Imaging the Earth's magnetosphere from space will enable scientists to better understand the global shape of the inner magnetosphere, its components and processes. The proposed Inner Magnetosphere Imager (IMI) mission will obtain the first simultaneous images of the component regions of the inner magnetosphere and will enable scientists to relate these global images to internal and external influences as well as local observations. NASA's Marshall Space Flight Center (MSFC) is performing a concept definition study of the proposed mission. As currently envisioned, the baseline mission calls for an instrument complement of approximately seven imagers to be flown in an elliptical Earth orbit with an apogee of seven Earth Radii (RE). Several spacecraft concepts have been examined for the mission. The baseline concept utilizes a spinning spacecraft with a despun platform, the second uses a three-axis stabilized spacecraft with a spinning platform, while the third option splits the instruments onto two small satellites; a spinning spacecraft and a complementary three-axis stabilized spacecraft. This paper will address the mission objectives, the rationale for using proven spacecraft designs, and the preliminary concept definition study team results for all three options.

Simulated images of extreme ultraviolet (EUV) emissions from energetic outfiowing ions have been constructed to study techniques for remotely sensing the dynamic behavior of hot plasmas in the near-Earth environment. These calculations include realistic assumptions about the energetic ion outflow from high latitudes and take into account the effects of cold plasmasphenc and ionospheric ions. The energetic ion outflow is determined from a statistical study based on five years of measurements from the Energetic Ion Composition Spectrometer on Dynamics Explorer 1. The simulated images change significantly with viewing geometry and certain spacecraft locations are clearly favorable for observing emissions from energetic ions. For example, for a near equatorial orbit, viewing locations greater than 9 Earth radii are required to observe outfiowing ions above the cold plasmaspheric background. We will discuss other important considerations for magnetospheric imaging including the sensitivity requirements of the detector. In particular, we consider the performance of multi-layer optics for EUV wavelengths.

We describe a new imager suitable for measurements of magnetospheric neutrals with energies from about 100 eV to about 10 keY; an energy range adequate for imaging the plasmasheet neutral atoms out to about 10 RE. The instrument, an outgrowth of a study of atom-surface collisions in support of satellite drag calculations, separates incident photons from neutral atoms by surface scattering and conversion of the neutrals to ions. Subsequently, the. ions formed on the first surface are accelerated through a light rejection section which also disperses the ions according to energy. The dispersed ion beam is then allowed to impact a second surface where a start pulse is generated to obtain ion velocity and energy/charge. The second surface is chosen to give large secondary electron emission without regard to charge state of the particles reflected from it. The reflected particles are detected a second and final time in a position sensitive detector as drift in a field free region. This last detection generates the stop pulse for time-of-flight (velocity) determination. The data supporting the proposed ILENA design is presented in the first part of the paper. INTRODUCTION Global imaging of energetic magnetospheric ions (H and O) has been demonstrated recent1y1'2'. Low energy atom imaging with Carbon foils has been demonstrated in the laboratory for energies down to 1 keY , wherethe sensitivity becomes very small. The question arises whether atom-surface scattering may offer a viable detection mechanism for time-of-flight spectrometry of low energy neutral atoms and whether this might provide a lower energy limit for detection, perhaps near 100 eV, with good energy resolution (small straggling effects). This paper reviews a number of neutral atomsurface collision properties and the design of an imager of low energy neutral atoms (ILENA) that follows from these properties. ILENA differs from other approaches in that it depends on the reflection of the incident atoms from a surface. Hence, it is important to demonstrate (a) that neutral atoms are scattered into a narrow lobe (quasi-specular reflection), (b) that a significant fraction of the reflected particles change charge (positive or negative), and (c) that the secondary electron yield at the surface is sufficiently large to ensure high detection efficiency. We have begun such studies at our laboratory and report here on the quasi-specular character of the scattering at energies below 100 eV. We review older laboratory data which indicate that items (a), (b) and (c) above are favorable for energies from <100eV up to 10 keV or more. The specific application to ILENA is deserving of immediate attention. The data reveals that the main advantage of ILENA is the extension of the energy range of neutral atom imaging to the vicinity of 100 32/SPIE Vol. 1744 Instrumentation for Magnetospheric Imagery (1992) ° 19401 70/92/$4.OO IMAGER OF LOW ENERGY NEUTRAL ATOMS (ILENA): IMAGING NEUTRALS FROM THE MAGNETOSPHERE AT ENERGiES BELOW 20 KEy Federico A. Herrero and Mark F. Smith Laboratory for Extraterrestrial Physics, NASA Goddard Space Flight Center Greenbelt, Maryland 20771 Mailing address: Code 696, NASA Goddard, Greenbelt, MD 20771 Tel.(301)-286-5660, FAX (301)-286-9240 ABSTRACT We describe a new imager suitable for measurements of magnetospheric neutrals with energies from about 100 eV to about 10 keY; an energy range adequate for imaging the plasmasheet neutral atoms out to about 10 RE. The instrument, an outgrowth of a study of atom-surface collisions in support of satellite drag calculations, separates incident photons from neutral atoms by surface scattering and conversion of the neutrals to ions. Subsequently, the ions formed on the first surface are accelerated through a light rejection section which also disperses the ions according to energy. The dispersed ion beam is then allowed to impact a second surface where a start pulse is generated to obtain ion velocity and energy/charge. The second surface is chosen to give large secondary electron emission without regard to charge state of the particles reflected from it. The reflected particles are detected a second and final time in a position sensitive detector á.s drift in a field free region. This last detection generates the stop pulse for time-of-flight (velocity) determination. The data supporting the proposed ILENA design is presented in the first part of the paper.

