NASA's Office of Planetary Science and Astrophysics has placed the inner magnetosphere imager (IMI) third in its queue of intermediate-class Space Physics Division missions for launch in the 1990's. The George C. Marshall Space Flight Center (MSFC) is performing a concept definition study of the proposed mission. An instrument complement of approximately seven imagers will fly in an elliptical Earth orbit with a seven Earth Radii (RE) altitude apogee and approximately 4,800-km altitude perigee. Several spacecraft concepts were examined for the mission. The first concept utilizes a spin stabilized spacecraft and a complementary three-axis stabilized spacecraft. The second concept places all of the instruments on a spinning spacecraft with a despun platform. Launch options being assessed for the spacecraft range from a Delta II for the single and dual spacecraft concepts to dual Taurus launches for the two smaller spacecraft. This paper will address the mission objectives, the spacecraft design considerations, the results of the MSFC concept definition study, and future mission plans.
We propose a small Explorer (SMEX) IMAP mission to provide the first global magnetospheric images that will allow a systematic study of major regions of the magnetosphere, their dynamics and their interactions. The mission objective is to obtain simultaneous images of the inner magnetosphere (ring current and trapped particles), the plasmasphere, the aurora, and auroral upflowing ions. The four IMAP instruments are (1) a Low Energy Neutral Particle Imager (LENPI) for imaging H and O atoms, separately, in the energy range of approximately 1 - 30 keV, in several energy passbands, (2) an Energetic Neutral Particle Imager (ENPI) for imaging H atoms in the energy range approximately 15 keV - 200 keV and, separately, O atoms in the energy range approximately 60 keV - 200 keV, each in several energy passbands, (3) an Extreme-Ultraviolet Imager (EUVI) to obtain images of the plasmasphere (the distribution of cold He+) using He+ (30.4-nm) emissions; and (4) a Far-Ultraviolet Imaging Monochromator (FUVIM) to provide images of the aurora and the geocorona. All images will be obtained with time and spatial resolutions appropriate to the global and macroscale structures to be observed. The results expected from IMAP will provide the first large-scale visualization of the ring current, the trapped ion populations, the plasmasphere, and the upflowing auroral ion population.
Japan's spacecraft PLANET-B will be launched to Mars. An EUV measurement of 30.4 nm wavelength is proposed for an interplanetary He II observation during the cruising phase from the Earth to the Mars. This measurement will help our understanding of the creation and loss of interplanetary He II. Another objective is the imaging of the plasmasphere and magnetotail of the Earth. The EUV scanner we have designed for PLANET-B mission will provide the opportunity to observe both interplanetary and magnetospheric He II.
The proposed HI-LITE Explorer will investigate the global ion outflow from the high-latitude ionosphere, its relationship to auroral features, and the consequences of this outflow on magnetospheric processes. The unique nature of the HI-LITE Explorer images will allow temporal and spatial features of the global ion outflow to be determined. The mission's scientific motivation comes from the fundamental role high-latitude ionospheric ions play in the dynamics of the solar wind driven magnetospheric-ionospheric system. These outflows are a major source of plasma for the magnetosphere and it is believed they play an important role in the triggering of substorms. In addition this paper describes the HI-LITE spacecraft and instruments.
The imaging of magnetosphere in energetic neutral atom (ENA) fluxes is recognized as a powerful experimental tool in the study of global magnetospheric processes. Intensity of ENA fluxes is typically very low. ENA's cannot be collected and concentrated by diffracting and refracting elements as it is done in optics, and therefore an imaging system on the basis of the pinhole camera should be used. There were several suggestions to use a coded-aperture technique to enhance geometrical throughput and, consequently, sensitivity of the instruments. The coded-aperture technique is reviewed and its application to the planetary magnetosphere imaging is considered. Computer simulation demonstrates advantages and limitations of the technique and promising applications are identified.
Recently proposed low energy neutral atom (LENA) imaging techniques use a collisional process to convert the low energy neutrals into ions before detection. At low energies, collisional processes limit the angular resolution and conversion efficiencies of these devices. However, if the intense ultraviolet light background can be suppressed, direct LENA detection is possible. We present results from a series of experiments designed to develop a novel filtering structure based on free-standing transmission gratings. If the grating period is sufficiently small, free standing transmission gratings can be employed to substantially polarize ultraviolet (UV) light in the wavelength range 300 angstroms to 1500 angstroms. If a second grating is placed behind the first grating with its axis of polarization oriented at a right angle to the first's, a substantial attenuation of UV radiation is achievable. The neutrals will pass through the remaining open area of two gratings and be detected without UV background complications. We have obtained nominal 2000 angstroms period (1000 angstroms bars with 1000 angstroms slits) free standing, gold transmission gratings and measured their UV and atomic transmission characteristics. The geometric factor of a LENA imager based on this technology is comparable to that of other proposed LENA imagers.
