Satellite observations and instrumentation for measuring energetic neutral atoms
Henry D. Voss, Joseph Mobilia, Henry L. Collin, William L. Imhof
Lockheed Palo Alto Research Laboratory
Space Sciences Laboratory
3251 Hanover Street
Palo Alto, California 94304
Abstract. Direct measurements of energetic neutral atoms (ENA) and ions have been obtained
with the cooled solid state detectors on the low-altitude (220 km) three-axis stabilized S81-1/
stimulated emissions of energetic particles (SEEP) satellite and on the spinning 400 km x 5.5
Re (where Re is Earth radii) Combined Release and Radiation Effects Satellite
(CRRES). During magnetic storms ENA and ion precipitation (E>10 keV) are evident over the low-
altitude equatorial region based on data from the SEEP (ONR 804) spectrometers and CRRES ion mass
spectrometer (IMS-HI) (ONR 307-8-3) ion composition and ENA instrument. The IMS-HI neutral atom
spectrometer covers the energy range from 20 to 1500 keV with a geometrical factor of 10-3
cm2 sr and uses a 7-kG magnetic field to screen out protons less than about 50 MeV.
During the strong magnetic storm of 24 March 1991 the first ENA and ion mass composition
measurements were obtained of ring current particles below the inner belt and these fluxes are
compared to the IMS-HI flux measurements in the ring current. Recently, an advanced spectrometer,
the Source/Loss-cone Energetic Particle Spectrometer (SEPS), has been developed to image electrons,
ions, and neutrals on the despun platform of the POLAR satellite (~ 1.8 x 9 Re) for
launch in the mid 1990s as part of NASA's International Solar Terrestrial Physics/Global Geospace
Science (ISTP/GGS) program. To improve particle imaging by increasing sensor spatial resolution and
sensitive area, a 256-element solid state pixel array having 6.25 cm2 area has been
developed for SEPS along with three new low-power application-specific integrated circuit (ASIC)
Subject terms: magnetospheric imagery; atmospheric remote sensing; ring current; energetic neutral
atoms; position-sensitive Si detectors.
Optical Engineering 32(12), 3083-3089 (December 1993).
Paper MI-001 received May 28, 1993; revised manuscript received July 2, 1993; accepted for
publication July 21, 1993. This paper is a revision of a paper presented at the SPIE conference on
Instrumentation for Magnetospheric Imagery, July 1992, San Diego, Calif. The paper presented there
appears (unrefereed) in SPIE Proceedings Vol. 1744.
© 1993 Society of Photo-Optical Instrumentation Engineers. 0091-3286/93/$6.
Energetic neutral atoms are an important tracer of energetic ion collisions with neutrals in solar
and planetary plasmas.1 Global scale images of the Earth's ring current (2 < L < 4) may
be remotely sensed from either above the Earth's radiation belt2 or from below the
radiation belt at low altitude.3 Neutral atoms can therefore provide a mapping of the
internal composition, spatial geometry, and temporal changes of the ring current, weighted by the
appropriate cross sections and neutral hydrogen density; it provides a powerful method of studying
The source of energetic neutral atoms (ENA) is believed to be a double-charge-exchange process
of ions originating in the ring current,4,5 as illustrated in
Fig. 1. ENA has been
observed by Roelof et al.6 using the Interplanetary Monitoring Platform (IMP) 7/8 and
the International Sun-Earth Exploror (ISEE) 1 data. New ENA and ion composition measurements are
described in this paper using the Combined Release and Radiation Effect Satellite (CRRES) ion mass
spectrometer (IMS-HI) instrument. The trapped ions of the ring current, by charge exchange with
thermal hydrogen atoms of the geocorona, become high-velocity neutral atoms that are focused, for
those directed toward the Earth, in the equatorial atmosphere where they again become ions by
ionization collisions. The injected energetic neutrals at low altitudes produce the equatorial
precipitation zone7 and a temporary low-altitude ion belt between 200 and 1000 km, as
2 Ring Current Neutrals at Low Altitude
The low-altitude ion belt cannot be the result of stably trapped charged particles because the ion
lifetime is much less than the drift period. The low-altitude equatorial ion zone has been observed
using satellite measurements by Hovestadt et al.,8 Moritz,4 Mizera and Blake,
9 Butenko et al.,10 Scholer et al.,11 Meier and Weller,12
Voss et al.,13 and Miah et al.14 The zone typically covers the region
of ±20 deg in latitude from the geomagnetic equator.
Low-altitude rocket15 and satellite measurements4 at the equator indicate
that the ions are concentrated at 90-deg pitch angle with an altitude profile that increases
rapidly in intensity between 180 and 260 km and becomes nearly independent of altitude thereafter.
Because the loss process for ions increases with atmospheric density, the source must also increase
with atmospheric density to maintain a constant flux profile between 300 and 1000 km. The inferred
source mechanism is the charge exchange process of ring current ions, which is illustrated in
The low-altitude energy spectrum is similar to that of the ring current, with most of the
particles having energies between 10 < E < 100 keV and with the spectrum extending above 1 MeV.
