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The TENACIUS mission will provide new knowledge of the inner magnetospheric energetic particle populations through both in situ and remote sensing techniques. The mission payload consists of energetic neutral atom imagers, energetic charged particle, and optical sensors that will image the Earth's geocorona and inner magnetospheric neutral particle populations while also providing in situ measurements of energetic charged particles. TENACIUS is optimized to measure local and remote particle populations from low-Earth orbit. With a July 2000 launch, TENACIUS will operate during the NASA IMAGE and TWINS missions, thereby augmenting the high-priority scientific measurements they, and other contemporaneous imagers provide. The TENACIUS mission provides a significantly different and critically important vantage point than its high-altitude counterparts. These unique data will allow us to obtain entirely new tomographic views of magnetic substorms and storms and to explore the low-altitude extension of the ring current and plasma sheet that can be imaged easily with high temporal and spatial resolution.

12 October, 1998

1. The Significance of and Scientific Basis for Magnetospheric Imaging

Great progress has been made recently in magnetospheric imaging. Beautiful images of energetic neutral atoms (ENAs) created by ion collisions with the Earth's tenuous upper atmosphere have been collected by several satellites, including NASA’s POLAR satellite. These new global images demonstrate the feasibility and scientific usefulness of remote sensing to understand quantitatively the magnetospheric response to solar events (Jorgensen et al., 1997; Henderson et al., 1997). In the Tomographic Energetic Neutral Atom Comprehensive Imaging University Satellite (TENACIUS) mission, we propose a suite of instruments that will substantially increase our knowledge and insight into the magnetospheric responses by providing in situ measurements of particle fluxes and supporting optical emissions to complement ENA imaging. Ultimately, with this new knowledge, we will be able to better understand and predict the magnetic substorms and storms that energize and transport energetic particles in the geospace environment.

The power of ENA measurements from spacecraft has been amply demonstrated recently on orbit (ASTRID, POLAR, GEOTAIL) - but particularly so at high altitudes by the CEPPAD/IPS on the NASA POLAR spacecraft. The TENACIUS ENA imager we propose is based directly on the CEPPAD experiment. A schematic of how POLAR remotely senses the ENAs produced by charge exchange between the geocorona and the energetic ions of the ring current is shown in Figure 1. This figure illustrates how a two dimensional image of the optically-thin ENA emissions can be formed from a single vantage point.

From high apogee orbits, ENA images can be obtained on a global scale. However, ENA emissions are optically thin and involve the convolution of three-dimensional neutral and ion density distributions, interacting with an energy-dependant charge exchange cross-section. Thus, data from a single ENA sensor provides a non-unique determination of the important ion fluxes along the line-of-sight. Ideally, multiple measurements are required to obtain an unambiguous spatial distribution. With the IMAGE and TWINS missions, NASA has taken the necessary first step toward stereoscopic imaging of geospace with neutral atoms, a high scientific priority identified by NASA in their Sun-Earth Connection Roadmap and also by the NAS/CSSP. However, as Gruntman (1997) points out in his excellent review article on ENA imaging, views from both the "outside looking in" (POLAR, IMAGE, TWINS) and from the "inside looking out" (our proposed TENACIUS mission) are critically needed in order to extract the most meaningful physical information from the images. This need is driven by the fact that the highest spatial resolution and tomography can be obtained only from low altitudes where the optically thin emissions are the brightest.

We note that dynamic magnetospheric regions connect magnetically to low-altitudes at mid- to high-magnetic latitudes. The feet of the flux tubes threading the inner magnetosphere and inner magnetotail connect to the ionosphere in regions where the geocoronal density is high. Therefore, ENA emissions are focused in these regions and the emissions are bright. A low altitude spacecraft has two important advantages over those imaging from high apogee. First, it is closer to the source region which means that the emissions will be bright and will provide higher spatial resolution images for the same angular resolution imager. Second, the spacecraft location is rapidly changing which means that the imager will provide a sequence of images of the same emission region but from lines-of-sight that are constantly changing. These two features allow for tomographic inversions to be performed with TENACIUS, just in the same way that the BU TERRIERS STEDI mission will provide tomographic 3-D images of the upper atmosphere and ionosphere.

1. Overarching Scientific Goals and Objectives

Two of the most important dynamic features of the magnetosphere are geomagnetic storms, characterized by ring current intensifications with Dst variations, and substorms, characterized by magnetotail reconfigurations with auroral electrojet variations. Each event is a global magnetospheric response to changes in the solar wind and interplanetary magnetic field. For the past thirty years, we have been limited in our ability to understand the global nature of these phenomena. While we have amassed a great volume of single-point, in situ measurements that provide the global picture in an average, static sense (space climatology), we have lacked an instantaneous view of large-spatial-scale magnetospheric dynamics (space weather). This lack of a global perspective has limited our analysis of the in situ measurements. In addition, the lack of these global data limit our ability to differentiate between various physical models that describe magnetic storms and substorms. These limitations include inadequate global knowledge of particle origins, loss processes, transport mechanisms and their time dependence.