The measurement of neutral atom fluxes generated by charge exchange with the Earth's geocorona has recently been shown to provide the capability to image the magnetosphere. Here we investigate neutral oxygen fluxes, produced by charge exchange from the cusp/cleft ion fountain population. Using an empirical cusp/cleft ion fountain model, an empirical variation of the geocoronal neutral hydrogen density with distance, and typical values for charge exchange cross-sections, line-of-sight integrations are performed to calculate the neutral oxygen flux at arbitrary locations in space. The resulting images are evaluated for a set of orbital positions of the proposed HI-LITE small explorer spacecraft. It is shown that the resulting neutral oxygen fluxes are high enough for imaging with a low energy neutral atom imaging instrument (ILENA) on board the spacecraft.
The potential scientific return from low energy neutral atom (LENA) imaging of the magnetosphere is extraordinary. The technical challenges of LENA detection include (1) removal of LENAs from the tremendous ambient UV without losing information of their incident trajectories, (2) quantification of their trajectories, and (3) obtaining high sensitivity measurements. Two techniques that have been proposed for this purpose are based on fundamentally different atomic interaction mechanisms between LENAs and a solid: LENA transmission through an ultrathin foil and LENA reflection from a solid surface. Both of these methods provide LENA ionization (for subsequent removal from the UV by electrostatic deflection) and secondary electron emission (for start pulse generation for time-of-flight and/or coincidence). We present a comparative study of the transmission and reflection techniques based on differences in atomic interactions with solids and surfaces. We show that transmission yield an order of magnitude greater secondary electron emission than reflection methods. Transmission methods are shown to be sufficient for LEAN energies of approximately 1 keV to greater than 30 keV.
We describe an instrument concept for measuring low energy neutral H and O atoms with kinetic energies ranging from about 10 eV to several 100 eV. The instrument makes use of a low work function surface to convert neutral atoms to negative ions. These ions are then accelerated away from the surface and brought to an intermediate focus by a large aperture lens. After deflection in a spherical electrostatic analyzer, the ions are post accelerated to approximately 25 keV final energy into a time-of-flight mass analyzer. The latter consists of a thin carbon foil at the entrance that provides the secondary electrons for the start signal, a drift space, and a stop microchannel plate that detects the primary particles. Mass resolution is adequate for resolving H, He, and O, and the isotopes D and 3He. The image created by the spherical electrostatic analyzer is arc shaped with initial incident direction dispersed in azimuth and energy dispersed radially. Energy and azimuth information are obtained by position imaging the secondary electrons produced at the foil. A large geometric factor combined with simultaneous angle-energy-mass imaging that eliminates the need for duty cycles provide the necessary high sensitivity. From a spinning spacecraft this instrument is capable of producing a two-dimensional map of low energy neutral atom fluxes.
We describe a novel technique which enables us to conduct two-dimensional imaging spectroscopy using a one-dimensional imaging spectrometer. A typical imaging spectrometer obtains a series of one-dimensional monochromatic images containing the entire field of view, but the spatial information in the dispersion direction is lost. By rotating the instrument (hence rotating the field of view) we compile a series of time dependent profiles. We use a numerical tomographic reconstruction method to recover the second spatial dimension and thereby obtain two-dimensional monochromatic images of the field of view. This technique is more sensitive and hence collects a full two-dimensional image more quickly than a conventional push-broom scan. We present the results of numerical simulations and discuss the prospects of this method for magnetospheric imaging applications.
The development of instrumentation for magnetosphenc imagery and the design of future missions demands increasingly realistic simulations of the EUV and ENA emissions from magnetospheric ion populations. Relatively "cold" ion populations (E<5OeV) that fill the plasmasphere can, in principle, be imaged in the re-radiated solar lines of He+(304A) and O+(834A). "Hot" ion populations (E<lkeV) can be imaged using the energetic neuiral atoms produced when energelic singly-charged ions are neutralized in a charge-exchange collision with the cloud of exospheric H-atoms (the hydrogen geocorona) that suffuses the magnetosphere. We have responded to the need for increasing realism in simulations by incorporating elements of the Rice Convection Model (RCM) of magnetospheric dynamics into our images. The model, developed over the past decade by the Rice University, takes as its inputs the variation of measured magnetospheric indices and follows the transport and energy changes of the ion populations over the evolution of magnetic storms. We actually use a stream-lined version of the RCM, called the Magnetic Specification Model (MSM) that does not compute a self-consistent electric field, but utilizes phenomenological convection pauerns. The result is a sequence of simulated images as they would be obtained throughout a magnetic storm along a representative spacecraft orbit. These images set the requirements for sensitivity, angular resolution, energy pass-bands, etc., that must be met by imaging instruments on future magnetospheric missions. We then address the critical question of the extraction from the images of physical parameters describing the ion distribution functions. We report our progress in the development of computer-automated algorithms that extract the optimal set of parameters by minimizing a difference function between images simulated from a mathematical model of the ion distribution and "data" images simulated from MSM runs using inputs from actual geomagnetic storms.
