ENERGETIC NEUTRAL ATOM IMAGING BY THE ASTRID MICROSATELLITE
S. Barabash1, P. C:son Brandt1, O. Norberg1, R. Lundin1, E. C. Roelof2, C. J. Chase2, B. H. Mauk2, H. Koskinen3
1Swedish Institute of Space Physics,
Box 812, 98128, Kiruna, Sweden, E-mail: email@example.com
2The Johns Hopkins University / Applied Physics Laboratory, Johns Hopkins Rd., Laurel, MD 20723-6099, USA
3Finnish Meteorological Institute, Box 503, SF-00101, Helsinki, Finland
The microsatellite Astrid carried the first instrument (PIPPI, Prelude in Planetary Particle Imaging) specifically designed to perform energetic neutral atom (ENA) imaging. It made measurements from a low altitude (1000 km) polar orbit in the energy range ~13 - 140 keV. The ENA images, obtained from near-pole vantage points, adequately reflect general morphological features of the ring current such as a global dawn - dusk asymmetry. The detected ENA peak fluxes (500 - 2000 cm-2s-1sr-1keV-1 for 26 - 37 keV ) and structure of the ENA images correlate well with magnetospheric activity throughout the entire data set. The Astrid results demonstrate a considerable potential for ENA imaging from low altitude polar orbits. High ENA fluxes, large angular size of the generation region and simultaneous sampling over all local times are major advantages of such imaging.
Any energetic plasma immersed in a neutral gas will emit energetic neutral atoms (ENAs) generated by the charge - exchange process. ENAs are not affected by electromagnetic fields and propagate essentially rectilinearly, like photons. A direction-responsive neutral particle detector can thus image the emitting region. The main region of the Earth's magnetosphere which is occupied by a hot plasma with a significant neutral background is the ring current. High energy ( > 10 keV) neutral atoms (HENAs) emitted from the ring current have, indeed, been detected by charged particle detectors on several occasions (Hovestadt and Scholer, 1976; Roelof et al., 1985; Voss et al., 1993), but not, as yet, by a specifically designed ENA imager. The most advanced of these early analyses produced the first-ENA image of a storm-time ring current from ISEE 1 data (Roelof, 1987) and can be considered as a proof-of-concept of ENA imaging.
So far, general attention has concentrated on the ENAs generated within the high altitude ring current, because they are supposed to be used in global magnetospheric imaging from high altitude ( > 20000 km) spacecraft. However, in the auroral region where the ring current/radiation belt particles plunge into the dense exosphere/upper atmosphere, the charge-exchange process should be much more effective and the ENA emissions much more intense. Despite that McEntire and Mitchell (1989) mentioned high ENA flux and two dimensional character of ENA emissions in auroral/sub-auroral zone (due to the very quick rise of exospheric densities), up to now ENA imaging from low altitudes and polar latitudes has not been considered in details and advantages of such vantage points have not been realized. Only after the flight of Swedish microsatellite Astrid, has Roelof (1996) developed a basic approach to the analysis of such images.
This report discusses the basic features of ENA imaging from a low altitude (~1000 km) polar orbit and presents the first images obtained by the ENA camera PIPPI (Prelude In Planetary Particle Imaging) onboard the low altitude orbit Swedish microsatellite Astrid.
INSTRUMENTATION AND ORBIT
PIPPI consists of two sensor heads, PIPPI-SSD and PIPPI-MCP to measure ENA in the energy ranges 13 - 140 keV and ~0.1 - 70 keV respectively. In this report we will deal with data from only high energy sensor head. In the PIPPI - SSD charged particles are removed by a two-level, electrostatic deflector, consisting of 4 disks. Neutrals in the range of energy 13 - 140 keV are detected by 14 Si solid state detectors (SSDs), located on two levels in order to increase the geometrical factor of each sensor (Figure 1). The fifteenth sensor looking towards the Sun is blind (the corresponding apertures were not cut) and used to monitor electronic internal noises. Eight plastic spokes between each pair of disks divide the 2 field of view into 8 sectors providing an angular resolution of 2.5° x 25° (FWHM) per pixel. For one 180° turn around an axis lying in the plane of the instrument, a half spacecraft spin, almost the entire 4 space is covered by all sensors. All detectors are sampled simultaneously during 31.25 ms. Each pulse is discriminated in 8 levels to give the energy spectrum. The direction is given by the sensor number. The geometrical factor is 2.5 x 10-3 cm2sr / pixel and the energy resolution 30%. The energy levels were calibrated with the use of electrons. The proton energy bins were recalculated knowing the proton loss in the SSD dead layer. For more details, see Barabash (1995).
Fig. 1. Cross section view of the PIPPI instrument with principal components.