Unfortunately, ions and magnetic fields are "invisible." These quantities are dynamic and can only be measured when we fly through them with a spacecraft. Single point measurements thus cannot separate temporal from spatial variability, nor can they provide a global view. The novel approach of ENA imaging however gives us the vision we need. Recently, our team has demonstrated with the POLAR instrument on which TENACIUS is based, that both storms and substorms have strong and repeatable ENA signatures. With the advent of global ENA imaging, we are now on the threshold of major advances in our understanding of these science topics.

The TENACIUS mission will provide new knowledge of the inner magnetospheric energetic particle populations and their dynamics through both novel remote sensing tomography of ENAs and through in situ measurements. The mission payload consists of an ENA imager, energetic charged particle telescopes, and optical spectrographs that will image the Earth's geocorona and the inner magnetospheric particle populations while simultaneously providing in situ ion and electron measurements. The TENACIUS mission will be operating during the NASA MIDEX IMAGE and TWINS missions and will augment the already high-priority scientific measurements they will provide. The TENACIUS mission, with a unique low-altitude view different from IMAGE and TWINS, will allow us to best interpret the joint TENACIUS-IMAGE-TWINS particle images. This will allow us to image the spatial distribution and spectral and temporal evolution of ions during storms and substorms with high spatial resolution.

The TENACIUS goals will be accomplished onboard a spinning low-Earth orbit spacecraft in a polar near-sun-synchronous orientation. From low altitudes, significant portions of the inner magnetosphere ring current regions and near-Earth plasma sheet can be viewed with ENA and optical imagers along and near the orbit track. At low altitudes, the emissions are exceptionally bright both because of proximity to the source region and to the high geocoronal densities. Closeness to the source region also permits higher spatial resolution than would be possible from high apogee satellites such as POLAR, IMAGE, or TWINS. In addition, charged particle fluxes can be sampled along the 90 minute orbit. The ENA and optical images acquired from this low-altitude vantage point complement those from high apogee orbits; it is uniquely different in that it is from the inside looking out, rather than the outside looking in. Also, since the spacecraft moves through the region rapidly, it remotely samples the same region of space from changing view paths, thus allowing for true tomography of the emission regions. We stress that a three-dimensional, time dependent picture of the near-Earth plasma is essential to the physics of storms and substorms; only at low altitudes do these regions project to volumes that can be imaged at high resolution. Together, the ENA images and the particle data, along with other expected ENA data sets (such as IMAGE or TWINS), will be used to extract globally the energetic particle populations and their dynamics.

We propose to assemble a suite of sensors optimized to observe the dynamics of energetic charged particles and their interaction with neutral particles within the earth's magnetosphere from a unique low-altitude vantage point. These sensors and the satellite that will carry them, all have excellent and proven NASA flight heritage. They will perform the following measurements with the stated goals (also outlined in Table 1), at an exceptionally low cost to NASA:

• obtain high-energy-spectral resolution images of ENAs with excellent time resolution, broad spatial coverage and exceptional spatial resolution in three dimensions;
• obtain ultraviolet (UV) images of major neutral species with comparable time resolution and comparable or better spatial resolution than ENA images both for ENA inversions and to determine the response of the upper atmosphere to magnetic storms and substorms;
• obtain multiple line-of-sight views of the ENA source regions along the short-period orbit to allow for true tomographic inversions of the low-altitude extension of magnetospheric particle populations (especially for L < 6.6 R_E);
• provide data bases for comparison with data from other available satellites to permit stereo ENA views of the magnetosphere
• obtain in situ measurement of the charged particle energy distribution in regions with UV and ENA images; our orbit permits portions of the imaged regions to be directly sampled;
• quantitatively relate high spectral/spatial/temporal resolution ENA image intensities to global particle fluxes and their dynamics, both locally and remotely driven.

• Substorms and Tail Dynamics

• Geocoronal Physics

• Auroral Structure and Acceleration

1. Geomagnetic Substorms

During a magnetospheric substorm, magnetic energy stored in the magnetotail lobes is converted, in part, to plasmasheet flow and inner magnetosphere plasma thermal energy. This energy conversion process is rapid, happening on the time scales of minutes. It produces large disturbances in the magnetotail magnetic fields and particles over broad spatial scales, perhaps starting as a highly-localized site of acceleration and thermalization and growing in scale over a short time. Because the magnetosphere is electrodynamically coupled, the energy dissipation from this reconfiguration is global, involving virtually all regions of the tail as well as the auroral ionosphere, an essential component of a substorm. The relative timing of substorm phenomena in the deep magnetotail (>40 R_E), the mid-tail plasmasheet (20-40 R_E), the near-tail plasmasheet (10-20 R_E), the inner edge of the plasmasheet (5-10 R_E), and the auroral ionosphere provide important information about the source of particle energization during substorms and the injection of hot plasma into geostationary altitudes. Statistical studies have established the average properties of substorm injections including their strength and radial and local time extent. However, the question remains as to how representative the average substorm is in terms of dynamics and spatial extent. These questions can only be answered definitively by remotely observing the system globally and analyzing the in situ measurements in the context of the global evolution. TENACIUS imagers provide immediate insight into these decade old questions.