Radiation at He+ at 30.4 nm, which is emitted close to the Earth, comes from three distinct regions; the ionosphere, the plasmasphere and the polar cap. Published observational data on He+ 30.4 nm have shown that the intensities from polar regions are relatively smaller than the other regions. Polar emissions are believed to be due to resonant scattering of ion outflow in sunlight. A 1982 rocket flight from Poker Flat, Alaska has shown that line-of-sight 30.4 nm emission rates are relatively strong in the direction of the pole. Since the roll of the rocket afforded many different observing directions, we have used the variety of viewing geometries to extract ionospheric source densities from the photometric intensity data. We have assumed that the He+ densities vary with distance along dipole field lines according to a particular functional form, and then we proceeded to extract the source densities by a matrix inversion method. The results give density variations over a range of latitudes including samples from each of the regions mentioned above. The method obtains good fits of the observed profiles of intensity versus observation angle.
The concept of using solar EUV line resonance scattering to image ion populations in the magnetosphere has been studied extensively in the last decade. Global magnetospheric EUV images can display the effects of scatterer density, injection, the geomagnetic field, and the changing perspective of earthshine source intensity with altitude, latitude, and local time. Successful use of these images for magnetospheric plasma diagnostics or for examining the morphology of the magnetosphere will depend upon properly accounting for these effects and incorporating them into the models used to interpret such images. The importance of each of these effects is examined for varying levels of magnetospheric activity and different observer perspectives. This work utilizes oxygen 834 angstroms resonance scattering as a testbed for this examination. A Tsyganenko 1987 magnetospheric model is employed to study the effects of different levels of magnetospheric activity. The resonance scattering formulation employed assumes an optically thin magnetospheric medium, with full inclusion of Doppler shift effects, for 834 angstroms radiation; Monte Carlo techniques are used for the construction of simulated magnetospheric images.
Photometric imaging of ionospheric/magnetospheric O II emission at 83.4 nm is a primary objective for mapping the distribution of O+ ions. However, instrumental sensitivity has been a major barrier to realizing this goal. We report an instrumental design employing a low focal ratio camera where the reflecting surfaces act as both narrowband reflection filters at 83.4 nm and wide field of view imaging system. The design includes coatings with reflectances that are relatively insensitive to the angle of incidence of light. The peak reflectance per mirror is more than 60% at 83.4 nm with the average reflectance for out-of- band wavelengths less than 5%. The net reflective transmission for the three imaging mirrors is greater than 20% with 6.8 nm bandwidth and 0.01% maximum transmittance for out-of- band wavelengths.
We have developed techniques for depositing a uniform multilayer on a highly curved mirror surface. The multilayer was designed for normal incidence reflection of the emission line of He II at a wavelength of 304 angstroms. The mirror was of a design suitable for broad field imaging of the resonantly scattered solar He line from He+ in the earth's plasmasphere. A spherical proto-type mirror was chosen for the tests having a radius of curvature of 9.8 cm (focal length of 4.9 cm) and a diameter of 14 cm. The sagitta for this highly curved mirror is 2.8 cm and the angle that the mirror surface makes with its axis varies from 90 deg to 45 deg, center to edge. This poses a challenge to produce a multilayer that is uniform in response over the mirror's surface. For normal incidence reflection of 304 angstroms photons, molybdenum and silicon were chosen as the multilayer materials. A vacuum sputtering process, involving planar magnetrons, was used to fabricate the multilayers. Details of the deposition techniques, results of subsequent testing and ways of further improving the uniformity will be presented.
The study of upflow of ions from the high latitude ionosphere into the magnetosphere and the consequences of this outflow is the primary objective of a proposed small explorer mission. We describe an extreme ultraviolet instrument for imaging upflowing O+ ions which is part of this mission. The instrument is designed to measure solar 834 angstroms emissions resonantly scattered by upflowing O+ ions. Preliminary calculations of the maximum intensity expected from this process yield a value of approximately 0.1 Rayleigh. The technical challenge of measuring this signal in the presence of the bright background due to lower thermospheric airglow and aurora (approximately 1000 Rayleigh) was addressed by the choice of a favorable observational geometry. In addition, the presence of airglow signals in the nearby spectral region must be discriminated against to obtain a high signal-to-noise O+ image. Our instrument uses a normal incidence spherical mirror in prime focus configuration with a curved focal plane detector. The mirror will be coated with a multi-layer coating; further spectral discrimination will be obtained through the use of thin-film metal filter and an appropriate detector photocathode.
