Pages 1061-1066


P. C:son Brandt1, S. Barabash1, O. Norberg1, R. Lundin1, E. C. Roelof2, C. J. Chase2, B. H. Mauk2, M. Thomsen3

1Swedish Institute of Space Physics, Box 812, S-981 28 Kiruna, Sweden, E-mail:
2Applied Physics Laboratory, The Johns Hopkins University, Johns Hopkins Rd., Laurel MD   20723-6099, USA
3Los Alamos National Laboratory, MS D466, Los Alamos, NM87545, USA


The generation of energetic neutral atoms (ENA) through charge exchange processes between hot ( 1 keV) plasma and cold (< 0.1 eV) exospheric neutrals makes possible imaging of hot magnetospheric plasma. Here we report on the ENA observations from a polar 1000 km orbit carried out by PIPPI (Prelude In Planetary Particle Imaging), the ENA imager onboard the Swedish micro-satellite Astrid. We analyse ENA images in the energy range 26-52 keV obtained from the 8 February 1995 during a moderate geomagnetic storm (Dst˜ -80 nT). The equatorial ion distribution is modeled by a 13 parameter model using a three component (H, He and O) Chamberlain exosphere model with parameters from the MSISE90 extended thermospheric model. The equatorial ion distribution, deduced from the ENA-images through forward modeling is located at MLT (magnetic local time) 1900 hours and occupies L-shell 4-6. Very low ENA fluxes were detected at energies >37 keV. It is suggested that the ENAs are produced by precipitating/mirroring ions on auroral/subauroral field lines coming from the near Earth current sheet charge exchanging with exospheric neutrals at near-exobase altitudes (300-400 km).


Global information on the dynamics and morphology of the hot ( 1 keV) singly charged magnetospheric ion population such as the plasma sheet and ring current can be obtained through imaging of ENAs (energetic neutral atoms) produced by charge exchange with cold (< 0.1 eV) exospheric neutrals. Since the ENAs are not affected by magnetic- or electric fields and since their energies are greater than their escape energies (0.7 eV for hydrogen) the ENAs travel in straight paths from the production region to the observer, so that ENA imaging resembles in some extent to optical imaging.

The ENA differential flux produced by charge exchange between one, singly charged ion species and one neutral species can be written


where jION is the ion differential flux, 10 is the charge exchange cross section and nn is the neutral number density. The integral is taken over the line of sight (LOS) from the observer to infinity. For simplicity only single charge exchange process is taken into account in (1), thus, it neglects the fact that ENAs can be lost through stripping (electron loss) once they have been produced.

The first ENA measurements indicating the feasibility of ENA-imaging were made by energetic charged particle detectors onboard the IMP-7/8 and ISEE-1 spacecrafts (Roelof et al., 1985). The equatorial ion distribution was deduced through forward modelling from the first ENA image of the storm-time ring current obtained by the ISEE-1 spacecraft (Roelof, 1987).

Typical ENA fluxes are rather weak due to a small value of charge exchange cross section (approximately 10-16 cm2 for H + H+ at 40 keV) and low neutral density (typically 104 cm-3 for H at 1000 km). However, intense ENA fluxes (103 cm-2s-1 sr-1 keV-1) are expected from high latitudes (50°-70°) and near exobase altitudes (300-400 km) where trapped/precipitating energetic ions meet a relatively high neutral density.

Moreover, one can expect the ENA emission to be enhanced during periods of enhanced ion precipitation, such as storms and substorms. In this study we present the first ENA images during a magnetic storm on the 8 February, 1995 obtained by PIPPI, the first satellite borne instrument dedicated to ENA imaging.


The ENA imager, PIPPI, detects ENA from 13 keV up to 140 keV with eight energy steps. The energy channel width used throughout this analysis is calculated for protons. The intrinsic detector resolution (E/E) was approximately 30%. 14 solid state detectors are placed on a cylinder to view 322° in a two dimensional plane. Thus, as the satellite revolves half a spin PIPPI obtains full sky coverage in approximately 1.5 s with an aperture angular resolution 2.5°x25° (full width at half maximum). The lowest energy channel, 13-26 keV, is excluded from the present analysis due to uncertaintity in the lower energy threshold. Ions and electrons are filtered away by an electrostatic deflection system. For further details see Barabash (1995).