Energetic neutral atom (ENA) and low energy neutral atom (LENA) imaging of space plasmas are emerging new technologies which promise to revolutionize the way we view and understand large scale space plasma phenomena and dynamics. ENAs and LENAs are produced in the magnetosphere by charge exchange between energetic and plasma ions and cold geocoronal neutrals. While imaging techniques have been previously developed for observing ENAs with energies above several tens of keV, most of the ions found in the terrestrial magnetosphere have lower energies. We recently suggested that LENAs could be imaged by first converting the neutrals to ions and then electrostatically analyzing them to reject the UV background. In this paper we extend this work to examine in detail the sensor elements needed to make an LENA imager. These elements are 1) a biased collimator to remove the ambient plasma ions and electrons and set the azimuthal field-of-view; 2) a charge modifier to convert a portion of the incident LENAs to ions; 3) an electrostatic analyzer to reject UV light and set the energy passband; and 4) a coincidence position detector to measure converted LENAs while rejecting noise and penetrating radiation; all are flight proven technologies. We also examine the issue of LENA imager sensitivity and describe ways of optimizing sensitivity in the various sensor components. Finally, we demonstrate how these general considerations are implemented by describing one relatively straighiforward design based on a hemispherical electrostatic analyzer

Detection of low energy neutral atoms (LENAs) produced by the interaction of the Earth's geocorona with ambient space plasma has been proposed as a technique to obtain global information about the magnetosphere. Recent instrumentation advances reported previously1 and in these proceedings (McComas et a!., Funsten et aL) provide an opportunity for detecting LENAs in the energy range of <1 keV to -5O keY. In this paper, we present results from a numerical model which calculates line of sight LENA fluxes expected at a remote orbiting spacecraft for various magnetospheric plasma regimes. This model uses measured charge exchange cross sections, either of two neutral hydrogen geocorona models, and various empirical models of the ring current and plasma sheet to calculate the contribution to the integrated directional flux from each point along the line of sight of the instrument. We discuss implications for LENA imaging of the magnetosphere based on these simulations

Magnetospheric imaging has been proposed using remote detection of low energy neutral atoms (LENAS) of magnetospheric origin. In the detector, LENAS can be removed from the immense ambient UV by charge state modification (ionization) using a carbon stripping foil and can be subsequently deflected into an EIq analysis section. The LENA detector efficiency is linearly proportional to the ionization probability of neutrals as they transit the foil. In this study, we present equilibrium charge state and scatter distributions for 1-30 keV atomic hydrogen and oxygen transiting nominal 0.5 cm2 carbon foils. The fraction of hydrogen exiting a foil as H ranges from approximately 5% at 1 keV to 41% at 30 keV. The fraction of oxygen exiting the foil as O ranges from approximately 2% at 10 keY to 8% at 30 keV. Results obtained after coating the exit surface of foils with either aluminum (which forms aluminum oxide when exposed to air) or gold suggest that the intended alteration of the exit surface chemistry has no effect on the charge state distributions due to foil contamination from exposure to air. Scattering that results from the projectilefoil interaction is shown to be independent of the charge state distribution, illustrating the distinctly different interaction mechanisms associated with charge exchange and scattering.