The microsatellite ASTRID (Grahn and Rathsman, 1995) was launched on January 24, 1995 into a 1000 km circular orbit with 83° inclination and 105 min period. The payload was operational during the first 5 weeks of the mission. The data-taking covered from approximately 90° down to 40° magnetic latitude and, mainly, the dusk sector between approximately 15 - 18 hours MLT. The spacecraft was spinning with 3 - 4 s spin period resulting in an elevation pixel of 4°. Since this is close to the inherent angular resolution of the instrument, the absolute accuracy is about one pixel. The absolute spacecraft attitude was derived on the ground from the solar sensor and magnetometer data.
ENA IMAGING FROM THE ASTRID ORBIT
Figure 2 describes the general schematics of ENA imaging from the Astrid orbit in two different projections. Figure 2a shows a typical Astrid orbit in the MLT - SM (solar magnetospheric) latitude coordinates. A range of L - shells between 4 - 8 corresponding to the ring current location is shadowed. A ring current intensification, due to an injection occurring in the near-dusk sector, as shown, causes an increase in the trapped / precipitating ion flux at low altitudes and, hence, an increase in the ENA production. These ENA emissions can be remotely detected from the polar part of the Astrid orbit. The situation in the Astrid orbit plane (Figure 2b) is more interesting. The most intense ENA production takes place within a comparatively narrow layer of altitudes because of the exponential variation of the neutral density with altitude. A vertical size of this layer is about the scale height of the exosphere, i.e., several hundred kms. As pointed out by Roelof (1996) the ENAs generated at lower altitudes may pass through even lower altitudes before they are detected by the instrument. The neutral density along the ENA trajectory may therefore be much higher than in the generation region and the ENA flux attenuation due to electron stripping starts to play a significant role in the ENA absorption region (Figure 2b). This effect decreases the vertical size of the visible ENA generation region. Moreover, as the look direction goes from zenith to low altitudes, the ENA emission region progresses from optically thin to optically thick. This effect decreases even more the thickness and distance from the spacecraft of the ENA generation region. Thus for vantage points near the pole the region of the ENA generation should look like a narrow band encircling the point of observation, from dusk to dawn. If an MLT asymmetry in ion distribution is present as shown in Figure 2a, the band in the ENA image is also asymmetric since the neutral density distribution can be assumed to be azimuthally symmetric to first approximation. Out of the polar part of the Astrid orbits (less than 80° latitude), the visible asymmetry may be caused by the attenuation of ENA fluxes coming from the dawn - midnight sector because these atoms travel at very low altitudes at certain parts of the trajectories (Figure 2b). For some vantage points the asymmetry may result from an anisotropy in the ion pitch-angle distribution (empty loss cone) because a shell with a fixed L may be imaged from different pitch-angles at different MLT .
Fig. 2. General schematics of ENA detection from the Astrid orbit. (a) shows a typical Astrid orbit in the MLT - SM latitude coordinates. A range of L - shells between 4 - 8 is shadowed. (b) presents the Astrid orbit in the altitude - SM latitude projection. The most intense ENA production takes place within a comparatively narrow layer of altitudes.
The reasoning above and numerical simulations (C:son Brandt et al., 1997) give an idea of what the ENA images from the Astrid orbit should look like. We have analyzed all available data and specified the all-sky images with the ENA structure on them. For well-pronounced events the accumulation time was about 1 min although some orbits required accumulation up to 5 min to identify an ENA image. Figure 3 presents the peak count rates over the ENA structures for the energy 26 - 37 keV from the polar part of the Astrid orbit (rhombuses).
Fig. 3. Peak count rates from ENA structures on the all - sky images for the energy 26 - 37 keV from the polar part of the Astrid orbit (rhombuses). The peaks correspond to orbits 85, 114, 209, 291. The Dst index for the same period is also shown by the thin line (courtesy of WDC-C2, Kyoto, Japan).
The number is proportional to the ENA flux and characterises the "brightness" of ENA images. The thin line in Figure 3 gives the Dst index for the same period. We show also the data for the initial phase of the mission when the deflection system had no voltage applied. Energetic charged particle fluxes are low in the polar cap and it is natural to assume that the majority of the counts recorded resulted from ENAs generated in the subauroral / auroral region. Four well-pronounced events with increased count rates can be identified. They are marked by the orbit number. The observed increases in the ENA emissions for orbits 85, 209, 291 correlate well with decreases in the Dst index. The dependence of the total ENA flux generated within the inner magnetosphere on the Dst index is not, however, a simple proportionality because the Dst index characterizes the total energy of particles in the ring current (the Dessler-Parker-Sckopke relation). The bulk (~50%) of the ring current energy is contained in the 50- to 250-keV energy range (Williams, 1987) while the total ENA flux depends rather weakly on this population due to drop of the charge - exchange cross section with increasing energy. However, during storms the ion fluxes over the entire energy range increase and so does the ENA flux, because a dominant loss mechanism of hot plasma during magnetic storm and subsequent recovery is through charge - exchange (Jordanova et al., 1996). Thus, the Dst index may be considered as an indicator of the "ENA activity". The event on orbit 114 seems to be related to the recovery phase of the moderate storm on January 30.