The precipitation rate is of the order of 10-3 ergs cm-2 s-1 for
disturbed magnetic conditions. The low-altitude ion zone is also important as a background
environment for operating energetic particle or x-ray spectrometers.
3 S81-1/SEEP Satellite Observations
From May to December 1982, the stimulated emissions of energetic particles (SEEP) payload on the
three-axis stabilized S81-1 polar satellite made high-sensitivity measurements of precipitating
energetic electrons and ions in the altitude region 170 to 270 km. The payload included an array
of passively cooled solid state detectors16 covering the energy range from 2 to 1000 keV
in 256 channels. The sensors had a geometrical factor of 0.17 cm2 sr and a field of view
of ±20 deg.
The SEEP data during the 13 to 14 July magnetic storm13 are shown in
Fig. 2 and
Fig. 3. The
prominent equatorial zone is observed between geomagnetic latitudes of ±20 deg and is stable
and repeatable in form during each equatorial pass. The twin peaks in the ion flux (i.e., maxima
displaced ±10 deg in latitude about the geomagnetic equator) are consistent with the north and
south sidelobes of the temporary ion radiation belt
(see Fig. 1).
The temporal variations of ions in the equatorial zone are shown in the top panel of
These fluxes are expected to vary in relation to the fluxes in the ring current. The magnetic
indices Kp and Dst are shown in the lower panel. Near 01:00 hours universal time (UT) on 14 July
1982, the storm reached a maximum (Dst=-325) and the equatorial zone ion flux was observed to
simultaneously increase by a factor of about 500 over prestorm levels. The energetic equatorial
neutrals and ions, however, remained high (~103 cm-2 s-1) after the
storm and confirm the development of a low-latitude (L~1.14) ion radiation belt during the storm's
main phase that decays relatively slowly in time producing the equatorial zone ionization. The
similarity of the day and night intensity and slow temporal variation of energetic particles in the
equatorial zone indicate the global extent of the ion precipitation.
4 CRRES lon Mass and Neutral Atom Spectrometer
The contiguous mapping of the particle distribution by the Combined Release and Radiation Effects
Satellite (CRRES) over the radial distance range from 400 km to 5.5 Re near the
equatorial plane provides a comprehensive database that can be used for studies of the radiation
belt and ring current. The primary objective of the medium energy ion mass spectrometer (IMS-HI)
on CRRES was to obtain the necessary data to construct models of the energetic ion (10 to 2000 keV
amu/q2) and neutral atom (20 to 1500 keV) environment of the Earth's radiation belts. The
spectrometer17 measured the energetic ion composition, energy spectrum, charge, and
pitch angle distribution with good mass, temporal, and spatial resolution. Additionally,
simultaneous mass and energy analysis was obtained with geometrical factors between 10-3
and 10-2 cm2 sr.
The instrument principle of operation is illustrated in
and is based on ion momentum
separation in a 7-kG magnetic field followed by energy and mass defect analysis using an array of
cooled (-50o C) silicon solid state detectors. The entrance collimator consists of a
series of rectangular baffles that define the ion and neutral beam angular resolution and a broom
magnet to reject electrons with energy less than 1 MeV. A seventh sensor, located directly in line
with the collimator, measures energetic neutrals and has an ion rejection of approximately 50 MeV
amu/q2. In principal the spin of the satellite and orbit motion can be used to raster
scan a portion of the magnetosphere to obtain a neutral atom image. In practice, however, the
relatively low neutral atom flux (103 cm2 s-1 sr-1), the
IMS-HT ENA small geometric factor (10-3 cm2 sr), and the background of the
radiation belts make this a difficult task from CRRES.
Near perigee (<1000 km), when CRRES is below the inner radiation belt, the IMS-HI instrument is
able to directly view the ring current neutrals and ion composition of the low-altitude ion belt.
Above the inner belt, the IMS-HI instrument is able to make cross-sectional cuts of the ring
current ion composition. About 1 day after the large magnetic storm of 24 March 1991 the ring
current ions were observed to increase and move to lower altitudes. The H+ flux profiles
of the ring current are shown in
The ring current flux at this time extends down to L~2.0
and is dominated by hydrogen at E~60 keV. Energetic O+, O++, and
He+ are one to two orders of magnitude less in flux at these energies from the IMS-HI
The ENA transported to low altitude from the ring current is again charge exchanged below 1000
km and temporarily becomes the low-altitude ion belt. Because the ions cannot support drift motion
in much of the belt the loss rate is high. The steady-state ion population to first order is a
mapping of the source composition multiplied by the appropriate lifetime with respect to the
atmospheric loss rates. A mass spectrogram of the low-altitude ion belt for detector 2 of IMS-HI
is shown for the first time in
Fig. 6 for 25 March 1992.
Also shown for comparison is the mass
spectrogram of detector 2 near the inside edge of the ring current at L~2.2. At this time the
prominent ion, inferred from the ring current ENA is H+ at 60 keV. The average
low-altitude hydrogen count rate is about 2000 times lower than the average count rate in the ring
current for the IMS-HI spectrometer. The ion flux is observed to be altitude independent between
400 and 1000 km.