Studies have show that Earth passages of fast coronal mass ejections (CMEs) trigger geomagnetic storms. Early identification of fast Earth-directed CME can help provide storm warnings, but detection of such by coronagraphs is extremely difficult. We suggest that energetic hydrogen atoms (EHA) between 2 and 10 keV produced during the transit phase of an Earth-directed CME by recombination between protons and electrons in the CME can travel ahead of the CME and act as harbingers of a magnetic storm. This forecasting scheme should work if enough EHA are produced, because while CMEs decelerate continuously after their ejection, the EHA fluxes produced in the initial phase of fast CMEs propagate at their initial high speeds (> 1 X 10³ km s^-1). Model simulations support this proposed mechanism. A coarse measurement of the CME-produced ENA at 1 AU could provide storm warning hours in advance, and finer measurements could yield detailed information on the likely geomagnetic effectiveness of a CME, as well as the evolution and propagation of CME between the Sun and Earth.

Direct measurements of energetic neutral atoms (ENA) and ions have been obtained with the cooled solid state detectors on the low altitude (220 km) three-axis stabilized S81-1/SEEP satellite and on the spinning 400 km X 5.5 Re CRRES satellite. During magnetic storms ENA and ion precipitation (E > 10 keV) is evident over the equatorial region from the LE spectrometer on the SEEP payload (ONR 804). The spinning motion of the CRRES satellite allows for simple mapping of the magnetosphere using the IMS-HI (ONR 307-8-3) neutral spectrometer. This instrument covers the energy range from 20 to 1000 keV and uses a 7 kG magnetic field to screen out protons less than about 50 MeV. ENA and the resulting low- altitude ion belt have been observed with the IMS-HI instrument. Recently, an advanced spectrometer (SEPS) has been developed to image electrons, ions, and neutrals on the despun platform of the POLAR satellite (approximately 1.8 X 9 Re) for launch in the mid-90's as part of the NASA ISTP/GGS program. For this instrument a 256 element solid state pixel array has been developed that interfaces to 256 amplifier strings using a custom 16 channel microcircuit chip. In addition, this instrument features a motor controlled iris wheel and anticoincidence electronics.

We propose to perform in situ measurements of precipitating and escaping energetic neutral atoms (ENA) of energies between approximately 5 and 200 keV. The interface characteristics of this new type of instrument, named ISENA (Imaging particle Spectrometer for Energetic Neutral Atoms), are consistent with the SAC-B spacecraft specifications, so that it could be included in its scientific payload. The main technical properties of this experiment are here briefly described.

Japan's spacecraft PLANET-B will be sent to the Mars in 1996. We are proposing the Martian ionosphere and the magnetosphere imaging using the extreme ultra violet (EUV) light on this mission. Our main target is the 84.3 nm light scattered by the oxygen ions. Interesting topics related to the imaging are; (1) the density profile of the oxygen ions in the Martian ionosphere, (2) oxygen ions which are reported to outflow from the nightside ionosphere to the Martian tail with about 1 keV energy, and (3) pick-up ions created at the dayside of the Mars. Also proposed is the measurement of the EUV light scattered by Helium ions in the interplanetary space during the cruise phase from the Earth to the Mars.

We describe the WIDe-angle GEocoronal Telescope or WIDGET. The telescope is designed to image the cold plasma constituent of the magnetosphere, the plasmasphere, by imaging the He II 304 angstroms emission. It utilizes a spherical normal incidence f/2 mirror with a microchannel plate imaging detector positioned near the focal point of the mirror. The instrument has a 30 degree(s) field of view with a .5 - 1 degree(s) angular resolution. The mirror has a multilayer coating which, combined with an Al/C filter and the detector response, provides a peak sensitivity of approximately 0.5 ct/sec/Rayleigh/bin at 304 angstroms with a bandpass of about 60 angstroms. The instrument will be flown on a sounding rocket in the Fall of 1992 as a test of its capabilities. It can easily be adapted to different wavelengths (584, 834, and 1216 angstroms) using suitable mirror coatings and filters. In this way, a series of WIDGETs could image different aspects of the inner magnetosphere simultaneously, thus providing a more unified picture of this complex region.