The Swedish micro satellite Astrid was launched from Plezetsk, Russia into a 1000 km altitude orbit with an 83° inclination on January 24, 1995. From now on we will use Astrid orbit numbers instead of date and time. The corresponding dates and times can be found in Table 1.

ENA Observations

Figure 1 (a) shows an ENA image obtained by the satellite at vantage point at SM (solar magnetic) latitude 84° and longitude 64° during a 1 min integration time at 26-37 keV. The image is presented in a so-called spherical fish-eye projection with the centre of the image being the spin axis direction. In this type of projection the entire sphere is projected down to a plane. The ellipse represents the Earth limb and the line inside the ellipse represents the terminator. The "A" (on the left) marks the anti-sunward direction and the "S" in the centre marks the solar direction. The magnetic north direction and nadir is marked by "M" and "E", respectively. The dusk sector is on the left side and the dawn sector is on the right. The shaded area is UV contamination from the sunlight and its reflection off the dayside of the Earth.

Fig. 1.

ENA fluxes are seen on the dusk side of the image. It will be shown through simulation that these fluxes are neutral atoms due to precipitating/mirroring ions charge exchanging with the neutral exosphere.

ENA Flux and Magnetospheric Conditions

The magnetosphere encountered a large magnetic cloud on the 8 February, 1995, as has been observed by the WIND spacecraft (R. Lepping, Private communication). This resulted in a geomagnetic storm with an enhanced ring current. In Figure 2 (a) we show the observed peak differential ENA flux, also listed in table 1, in the energy range 26 to 37 keV. The one count level is shown as dashed line. Figure 2 (b) shows the one hour averaged Dst index and 2 (c) the 3 hour averaged Kp index.

Lastly, Figure 2 (d) shows the z-component of the interplanetary magnetic field in geocentric solar ecliptic coordinates (IMF-Bz) obtained by the magnetic field instrument on WIND. Between orbit 165 and 195 the peak differential flux of observed ENAs was comparatively faint but stable around 200 (cm2 s sr keV)-1 as were the magnetospheric conditions (see also Table 1). On the same day as orbit 208 and 209 Dst decreased from -48 nT at 0600 UT to -77 nT at 1000 UT hours what can be identified as the main phase of the storm. During orbit 208 a peak differential flux of about 1340±280 (cm2 s sr keV)-1 was detected. Poisson counting statistics has been used to define the error in the ENA peak flux. One orbit later (145 min) the particle flux was 1460±290 (cm2 s sr keV)-1, consistent with the ENA emissions being comparatively stable during the intervening 145 minutes. However, in the next available data set from orbit 224 no ENA fluxes above the one-count-flux-level were detected from approximately the same vantage point and the Dst index was back at the same level as during orbit 195, i.e. a recovery phase of approximately 20 hours. Kp was enhanced from 1 to 4+ on 8 February (orbit 208 to 209) and then decreased to zero the following day (orbit 224). IMF-Bz was fluctuating around 0 nT most of the time on 7 Feb.


Table 1. Astrid orbit number and times, ENA peak differential flux. Integration time is 1 min for all orbits. Note that ENA fluxes before and after the 8 Feb are small. Zero values are set when the signal to noise ratio was less than 0.5.


Date, UT


Flux @ 26-37 keV (cm2 s sr keV)-1


5 Feb 0830




5 Feb 1020




6 Feb 1240




7 Feb 1136




7 Feb 1250




8 Feb 1153




8 Feb 1338




9 Feb 1556




The IMF-Bz was stable around -10 nT for 6 hours on the 8 February, then it started steadily turning northward marking the beginning of the recovery phase as seen in Figure 2 (b) and (c). The observed increase in ENA fluxes also correlates well with the ion measurements at geosynchronous orbit. The magnetospheric plasma analyser onboard the geosynchronous satellite LANL (Los Alamos National Labortory) 1989-046 observed a one-order-of-magnitude-increase on the 8 February in 4.7 and 5.7 keV ion fluxes from 3x104 at 0000 UT to 3x105 (cm2 s sr keV)-1 at 0400 UT and MLT 1700 h.