As noted previously, the promise of ENA imaging has been made clear recently by the POLAR spacecraft. POLAR has provided the first glimpse of the substorm injection region through global ENA imaging [Henderson et al., 1997; Spence et al., 1997]. During the substorm life-cycle, ENA emissions are observed to first brighten weakly at the inner edge of the plasma sheet during a substorm growth phase, then intensify in a localized region near pre-midnight at substorm onset, and finally spread azimuthally (primarily to earlier local times) as the substorm enters the recovery phase. Thereafter the injected ions drift westward and decay while the associated ENA emissions subside. Simultaneous measurements at geostationary orbit confirm the tail stretching and subsequent ion injection while auroral images record the auroral arc brightenings and surge evolution. Despite the relative simplicity of the POLAR ENA measurements, significant qualitative understanding of the substorm injection process has already been made. The phenomena seen in the auroral zone and magnetotail are linked by the global images and the comprehensive view of spatial and temporal evolution allows us to test various substorm hypotheses. However, these analyses are hampered by the inherent single point POLAR ENA measurements. Even with the multiple ENA data sets promised with IMAGE and TWINS, all these views are from the outside looking in and lack the ability to image with high spatial resolution the near-Earth extensions of these magnetospheric regions. For example, the middle to distant tail viewed at the magnetic equator will be virtually invisible in ENA, not primarily because there are no energetic ions, but rather owing to the very low geocoronal densities near the magnetic equator at such great distances. On the other hand, these same regions are visible from low altitudes - a small pitch-angle ion from the tail can charge-exchange in the high geocoronal densities near the Earth and thus be seen with a low-altitude ENA imager. The low-altitude, "inside-out" views provided by the TENACIUS ENA imager will allow for substantially improved and routine observations of these regions and will complement nicely the global imaging provided by IMAGE and TWINS. Indeed, linking the high and low altitude signatures of substorms is extremely difficult with traditional methods and has led to great uncertainty in substorm model differentiation. A critical outstanding question to be answered is whether the substorm is initiated in a near-Earth current sheet disruption region that propagates tailward or is it initiated first at a near-Earth reconnection site whose effects propagate earthward? The next-generation TENACIUS and IMAGE images should allow us to immediately establish the critical direction of propagation during a substorm and thus differentiate between competing substorm theories.

2. Magnetic Storms

A geomagnetic storm is characterized by the injection and energization of ions deep within the trapping region of the inner magnetosphere (L<6.6 R_E). This trapped ion population decays to some resting state typically on the order of a few days. The storm time ion population is sufficiently energized such that its net guiding center drift leads to a strong enhancement of the westward-flowing, azimuthal ring current. The strong current near the magnetic equator between 2 and 6 R_E depresses the Earth’s surface magnetic field which defines the classic magnetic storm index, Dst. Early studies demonstrated the relationship between the energy content of the ring current and the strength of Dst as well as the large azimuthal asymmetries that can arise during the injection process (see Figure 2). Kinetic particle models rather than MHD models of the ring current are warranted owing to the large gyroradii of the pressure-bearing ions. Such models have evolved over the last twenty years but all depend on the specification of initial injected particle populations and evolving electric and magnetic fields that the particles react to during a storm cycle. These models have provided a theoretical view of the global system but until recently, have had relatively little data to constrain and test our understanding of the physics of storm injection. Most comparisons have been done with one or at most a few satellites traversing only portions of the volume important for the storm time development.

With POLAR, the growth and decay of the energetic ion population during a magnetic storm was directly tracked on a global scale for the first time using ENAs. The high geometric factor, excellent energy resolution, and capable spatial resolution of the POLAR ENA imager permit us to create global images of the ring current at high time resolution (on the order of several spacecraft spins or about one minute). An example of one such sequence of ENA images from POLAR is shown in Figure 2 during a magnetic storm in January 1997. This spatially-smoothed image shows the emissions mapped to the symmetry plane of the magnetic equator. A clear rise and fall and large local time asymmetry of the ENA flux is apparent. The partial ring current is clearly seen as fresh ions during a storm surge are injected deep into the trapping region and then drift westward owing to gradient-curvature effects; there is even evidence of ionospheric extraction of energetic ions during the main phase (Sheldon et al., 1998) of another storm in 1996. These ions carry an azimuthally-localized current that is seen on the ground as an asymmetric ground magnetic disturbance in local time. However, from the ground there is no clear way to know the radial or azimuthal distribution of current. Presently, only a crude asymmetry index called ASY is ever routinely produced. On the other hand, our POLAR images provide immediate global context for the analysis of point measurements within the globally imaged region. In one image, we see the local time, radial, and spectral distribution of the ring current ions and can track them in time at relatively high time resolution. These data are allowing us to test our models of electric field penetration and the complex particle accelerations during storm main phase and recovery. With TENACIUS we will be able to obtain high spatial resolution information of the storm time ring current not possible from high apogee orbits.