EUV images from emissions of O⁺ (83.4 nm) and He⁺ (30.4 nm) distributions in the plasmasphere and trough regions are constructed for a satellite at 9 R_E. A diffusive equilibrium model is used to describe the density distribution along field lines for ions in the plasmasphere while a kinetic collisionless model is assumed to calculate ion densities in the high latitude regions beyond the plasmapause. In our model of the plasmasphere we assumed that ions move along a static dipole field. Observational data on O⁺ and He⁺ densities in the ionosphere are used as boundary conditions to calculate spatial distribution of densities along field lines. Assuming that the day and night boundary conditions are asymmetric and exobase densities vary with latitude we will discuss how this would be reflected in the intensities and structure of the magnetosphere images.

In a comparison paper, one of us (Curtis) discusses the development of aperture codes appropriate for instruments viewing an extended object--in this case, the earth's magnetosphere. The magnetosphere becomes an extended object when viewed at close enough range to extend laterally beyond the field of view (FOV) of a sensor which is imaging it. The imaging particles are UV photons or energetic neutral atoms (ENA), the latter created from magnetospheric ions after charge-exchange interactions with exospheric gas. Here we describe coded aperture sensors for photons or ENA which incorporate FOV limiters and subdivide the object field into a number of elements which is smaller than the number of detector pixels. A least squares fit to the data is made in reconstructing the object field. To test the performance of a sensor, it is necessary to simulate an object of relatively large angular width which exhibits no parallax effects when seen by different elements of the detector. To evaluate the optics and reconstruction algorithms, two 'breadboard' sensors have been constructed, one based upon a film camera and the other upon a UV-light sensitive microchannel plate detector system. Laboratory tests of these sensors are described.

A class of instruments designed for remote sensing of space plasmas by measuring energetic neutral atoms (ENA) uses a thin foil as both a signal generator and a light shield. An ENA imager must look directly at the ENA source region, which is also usually in intense source of H Ly(alpha) (1216 angstroms) photons. ENA are produced by charge exchange between energetic ions and the ambient neutrals, and both charge exchange cross section s and ion populations decrease with increasing ion energy. Therefore it is desirable to minimize the energy threshold for ENA detectors, at the same time maximizing the blocking of H Ly(alpha) . Optimizing filter design to meet these two contrary requirements has led us to measure the transmittance of thin C, Si/C, and Al/C foils at H Ly(alpha) . Our results indicate that (1) transmittance of < 7 X 10^-4 can be achieved with (mu) g/cm² Si on 1.7 (mu) g/cm² C; (2) an Si/C composite foil with a thin carbon layer is more effective in blocking UV radiation while having the lowest energy threshold of all the foils measured; and (3) transmittance of Si/C foils of known Si and C thicknesses cannot be accurately predicted, but must be measured.

In a companion paper, Curtis, et. al. discuss coded aperture sensors appropriate for viewing an extended object such as Earth's magnetosphere seen at relatively close range. The aperture codes used may be different from those generally found in X or (gamma) ray telescopes, where the field of view (FOV) may encompass an entire region containing some small number of point sources. The aperture codes described here find application in imaging an extended object which may have relatively low contrast, and whose lateral limits extend beyond the FOV of the sensor. Those elements of an extended object lying near the FOV limits are only partially coded, that is, flux from those elements cannot cast a shadow of the entire aperture code onto the detector, as can elements near the center of the FOV. This has consequences for the algorithms used to reconstruct the image. The object field is divided into a number of elements which is smaller than the number of detector pixels, and a least squares fit to the data is performed. A discussion of the methods used for choosing the matrices representing the aperture codes is given, and computer simulations of the effects of noise are described.

In preparation for the upcoming IMI mission to image the Earth's inner magnetosphere, we have simulated several images for two EUV emission lines in the plasmasphere. Two main candidates for remote sensing of the plasmasphere are He II 304 angstroms and O II 834 angstroms emissions, both of which are excited by resonantly scattered sunlight. The He II 304 angstroms feature typically has a brightness in the plasmasphere of about 5 Rayleighs, with no background from the disk of the Earth, making it an ideal emission for imaging the plasmasphere. On the other hand, the plasmaspheric O II 834 angstroms brightness is expected to be no more than about 1 Rayleigh. In addition, there is a background from the disk of the Earth (on the order of 1500 Rayleighs on the dayside) that must be accounted for. Despite the technical difficulties associated with observing a faint plasmaspheric emission component above the bright disk, the possibility of remotely imaging upflowing O⁺ ions pouring into the plasmasphere is compelling enough that the IMI mission payload is likely to include a 834 angstroms imager. We have simulated these two emissions (including the disk background at 834 angstroms) as seen from a nominal IMI orbit, using a constant model plasmasphere and upflowing ion rate. The resulting images show a rough idea of what may be expected from the IMI plasmasphere imagers.