The ion flux stayed at 2-3x105 (cm2 s sr keV)-1 until 1500 UT and MLT 0400 h, i.e. throughout orbit 208 and 209. The above observations show that there was a geomagnetic storm with a ion injection into the inner magnetosphere with a subsequent ring current intensification. During such conditions one would expect increased levels of dusk- nightside ion precipitation from the near Earth plasma sheet and/or the ring current.


One of the purposes of a spectral analysis of ENAs is to derive the spectral slope of the parent ion population. Despite that the PIPPI energy resolution was very crude it is still possible to justify probable ion populations by deriving the ion spectral slope. The energy spectra were sampled with eight energy steps from 13 to 140 keV. ENAs were detected only in the three first energy channels, i.e. up to 52 keV, with almost no counts in the third energy channel, 37-52 keV. The first energy channel, 13-26 keV, has been excluded from this spectral analysis due to uncertaintity in the lower threshold, thus, only channel 2, 26-37 keV, and 3, 37-52 keV, is used in this analysis. The spectra were obtained from the brightest pixel in the ENA image at 26-37 keV. Each image was obtained through 1 min accumalations as late in each passage as possible, so that the LOS from the satellite to the brightest ENA-emitting region did not cut too deep in the atmosphere, and, thus, minimizing the effects of stripping of the ENAs by the atmosphere. Almost no ENAs were detected in the third energy channel (37-52 keV), i.e. the ENA spectral slope is very steep.

The spectral slope of any particle population is defined as -ln(j)/ln(E), where j is the particle flux and E is particle energy. With this definition we get an ENA spectral slope of 7±4 at 38 keV (the center energy between PIPPI energy channel two and third) for the pixel displaying maximum count rate in the ENA image. One obvious candidate parent ion population is the proton ring current with an almost flat ion spectral slope at L=4-5 at MLT=2300 hours at 38 keV (Kistler et al., 1989) during the main and recovery phase of a magnetic storm. A second obvious candidate is the near Earth equatorial plasma/current sheet, with a proton spectral slope ranging from 3-12 at 38 keV(Christon et al., 1991 and Ipavich and Scholer, 1983) during geomagnetically active periods defined as AE100 nT.

By differentiating Eq. 1 the ENA spectral slope can be written, to a first approximation,


Since the charge exchange cross section decreases with energy for all ionic and exospheric species in question (H+, He+ and O+ on exospheric H, He and O) the ENA spectral slope is always greater than the parent ion spectral slope. A full derivation of the ion spectral slope from the observed ENA spectral slope requires therefore knowledge about exospheric and ionic composition. Assuming protons charge exchanging with exospheric oxygen (justified by simulations. See next section), producing hydrogen ENA, gives an ion slope of approximately 6, which is within the range of the spectral slope of the near Earth plasma sheet. Two essential questions are not resolved at this stage. First, is the plasma sheet spectral slope mentioned above a good represention of that of the actual ion population charge exchanging with the exosphere at low altitudes (< 500 km) as observed? And second, how is the ENA spectral slope modified if ENAs are stripped by the upper atmosphere? Since the charge exchange cross section decreases with energy high energy ions will penetrate deeper into the lower exosphere than ions with lower energy and same pitch angle (Roelof, 1996). Thus, high energy ENA will be produced deeper in the lower exosphere and, hence, be more effected by stripping on their way to the detector as they cut deeper into the lower exosphere. Now, the stripping cross section increases with energy for H+ on exospheric O (Allison, 1958), which is dominant near the exobase (300-400 km). The effect on the ENA slope is therefore additional steepening. This implies that ENA stripping may also explain the observed steep slope, but up to this point no quantative estimations of its effect on the ENA spectrum have been made. The derived ion spectral slope implies near Earth plasma sheet rather than ring current to be the parent ion population, but not until the above questions have been answered can quantitative conclusions regarding the origin and spectral characteristsics of the parent ion population be made.