Figure 2. Global ENA Images from POLAR During a Moderate Magnetic Storm

3. Current State of Magnetospheric Imaging at High and Low Altitudes

Over the last many years, the increasing sophistication of ENA imagers has led to better empirical global views of the stormtime ring current region. Early results from the ISEE spacecraft (Roelof et al., 1985; 1987) demonstrated the concept of quantitative use of ENA observations and ENA imaging. More recent results from the GEOTAIL spacecraft (Lui et al., 1996) demonstrated the ability of ENA measurements to provide not only energy but also ring current composition. Even at low altitudes, ENA imaging studies have already amply demonstrated the power of these data, both from free-flying spacecraft, such as ASTRID measurements of the ring current (Barabash et al., 1997; C:son Brandt et al., 1997), and from rocket experiments, such as "Poleward Leap" measurements of auroral proton arcs (Soraas and Aarsnes, 1997).

The most contemporary high-altitude ENA results are from the POLAR spacecraft (Jorgensen et al., 1997; Henderson et al, 1997; see Figure 3) which emphasize the power of 2D imaging from a POLAR orbit and the ability to instantaneously image the entire ring current throughout a magnetic storm (see Figure 2). The POLAR ENA images permit, for example, the direct correlation of globally-integrated ENA images and the Dst index, and also are allowing us to track the stormtime injection process visually. The azimuthal asymmetry can be seen to evolve during a storm, reflecting the aggregate effects of ion drifts and the establishment and loss of the partial ring current. Despite the power of the POLAR data, the quantitative physical interpretation of the ENA images is confounded by their inherently low spatial resolution. The current pixel size projects to an area that is large in comparison to the neutral density scale height and the ion pressure gradient scale length in the ring current regions. This shortcoming represents an underconstrained data set from which it is difficult to extract unambiguous information on the populations that convolve to produce ENAs. This same problem will be encountered even by the much more capable IMAGE and TWINS imagers. By making complementary ENA images at low-altitude, the 3-D multiview and higher resolution TENACIUS data will reduce these uncertainties and permit the important next level of quantitative analysis of stormtime injection both near the equator and possibly off the equator via parallel potentials.

For the TENACIUS mission, we shall be observing ENAs produced within geospace but from low-altitudes, a complementary vantage point to that of POLAR (the same as IMAGE and TWINS will provide). Do we have evidence that ENAs from these low-altitude regions will actually be observed? We have every expectation they should be. The equatorial distributions have fluxes near the loss cones - often comparable to the trapped fluxes - that will mirror at lower altitudes. These lower altitudes have much higher neutral densities, often by many orders of magnitude, so the commensurate rate of ENA production could be much higher. Furthermore, a low altitude spacecraft is considerably closer to this low-altitude source region so the emitted flux of ENAs should appear brighter. Such considerations have already been treated theoretically by several studies (Orsini et al., 1994; Millilo et al., 1996, Roelof et al., 1996) and these have shown that low-altitude imaging is not only viable but is highly desirable. Indeed, as noted above, both ASTRID and Poleward Leap observations demonstrate not only the existence of low-altitude ENAs but also their high intensity and also their scientific utility. Those are reviewed below. In addition, we present exciting new evidence from the long-running TIROS mission of considerable ENA fluxes detected at low altitude and their association with storms and substorms.

Barabash et al. (1997) used the ASTRID data to show a correlation, as in Figure 3, of the ENA flux measured from a low altitude orbit (1000 km) with Dst. Their instrument had a geometric factor of 2.5 x10^-3 cm²-sr (somewhat smaller than that of the POLAR CEPPAD imager). Within the energy range of 26-37 keV, they observed ENA rates of 100 counts/second even during intervals of very weak ring activation (Dst > -40 nT). These data were used to create maps of the ring current ions and used to do quantitative modeling of a small magnetic storm (Dst ~ -80 nT) on 8 February 1995. Simulation results (C:son Brandt et al., 1997) and forward modeling revealed that during the main phase the ENAs seen at low altitude were consistent with a strong duskside source near the equator between L=4 to 8 centered at 19 MLT with a spread of about one hour of MLT. This is similar to the strong partial ring current peak observed in ENAs near the equator by POLAR.

Soraas and Aarsnes (1996) have used ENAs measured from the Poleward Leap sounding rocket to probe the structure of proton arcs in the auroral zone. They used a solid state detecting system with a geometrical factor of 4.3 ´ 10^-2 cm²-sr but were constrained to measure from altitudes below 454 km, in regions of multiple scattering in the higher density low altitudes atmosphere. At energies of 20-50 keV, they observe highly directional ENAs coming from a proton arc with rates approaching 1000 counts/second. Using these data, the study was able to observe ENAs in an arc during the expansion phase of a substorm and provide a regional map of the proton precipitation with excellent energy spectral resolution. The study showed a remarkably good agreement between the observed ENA emissions compared to a simple model of proton precipitation, even in this lower altitude regime where multiple scattering can have a blurring effect.

We note also in a study presently underway at Boston University (Alothman et al., 1998), the discovery that even the NOAA TIROS spacecraft have been observing ENAs routinely from their 850 km polar orbit for decades. The TIROS orbit is at an altitude just midway between the rocket ENA measurements (<450 km) and the ASTRID measurements (1000 km). Like the Poleward Leap instrument, the TIROS energetic particle experiment is a simple solid state detecting system. However it possesses a higher energy threshold than we will fly on TENACIUS; therefore the rates that it measures should be a lower limit approximation to what we should expect. TIROS measures ENAs at many hundred counts per second while in the polar caps looking back toward the auroral zone and ring current. Figure 4 shows a plot of TIROS ENAs, similar to Figure 3, demonstrating the relationship of ENA count rate to AL and Dst during a substorm and a magnetic storm.