We report the design of an extreme ultraviolet (EUV) filter for He I at 58.4 nm and the design and fabrication of an EUV filter for O II at 83.4 nm. Both filters are designed as combinations of three narrow-band reflection filters. The net transmittance through both EUV filters is close to 10% with bandwidths less than 10 nm, and blocking better than 0.005% for out-of-band wavelengths. A theoretical calculation of the 83.4 nm filter predicts higher values for the peak transmittance than the measured spectral performance of the fabricated filter. Since aluminum is one of the film materials used for the fabrication of EUV filters, the aluminum film oxidation can be modeled in order to explain the discrepancy between the theory and experiment. The 83.4 nm filters were deposited in a conventional high vacuum coater with pressure of 10^-6 torr. In order to avoid aluminum oxidation and improve the performance of the narrow-band filters, an ultrahigh vacuum coater must be used with deposition pressures of less than 10^-10 torr. Since the filters operate at angles of incidence up to 50 degree(s), the optical components of a system can serve as both the filtering and imaging elements.

Our group has developed a pinhole Anger camera to detect and image auroral X-rays in the energy range 20 - 120 keV. We have flow this camera on four different occasions and have imaged auroral zone X-rays from dawn, dusk, and noon. The fourth flight was from a circum polar navigating balloon from Antarctica. Our data, which sampled a small number of the dynamic auroral precipitation temporal forms, show that auroral X-rays persistently include small spatial structures of approximately equals 20 km (at ionospheric heights). Our camera obtained energy spectral information as a function of space and time and shows that precipitation includes at least two energy components, one with an e-folding energy of a few keV and another with an e-folding energy of tens of keV. These X-ray images 'remote sense' the distant magnetospheric sources and indicate that there are at least two different source populations.

Precipitation of energetic electrons from the magnetosphere into the auroral zone produces x- ray bremsstrahlung. Although in-situ electron spectrometers can provide detailed information at the point of observation, only x-ray imagers can provide large scale maps of the 1 to 300 keV energy electron precipitation. X-ray imaging provides complete day and night coverage of the electron energy spectra at each position. Early x-ray images, such as those obtained from 1979 - 1983, served to demonstrate the importance of narrow elongated arcs of energetic electron precipitation in the auroral zone. They also characterized the spectral parameters and precipitation rates required for understanding source and loss mechanisms in the magnetosphere, but they were limited in field of view and to one map for each pass over the emitting regions. The Magnetospheric Atmospheric X-ray Imaging Experiment (MAXIE), soon to be launched on a TIROS satellite, will make time-space mappings by scanning a 16 pixel pinhole camera. These data will distinguish intensity variations of a fixed auroral feature from motion of a steadily radiating features. However, the spatial deconvolution is complex and features stay in the field of view for only approximately 10 minutes. These problems will be resolved by a high altitude (approximately 9 Re) imaging spectrometer PIXIE on the ISTP/GGS Polar Satellite to be launched in 1994. PIXIE's position sensitive proportional counter will continuously image the entire auroral zone for periods of hours. The resulting images will be important for understanding how the electrons are accelerated in the magnetosphere and why and where they precipitate into the atmosphere. Future needs and plans for next generation imagers will be discussed.

Imaging upflowing O⁺ ions of ionospheric origin and plasmaspheric O⁺ can be achieved through solar resonance scattering at 834 angstroms. Unfortunately, several strong background emissions, including the ones at 1025 angstroms and 1216 angstroms due to geocoronal hydrogen atoms, pose serious problems to its implementation. Most common optical coatings have higher reflectivity at 1025 angstroms and 1216 angstroms than at 834 angstroms. We have designed a multiple-layer coating which selectively reflects 834 angstroms radiation and suppresses 1025 angstroms and 1216 angstroms radiation. The structure of the coating material consists of a very thin (50 - 150 angstroms) method (nickel) layer, on top of a semitransparent dielectric material (magnesium fluoride), over an aluminum substrate. Three such coatings were produced at NASA Goddard Space Flight Center using an existing coating facility which is not optimized for thin coatings. In spite of such fabrication difficulties, we have obtained encouraging results.