We briefly describe a spectrograph concept designed for both high wavelength and high spatial resolution (in one dimension) of magnetospheric far and extreme ultraviolet (FUV and EUV) emission. We refer to the design as a Single Element Imaging Spectrograph (SEIS). It is a single bounce diffractive system which combines the spectral properties of a Rowland mount spectrograph with the imaging (spatial resolution) properties of a Wadsworth through the use of a toroidal diffraction grating. No primary optics are necessary making the system especially attractive for use in the EUV and FUV where low reflectivity of common optical coatings can severely limit instrument sensitivity.
The POLAR spacecraft of the Global Geospace Science Mission is scheduled for launch in the summer of 1994. Included in the payload is an advanced ultraviolet imager that will emphasize gathering data for quantitative interpretation of the simultaneously observed global auroral phenomenon. Significant advances have been made in the area of thin film filters, optics, and intensified-CCD focal plane detectors. These advances make possible imaging of weak but key diagnostic auroral emission features under fully sunlit conditions. The instrument's f/2.9 optical system, its highly efficient narrow band FUV filters, and high quantum efficiency focal plane detector combine to yield a noise equivalent signal of 4R per 37 second image. The instantaneous dynamic range is > 1000, and the 8 degree(s) field of view allows coherent global imaging of the auroral oval. The performance of the instrument and its components in extensive laboratory testing is discussed.
Previously flown satellite imaging experiments have demonstrated the suitability of the vacuum ultraviolet region for remote sensing observations of auroral particle precipitation. In the wavelength region 120 - 145 nm, a downward viewing imager is uncontaminated by the earth albedo and the intensity of the auroral emissions in most cases is competitive with the rescattered light even during daylit conditions. These features permit the quantitative imaging of the auroral regions during day and night conditions. An instrument suitable for such observation which has adequate wavelength resolution to separate key spectral features and simultaneously observe the Doppler profile of the auroral Lyman alpha line was designed. This instrument, in its simplest form, consists of an F3.8 Rowland circle spectrography with an FUV intensified CCD at the focal region. The entrance slit is perpendicular to the orbit plane and parallel to the spin axis of the satellite. The field of view of the instantaneous slit image is 50 degrees in the direction perpendicular to the rotational axis and 1 degree parallel to the spin axis [i.e. in the direction parallel to the orbital plane]. The spectrography produces a two dimensional spectral image where one dimension represents luminosity distribution and the other wavelength dependence. The UV intensified CCD is programmed to pick up the luminosity distribution of various key spectral regions. During the 360 degree rotation, a complete luminosity map of the 50 degree wide region under the satellite is recorded. Depending on the satellite rotation rate and the wavelength of interest, more than one complete rotation will be needed to achieve the desired signal to noise ratio.
We report designs of multilayer narrowband reflective 30.4 nm filters. The narrowband reflection filters are designed for angles of incidence (theta) o, ranging from 0 degree(s) - 30 degree(s). For example, the calculated reflectance of a 15-layer Au-Al multilayer which operates within the angular range 0 degree(s) <EQ (theta) o <EQ 10 degree(s) has for 0 degree(s) angle of incidence a reflectance peak value of 24% at 30.4 nm with a bandwidth of less than 5 nm. The average out-of-band reflectance is less than 2% for the wavelength range from 10 nm - 70 nm. Filters with more than 20% peak reflectance and less than 5 nm bandwidths are designed for incident angular cones from 0 degree(s) - 10 degree(s), 10 degree(s) - 20 degree(s).
Imaging of the terrestrial magnetosphere can be performed by detection of low energy neutral atoms (LENAs) that are produced by charge exchange between magnetospheric plasma ions and cold neutral atoms of the Earth's geocorona. As a result of recent instrumentation advances it is now feasible to make energy-resolved measurements of LENAs from less than 1 keV to greater than 30 keV. To model expected LENA fluxes at a spacecraft, we initially used a simplistic, spherically symmetric magnetospheric plasma model. We now present improved calculations of both hydrogen and oxygen line-of-sight LENA fluxes expected on orbit for various plasma regimes as predicted by the Rice University Magnetospheric Specification Model. We also estimate expected image count rates based on realistic instrument geometric factors, energy passbands, and image accumulation intervals. The results indicate that presently proposed LENA instruments are capable of imaging of storm time ring current and potentially even quiet time ring current fluxes, and that phenomena such as ion injections from the tail and subsequent drifts toward the dayside magnetopause may also be deduced.