In Figure 4, the solid colored crosses denote the average ENA rates measured by the TIROS spacecraft while in the polar caps looking back at the auroral zone and ring current regions. We can infer that these are ENAs by virtue of another detector looking toward the nadir which observed no discernible ion signal in the polar caps (squares). Over the course of several day periods, during the course of two magnetic storms in 1980, we can see that the ENA rate tracks both the Dst (black heavy line) and the AL indices (green line). Indeed, it is arguable that the ENA rates track the AL curve more closely than the Dst curve, suggesting that these are ENAs associated with higher latitude, auroral phenomenon, rather than lower latitude ring current phenomenon. From these powerful data, we therefore have every confidence that the unique TENACIUS perspective from low altitudes, with high spectral-, temporal-, and spatial-resolution ENA images coupled with optical data, will provide critical new information on both storm and substorm studies.

5. New Applications of Low-Altitude Imaging - Auroral Acceleration and Radiation Belts

The NASA Small Explorer, FAST, has shown that the auroral zone is a region of complexity with interesting microphysics coupling to macroscale phenomenon. Part of the auroral puzzle is the distribution in space and time of auroral arcs. While FAST can make very rapid measurements, it is still limited in its ability to unravel spatial versus temporal variations. While the bulk of auroral energies are below our instrument threshold, there does exist a finite component in our energy range (> 15 keV) that we are confident we can image. The TIROS and POLAR data indicate strongly that we will. Therefore, TENACIUS has the chance of revealing the structures associated with proton aurora on a macroscopic scale. Owing to the spacecraft motion, we will then fly through the structure, possibly coincidentally, and thus be able to make excellent progress in unfolding the surrounding ion fluxes for the first time, with tomographic inversion techniques. As FAST has revealed, TENACIUS will have the exciting possibility to image as a function of neutral atom energy two major auroral acceleration regions to complement classical optical images: the upward-accelerated ions associated with traditional auroral arc regions (from which auroral arc electric field structure may be inferred in 3D; and the so-called "black aurora" regions characterized by downward-going hot magnetospheric ions that neutralize at the feet of auroral field lines.

The mechanisms that form and accelerate the radiation belt particles can be different from the ring current. For example, both the CRAND source and the March 1991 solar wind shock produce radiation belt enhancements in a far different manner than the radial diffusion and adiabatic energization that forms the quiet time ring current. If the radiation belt ions are adiabatically transported to their low-L, high-B location, then one can confidently predict that the "source region" for the radiation belts has lower energy ions that are reduced by the ratio of magnetic field strengths, B_source/B_belt. Several regions have been proposed as sources of these ions, including the tail, the cusp and the boundary layer/ magnetosheath. None of these regions have high enough geocoronal densities to produce much ENA flux, but if they are mapped into the loss cone at low altitudes, a substantial ENA flux would result. The low altitude orbit of TENACIUS, then, is the only place where these putative source populations could be remotely sensed. Therefore it is of great interest to spectrally resolve ENA's arising from high latitudes and differing MLT, for then we may be able to map the precursors to a radiation belt enhancement and resolve their sources.

6. Geocoronal Science

As powerful as the ENA measurements are, they do not return the source ion distributions without deconvolving the densities of the neutral, electron-donor background gas. On this basis alone, a measurement of the geocoronal density is essential, including not only [H], but also [He] and [O] at low altitudes. We accomplish this task by using spectroscopic tomographic inversion methods pioneered by TERRIERS, and applied to the EUV sunlight scattered by these species described later in Section D.2a. In addition to these neutral gases which form a purely gravitationally bound atmosphere, both He⁺ and O⁺ are conceivably important electron donors for ENAs whose densities would track the magnetically confined plasmasphere. Sheldon (1994) has shown that He and He⁺ electron donor cross sections may actually dominate over H for select species and energy of ENAs. TENACIUS will have the ability to track photometrically the three most abundant neutral gases [H], [He], and [O], as well as the most important plasma species [O⁺] (excluding [H⁺] which is photometrically invisible). These geocoronal measurements will complement those being made from high altitudes with advantages similar to those described for the ENA measurements.

Finally, the geocorona is an extension of the upper atmosphere that causes satellite drag, which has high variability both annually and during the solar cycle. It should be noted that the scale height of the geocorona varies most rapidly at low altitude precisely where TENACIUS will have the best resolution. Thus, TENACIUS is uniquely suited to providing a 3-D, time variable, dynamic geocoronal model.

7. Complementarity to Other NASA Neutral Atom Imaging Missions

As noted above, TENACIUS is complementary with other planned NASA ENA imaging missions and should not be perceived as merely duplicating other related measurements. The TENACIUS measurements will be taken at low altitudes which is uniquely different than IMAGE and TWINS, the two major future missions. From these low altitudes, high resolution images can be obtained that are not possible from high apogees – these allow a range of important scientific studies elucidated in sections b through f. In addition, the TENACIUS data, when combined with these other contemporaneous data sets, will yield a meta-data set that is more powerful even than the sum of the individual components. Therefore, TENACIUS is not only capable of highly self-contained scientific investigations, but will also contribute significantly to the other main NASA ENA missions presently being developed. Table 2 reveals the complementarity of TENACIUS to these other planned NASA missions (and the ongoing POLAR mission).

