with 128 (0.125°) pixels on a side.
The Radio Plasma Imager (RPI) is a transmitter/receiver system that provides remote sensing measurements of plasma densities, structures and dynamics in the magnetosphere and plasmasphere. The instrument measures the time delay, angle-of-arrival, and Doppler shift of magnetospheric echoes over the frequency band from 3 kHz to 3 MHz. This frequency range makes possible remote sensing of plasma densities from 0.1 to 105 cm-3. Programmable operational modes will focus on specific magnetospheric and plasmaspheric features. RPI will have two crossed 500-m tip-to-tip thin wire dipole antennas in the spin plane, and a 20-m tip-to-tip dipole antenna along the spin axis. All three antennas will be used for reception to determine the angles of arrival of the echoes (Calvert et al., 1995; 1997).
The large distances, low power, and short antennas (relative to the wavelength) require onboard signal processing. Pulse compression and coherent spectral integration techniques will be used to increase the signal-to-noise (S/N) ratio. The nominal range resolution is approximately 500 km. The number of sounding frequencies selected for a given measurement, together with the coherent integration time, determines the time resolution.
Fig. 6. Simulated plasmagram showing the expected echo amplitudes and delay times as a function of delay and frequency. In addition, the corresponding radial Ne distribution is also shown.
Based on these simulations, RPI will determine the location of the magnetopause and plasmapause and their respective Ne values. It should be possible to deduce global-scale boundary structures using the directional and range measurements, as well as from a sequence of many plasmagrams along the IMAGE orbit.
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