The structure along the Earth's limb with higher intensity at the dusk sector is ENAs generated by trapped/precipitating ions. The general structure of the ENA images is as one expects from geometrical considerations, i.e., a dawn-dusk elongated asymmetrical narrow band at low altitudes. However, there are clear differences in details of the morphology (different thickness, different MLT coverage etc.) which reflect different characteristics of both the ion population and the upper atmosphere/exosphere (Roelof, 1996, C:son Brandt et al. 1996). Note that for the vantage points as high as 85° latitude effects of the obscuring atmosphere do not affect the MLT asymmetry in the source and the global dawn-dusk asymmetry clearly seen through all events corresponds to the ion distribution asymmetry. To come to the same conclusion by means of in situ measurements would take months of observational time, and even then, would only be a statistical result. That demonstrates clearly one of the most powerful features of ENA imaging.
Figure 5 gives fluxes and energies of HENAs averaged over several brightest pixels on images in Figure 4. The observed count rates were well above the one count levels for the specified times which, in turn, were close to the background level given by the fifteenth, blind detector. The shown background resulted from the electronic noise and penetrating radiation. As seen the HENAs were generally detected only in the first three energy channels and a typical HENA energy was less than 50 keV (due to an uncertainty in the lowest threshold the first energy channel ~13 - 26 keV is not shown). This is consistent with the exponential decreasing the H++H and H++O charge - exchange cross sections with increasing energy for the energy range of Figure 5 (see also discussion in C:son Brandt et al. (1997). The fluxes observed in the brightest pixels on every image were 500 - 2000 cm-2s-1sr-1keV-1 for the energy 26 - 37 keV that is very close to the fluxes predicted by McEntire and Mitchell (1989).
Fig. 4. The Astrid ENA images presented in a fish-eye projection. In these polar coordinates, the radius is given by the polar angle of each sensor in the frame related to the spacecraft spin axis, and the azimuth angle is the satellite spin angle. The spacecraft spin axis points approximately towards the Sun. The vantage points for different orbits is given in solar magnetospheric (SM) coordinates (latitude and longitude). The blue curve at the bottom part of the images is the Earth's limb. The line in the limb area is the terminator. A marks the antisunward directions, M the magnetic pole and E the nadir. A structure along the Earth's limb with higher intensity at the dusk sector is ENAs generated by trapped/precipitating particles.
We have presented the first ENA images from the near-Pole vantage points obtained by an ENA imager onboard a low altitude spacecraft. We have also given measured ENA fluxes and energies for four identified events. The ENA activity is observed to increase during a decrease in the Dst index, which indicates a link between ENA generation and magnetospheric activity. From this point of view, we have illustrated qualitatively by real measurements that ENAs are, in fact, a new window on plasmas in the magnetosphere, similar in some extent to the auroral display.
Finally, we would like to emphasize advantages of ENA imaging from low altitude polar orbits. As pointed out by McEntire and Mitchell (1989), due to a higher neutral density the ENA flux from the low-altitude precipitation region are more intense than from the equatorial ring current (L = 3 - 5). This sets less rigid requirements for the sensitivity of an ENA camera because a higher flux provides a better signal-to-noise ration under equal conditions. At the same time, the charged particle background is low over the polar cap and that makes identification of ENAs easier. The ENA generation region is comparatively large and occupies many pixels of an ENA camera even with a crude angular resolution. The ENA camera placed above the pole at low altitude makes an instantaneous image of the ring current ion population over all local times. Purely morphological features (even with no inversion applied) of polar ENA images provide global immediate characterisation of the ring current/radiation belt ion distribution over all local times.
The Swedish microsatellite Astrid as well as the PIPPI ENA experiment was financed by grants from the Swedish National Space Board. The Astrid project was initiated, managed and operated by the Swedish Space Corporation. Support to the Astrid project was also provided by the Finnish Meteorological Institute and the Academy of Finland. An excellent job by the engineering and programming staff at the Swedish Institute of Space Physics in Kiruna and the Finnish Meteorological Institute in Helsinki are gratefully acknowledged. The efforts of E. C. R., C. J. C., and B. H. M. were supported in part by Grants NAGW-2619 and NAGW-4729 from NASA to the Johns Hopkins University.
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