We propose to achieve the scientific goals and objectives outlined above with optimized instrumentation on TENACIUS. The instrumentation described below either has proven spaceflight heritage or is based, with low- or no-risk modifications, on hardware previously developed for NASA missions. The instrumentation includes an Imaging Energetic Particle Sensor, IEPS (for energetic protons, neutral atoms, and energetic electrons), and a pair of Tomographic Extreme-UltraViolet Spectrometers, TESS (for geocoronal imaging) each tuned either for daytime or nighttime observations. These instruments are described below; individual sensor specifications are summarized in accompanying tables.

The properties above have been chosen not only to minimize changes from the IPS design, but also to meet our science requirements. These are driven largely by the possibility of doing tomography from the unique low-altitude environment. The principle of tomography is really no different than the binocular vision most of us are born with. With one eye covered, we see only a flat image and must infer distance from size or overlapping images, but with both eyes open we are able to use parallax to judge distances much more precisely. In the same way, a slowly moving ENA imager, such as those on POLAR, IMAGE, or TWINS, sees a column integrated ENA map, and must infer the distance to the source from models of the emission region. Stereo images combined from multiple satellites helps greatly and that is the main thrust behind TWINS. However "depth" perception in stereoscopic vision is best when close to the source. We note however that TENACIUS reduces the uncertainty by a whole new major step from mere stereoscopy. It takes advantage of its rapid, low-altitude orbit to effectively snap a parallax picture, every 3 seconds and thus every 21 kilometers along its orbital track.

If the ENA source is changing rapidly, such as is seen in POLAR ENA substorm studies, three seconds between exposures might blur the image. However, we do know from POLAR/IPS images that substorm ENA bursts, which have shorter timescales than storms, persist for 20 or more minutes, with the onset occurring over several minutes which is much slower than the exposure time. However, during that time, the spacecraft will have traversed the feet of the auroral field lines and imaged the evolution from widely varying angles. This is critical for unraveling the complex line-of-sight inversions necessary to back out the fundamental ion flux distributions. The optimal viewing geometries provided at low altitudes just simply cannot be accomplished by the slowly-moving, high-apogee satellites such as POLAR and other planned ENA missions.

A second requirement for tomography is that our data rate be sufficiently high to invert these exposures. We appeal to the aforementioned low-altitude instruments to predict a data rate for the IEPS ENA imager. At the POLAR/IPS perigee (4000 km altitude) we typically observe ~4,000 counts/(sec -sr-cm²-keV) with a threshold energy of ~20 keV. The TIROS/MEPED instrument described in section D.1 is in a circular orbit at 850 km and it observes ~1,000 counts/(sec-sr-cm²-keV) with its threshold energy of ~30 keV. The ASTRID/PIPPI imager was in a circular orbit at ~1000 km and recorded fluxes of 500-2000 counts/(sec-sr-cm²-keV) in an energy band of ~26-37 keV. Note that the lower energy threshold of IPS more than compensates for its greater distance than TIROS from the assumed source. From these data, we can estimate a 10,000 counts/(sec-sr-cm²-keV) at the 15 keV threshold for TENACIUS. If we divide the spin plane into 32 sectors, then we have ~100 ms per sector with a 0.005 geometric factor resulting in about 5 counts per pixel, significantly above the expected cosmic ray noise of 0.1 counts/(sec-pixel) seen by IPS. At the lower average rates, summing of adjacent pixels may be required to improve counting statistics, but during moderate to large events, statistically significant images should be possible at our highest possible time resolution of ~3 seconds. Part of the Phase A study will be devoted to finalizing the IEPS geometrical factors based on the most contemporary rates and trading that off against the mass and volume constraints. At this juncture, we are confident that excellent images can be made routinely even with the present POLAR-designed geometric factor.

To best resolve particle pitch angle distributions and neutral atom images, the instruments will be mounted such that the long axis of the instrument (vertical axis in Figure 5) points nominally along the spacecraft spin vector. That is precisely the orientation used on the POLAR spacecraft. To avoid the sun, the instrument will be rotated slightly away from the spin axis. In each spacecraft spin, the ion and neutral telescopes will map out the unit sphere. To maximize spatial coverage, the ion and neutral sensors may be mounted back-to-back as an alternative configuration to the one shown in Figure 5. The single electron telescope will be mounted perpendicular to the spin axis and will thus cover all pitch angles once per spin. Other telescope alignments that minimize resources and maximum science may be possible and will be studied in the Phase A period; we expect to proceed though with the baseline configuration just described.