The simulation (Roelof and Chase, 1995) involves two steps: (1) modelling of the equatorial ion distribution and adiabatic mapping along given L-shells, (2) computation of the path integral giving the ENA flux including attenuation of the ion flux due to charge exchange and electron stripping of the ENA along the LOS in a three component (H, He and O) exosphere. The equatorial ion distribution is modelled by 13 parameters. Five are used to model the L-shell distribution, four models the local time distribution and three are used to model the pitch angle distribution. The last one sets the altitude of deepest penetration below which all ions are assumed to be absorbed by the atmosphere. This parameter then determines the equatorial loss-cone angle. Only five of the above 13 parameters have been used as free parameters. The first and second is the inner and outer L-shell of the injected plasma, the third and fourth is the local time position and gradient in terms of a harmonic function and the fifth is the altitude of deepest penetration. The rest has been set to presumed values corresponding to isotropic pitch angle distribution with empty loss cone, fixed gradients in the radial distribution (L-shell) and no second harmonic local time dependence has been used. The exospheric 3 component (H, He, O) model parameters are scale heights, exobase height and exobase densities. The scale heights for H, He and O were 936, 234 and 58 km and the exobase densities 9.75x104, 4.37x105 and 5.47x106 cm-3, respectively. The exobase was set to 300 km altitude. Exospheric parameters were derived from runs of the MSISE90 (Hedin, 1986) for MLT 2100 hours and 70° geographic latitude using geomagnetic and solar indices for 1200 UT 8 February, 1995.


Figure 1 (c) shows the modelled equatorial ion distribution corresponding to the simulated ENA image. The ion distribution is located at approximately L=4-8 in a 1-hour narrow interval (FWHM) centred around MLT 1900 h and the intensity is 2x106 (cm2 sr s keV)-1. The pitch angle distribution is assumed to be isotropic with an empty loss cone consistent with the observations of isotropic nightside ion precipitation studied by Seergev et al. (1983). The brightest emissions in the simulated ENA image (Figure 1 (c)) are located on auroral/subauroral L-shells (L=4-8), around dusk and at an altitude of approximately 300-400 km, which means that the brightest ENA emissions are produced near the exobase. The significantly thin layer of ENA emissions could not be reproduced with an exospheric model without oxygen suggesting that the thin layer is an effect of the much smaller scale height and, near exobase altitudes (300-400 km), higher density of oxygen.


The morphology of the ENA emissions implies that they are produced by precipitating and/or mirroring ions charge exchanging with the exosphere towards dusk on auroral/subauroral L-shells. The fact that little or no significant ENA fluxes were detected in the third energy channel (37-52 keV) may be an effect of, (1), the parent ion spectrum is very steep (ION 6) (See e.g. Ejiri et al. (1980) for the so-called "nose-events"), and/or (2), the ENAs are effected by the stripping (electron loss) in the lower exosphere (< 500 km). This issue is upto date still unresolved.

Precipitating/mirroring ions on auroral/subauroral L-shells in the dusk region have been frequently observed by many low altitude satellites, e.g. by the DMSP satellites (Newell et al. (1996)) or the NOAA-12 satellite (D. Evans, private communication). These ions are believed to map to the near Earth current sheet/plasma sheet. In order for the precipitating ions to be a good representation with conserved energy spectral characteristics from the near Earth currents sheet one has to assume that the ion energy spectra is not pitch angle dependent, i.e. isotropic pitch angle distribution. This is suggested to indeed be the case by Galperin et al. (1978) and pitch angle scattering of the near Earth plasma sheet ions has been proposed as the dominant source mechanism of these ion distributions by e.g. Sergeev et al. (1983). Thus the equatorial ion distribution shown in Figure 1 (c), given by forward modelling as discussed above may indeed be a good representation of the true equatorial ion distribution of the near Earth current sheet.


We have analysed ENA images obtained during a magnetic storm on the 8 February, 1995, by an ENA imager at 1000 km altitude. ENA emissions were observed to come from auroral/subauroral L-shells in the dusk region in the energy range 13-52 keV. The peak ENA flux correlated well with the decrease in Dst. Simulation and forward modelling gave a matching ENA image for an equatorial ion distribution at L=4-8 in a 1-hour MLT interval around MLT 1900 h. We suggest that the most intense ENA emissions are produced by the precipitating/mirroring ions in the dusk region on auroral/subauroral field lines charge exchanging with the oxygen exosphere near the exobase (300-400 km).


The Swedish micro satellite 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. An excellent job of the engineering and programming staff at the Swedish Institute of Space Physics in Kiruna and the Finnish Meteorological Institute in Helsinki are gratefully acknowledged. Thanks to the processing team at NASA/GSFC for providing WIND magnetic field key parameter data. 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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