The expected data rate for the IEPS sensor is approximately 3 kbps. This data is shared between the various instrument components and is commandable by uplinking a new telemetry table. The data are sampled at some high rate, stored in scalars, and then latched and moved into mass memory. The volume of the entire data set (21 look directions - 9 neutral, 9 ion, and 3 electron; each with eight energy bins; each sectored in nominally 32 sectors per spin) is far too large to fit our downlink limit. Therefore, we resample these data as we did on the POLAR/CEPPAD experiment; sampling the data into an 8-bit compressed format and undersampling those elements of the data cube that have fewer expected counts and allocating more telemetry to those parts of the data cube with more expected counts (i.e., at lower energies). For our calculations, we have used two ground contacts with TENACIUS per day and a 160 Mbit download per contact. Data downlink requirements for the IEPS is shown later in Section G.2.

In order to infer neutral atmospheric species and tracer ion densities, needed to deconvolve the ENA emissions, we propose to fly two tomographic EUV spectrographs (TESS). This instrument has already been designed to fly on TERRIERS where it too will measure the same UV emissions. The TERRIERS design is perfectly suited to TENACIUS. Atmospheric emis-sions of [O] (1304 Å), [He] (1168 Å), and [H] (1216 Å) are targeted as well as is [O⁺] (834 Å). Additional important atmospheric emission lines are possible with the TESS instrument and will be prioritized during the Phase A study.

The TESS design offers the best compromise between spatial imaging, spectral resolution, and high sensitivity through the use of a novel spectrograph mount. The system consists of two basically identical spectrographs; one nightglow instrument, and one with a narrower slit to reduce sensitivity and increase spectral resolution for daytime operation. Figure 7 illustrates the optical design, which consists of only two optical elements: a slit assembly and a grating. These instruments are the same as those to be flown on TERRIERS with the same data and telemetry interfaces preserved for TENACIUS.

We chose a Single Element Imaging Spectrograph (SEIS) (Cotton et al., 1994), because its imaging capabilities and large FOV offer the highest possible throughput with high spatial

resolution (0.4° ) in a very small configuration (150 mm Roland circle). The sensitivity of the

night TESS instrument is shown also in Figure 7. Furthermore, this design allowed us to vastly increase the science return by adding a nearly identical dayglow spectrograph for marginal cost.

Each TESS instrument will utilize a microchannel plate (MCP) detector with a wedge and strip readout. This type of detector has been flown many times, 12 times by our own group, including STP78-1, EUVE, UVX, DUVE, BEARS, CUBS and TERRIERS. The mechanical layout of a TESS is shown in Figure 7 while the electronics block diagram is shown in Figure 8.

The requirements on the imaging detector were intentionally minimized in order to simplify the readout and data handling, lower the data rates and power consumption, and increase the reliability. To cover the pass bands at 10 Å resolution, only 256 bins are needed (2.5 bins/resel for the normal incidence spectrograph and 8 bins/resel for the grazing incidence spectrograph), and a 40 mm diameter active area. We expect a maximum (burst) count rate of 20 KHz for the entire bandpass (20-1300 Å). This can be achieved with a microchannel plate (MCP) imaging, photon-counting detector with a wedge and strip readout. We chose this type of detector, because it offers very high signal-to-noise and the dynamic range that is necessary for the expected signal. Furthermore, the technology behind these detectors is very mature. We shall build the detector in house with our extensive experience with these types of detectors. The details of the operational characteristics of the wedge-and-strip readout are well known and life tests have been performed on MCP's in windowless EUV detectors, which show that they continue to operate nominally even after an exposure to 10¹³ counts/cm². This is more than adequate for the TENACIUS mission with the expected count rates mentioned above. (At 20 kHz, the detector can operate continuously for 2.0 years.)

The TESS detector bodies will be constructed of brazed ceramic/metal for ruggedness, simplicity and reliability. This type of construction has been used for the detectors in the ALEXIS satellite, identical to the TERRIERS satellite and in rocket experiments. These programs have demonstrated that the type of design scheme proposed is compatible with space-flight requirements (vibration, thermal, etc.) The microchannel plates will be held in place by an annular spring ring applying an even pressure around the circumference of the MCP stack. A stack of three back to back MCP's is baselined, each having a channel diameter of 12.55 mm, and a channel length to diameter ratio of 80:1. This MCP configuration has been used in 23 spacecraft, 1 shuttle, and 11 rocket experiments, and gives high gain (>2 ´10⁷) with tight pulse height distributions (<50% FWHM) and low background rates (<0.5 events sec^-1 cm^-2).

We intend to deposit a KBr photocathode on the MCPs to enhance the quantum efficiency between 800-1400 Å (about 20%). The two options in this region are KBr and CsI. We chose KBr since it offers better stability and a little higher quantum efficiency.

The wedge and strip position readout anode will be mounted on the signal feedthroughs about 1 cm behind the MCP stack. An appropriate design is the three-element anode which has been used extensively for other space missions. The detector will not be the dominating term in either of the spectrographs resolution budgets. The required resolution performance (90 nm) may be obtained with a standard wedge and strip design. The predicted resolution for a 35 mm diameter wedge and strip pattern on a quartz substrate is 50 nm. We have assumed input referred noise levels of 1000 e^- rms for the wedge and strip, and 1500 e^- rms for the zigzag. The latter noise levels are modest for charge sensitive preamplifiers when loaded with the <200 pF capacitance of this anode design.

We propose to calibrate the detector in flight by periodically taking dayglow spectra. Using the known lines (e.g. 834, 989, 1304, 1356 Å), we can detect and correct for changes in the electronics linearity, gain and DC offsets to the optical resolution of the spectrographs.

2. Mission: Technical Issues and Advantages of TENACIUS (LEO Orbit) for Magnetospheric Imagers

The instruments outlined above to carry out the mission goals all have excellent flight heritage. In some cases, the proposed instrument will need only modest modifications from a present design or combination of designs. Furthermore, the TENACIUS science team has excellent experience with the design, development, delivery, operation, and analysis of the data from these types of sensors.

We note that low Earth orbit is a novel, and in many ways an optimal, location to perform magnetospheric neutral atom imaging. We have noted the advantages previously. One concern is the multiple scattering of ENAs that can occur in the denser regions of the atmosphere. This question has been treated theoretically and shown that altitudes above 850 kilometers are nearly scatter-free. Altitudes at 1000 kilometers or higher are optimal from an ENA point of view, but higher altitudes introduce radiation damage effects to sensor electronics. Soraas and Aarsnes (1996) showed from their rocket data that even at 450 kilometers the effects of scattering were still surprisingly minimal. Indeed, Gruntman (1997) alluded to the fact that problems attributed to multiple scattering have probably been overestimated based on simple theory. Therefore we are targeting an apogee altitude of TENACIUS at 1000 km to be into a single encounter (optically thin) ENA regime. Anything above 850 km would be acceptable so we are actually very insensitive to exact altitude. Our insertion altitude is ~ 470 kilometers, so even in the worst case, the good science achieved by the rocket flight gives us excellent confidence for mission success.

The initial orbital insertion will place TENACIUS into a 10:30-22:30 UT sun-synchronous plane. During our motor burn to raise apogee, some of the Dv will go into a change in inclination. We estimate that an approximately four-degree plane angle change is possible with residual fuel beyond that for the 1000 km apogee raise maneuver. This change in sun-synchronicity will allow the orbit plane to slowly evolve with time at about 0.5 degrees/day. Therefore, we will cover all local times in one year and come back to the same configuration (with the same ascending node) in two years. This situation is desirable as it allows us to measure low-altitude ENAs at all local times. At the same time, we are almost totally insensitive to the rate of orbital motion; mission success is very weakly dependent on it. We will focus initially on substorm ENAs as the 22:30UT local time sector has been observed by POLAR to be the most probable location for ENA generation at substorm onset. As we move through other local time sectors throughout the two-year mission lifetime, different physics as appropriate will be emphasized.

Depending on the operating mode of TENACIUS, some initial processing of the data is performed by the on-board microprocessor. Each instrument may be handled separately: for the particle sensors, energy spectra and pixel maps of the sky are generated; for TESS, line emissions and background rates are selected out to correspond to a given portion of the sky. After these data are telemetered down, they are put through a multistage extraction, editing, and reduction process to produce a level 0 (zero) data product modeled on the TERRIERS. This process includes decommutation (unpacking) of the raw telemetry. A telemetry simulator will be used to verify software and system performance. While the level 0 data will be archived, data analysis will proceed interactively with an interactive level 1 interface and an archived level 2 product (data and software) for general use. These analysis programs will use a commercial data analysis tool, e.g., IDL - interactive display language of RSI, to minimize software maintenance and promote intrateam and extrateam (scientific community) availability over the WWW (as is done for POLAR). The level 2 product (data in geophysical units) includes all known corrections, e.g., dead time and instrument calibration, to the science data as well as engineering, attitude, and orbit data. For long term archiving, the data and software will be archived through a data base management system on optical disks. This final archive is expected to ready with a six-month latency; preliminary versions of the archival data will be available at a much quicker rate.

The TENACIUS Science team will initially consist of those individuals listed in Section H of this proposal. The team will function during Phase B/C/D to refine the scientific objectives of the mission in periodic meetings. Prior to Phase E the team will be expanded to include liaison members from the IMAGE and TWINS science teams. It will be the charter of TENACIUS Science Team to maximize the scientific return of the TENACIUS mission by a close coordination and interaction with the IMAGE and TWINS science teams. Specific background of Science Team members follows.

Theodore A. Fritz has decades of experience in the analysis of energetic charged particles. He is a co-Investigator on the POLAR CEPPAD experiment and has participated in ENA studies from that instrument. He has additional ENA analysis experience from the GALILEO EPD experiment. Finally, he is leading a new study of ENAs measured by the MEPED experiment on TIROS that he helped design. Harlan E. Spence will lead the science team, and has extensive experience with the design, fabrication and analysis of the ENA data from the POLAR/CEPPAD IPS experiment. Robert Sheldon has extensive experience with ring current modelling and knowledge of geocoronal models. He has been a key member of the POLAR CEPPAD and CAMMICE energetic particle teams and has participated in studies of ENA measurements from them. Jim Vickers (TESS Scientist) has great experience designing, building and analyzing data from UV optical sensors. He will lead the analysis of the TESS instrument. James D. Sullivan has years of experience in the design and analysis of energetic particle detectors and has been a key member of the CEPPAD and CAMMICE science teams. Douglas Zook will be a member of the science team and will coordinate the educational outreach activities of the TENACIUS team.