The ENA spectrum
was obtained directly with the IMS-HI neutral detector over the 400- to 900-km altitude region.
No charged particle background was evident in the data based on
pitch-angle plots. The integral ENA flux above 40 keV at 600 km is about 500 neutrals
cm-2 s-1 during this storm event. Because the neutral count rates were of
the order of 1 count per second for IMS-HI detector 7, the imaging of the ring current is very flux
limited. Future detectors must have a much larger geometric factor.
5 SEPS Energetic Particle imager
The Source/Loss-cone Energetic Particle Spectrometer (SEPS) is to be flown in the mid 1990s on the
POLAR Satellite in the National Aeronautics and Space Administration (NASA) International Solar
Terrestrial Physics (ISTP) program. Because SEPS is mounted on the POLAR spacecraft despun platform
and is capable of imaging particles over a 24- x 48-deg field of view in the nadir and zenith
directions the particle "source cone" and "loss cone" can be monitored continuously and
simultaneously. In addition, over the polar caps where charged particle backgrounds are relatively
low, SEPS will be able to image ENA.
The ion/neutral imaging telescope on SEPS has 128 XY pixel elements over a 6.25 cm2 area
(200 micrometers deep) solid state detector. A strong 2-kG broom magnet rejects electrons and low
energy protons in front of the entrance pinhole of the SEPS pinhole camera. A SiLi detector behind
the XY detector is used for anticoincidence. The instrument is designed to cover the proton energy
range from 30 keV to 10 MeV.
In addition to the SEPS ion/neutral imaging telescope, the electron telescope of SEPS can image
ions and neutrals above 300 keV in 256 pixels over 12.5 cm2 using the telescope
coincidence electronics. A system diagram of this telescope is shown in
Fig. 8. A small
motor-driven iris wheel is used to change the aperture size by up to two orders of magnitude on
command, thereby optimizing the instrument's geometrical factor (10-3 to 0.1
cm2 sr) and associated spatial resolution.
With the new SEPS capability, electron, ion, and limited neutral particle images are expected
with 100% duty cycle. The SEPS instrument mass is 3.4 kg, the power consumption 3.5 W, and the
telemetry rate is 1.2 kbits/s. The SEPS advanced imaging particle spectrometer design pioneers some
of the new technologies for mixed mode microcircuits and two-dimensional position sensitive
detectors. These are described in the next section.
6 Advanced Microcircuit Technology for Imaging Spectrometers
State-of-the-art sensors and electronics are required on future instrumentation to perform
energetic particle imaging and spectroscopy within spacecraft resources. Neutral atom imaging
requires large geometrical factor sensors with multi-pixels and good background rejection.
Background rejection can be improved by using anticoincidence detectors (as in SEPS), electric
and magnetic field deflectors in the optical path, coincidence time-of-flight techniques, and
coded aperture devices to improve SNR.
A recent review of instrumentation techniques for the remote sensing of space plasmas in the
heliosphere via energetic neutral atoms was presented by Hsieh et al.1
Much of the dproposed new instrumentation is based on rather sophisticated sensor electronics
demands. To simplify instrument development and greatly reduce instrument spacecraft resource
requirements the development of advanced microcircuits, solid state detectors, and microchannel
plate sensors has been pursued.
Figure 9 illustrates the key
modules which have been developed as part of the NASA ISTP/GGS SEPS program.
To minimize capacitance (i.e., low-energy threshold) and crosstalk a 6.25 cm2 x 200
micrometers thick solid state sensor was developed with Hamamatsu Inc. that has 256 pixels with
(Fig. 9). The proton dead zone for this
device was found to be 8 keV. The
energy resolution at room temperature is <2 keV full width at half maximum (FWHM).
Thousands of sensor elements require microcircuit technology to amplify and process the data. To
achieve this an advanced complementary metal-oxide semiconductor (CMOS) mixed mode (combined
analog and digital) gate array microcircuit was developed18 that is radiation hard.
It is called the KRAD gate array microcircuit. A gate array microcircuit consists of a matrix of
transistors and passive elements that are interconnected with only one custom metalization layer
to make an application-specific integrated circuit (ASIC). Because the matrix is mass produced and
well characterized (semicustom), the risk, cost, and turnaround time are significantly reduced as
compared to a fully custom microcircuit. The arrays are highly reliable because they significantly
reduce the number of interconnections and discrete parts while permitting a parallel system
architecture. The electronic weight, power, and size are significantly reduced with a semicustom
array. After initial design. circuit moditications require about a 3-week turnaround.
The first circuit described here on the radiation-hard array is a 16 channel charge amplifier chip
that features (1) 16 preamps and shaping gain stages, (2) 17 pulse peak detection circuits, (3) 32
individual comparators with digital-to-analog (D/A) control, (4) coincidence and anticoincidence
logic, and (5) microprocessor interface. The input preamplifier is designed to work with a solid
state detector or a microchannel plate in a dc coupled mode. The input tield effect transistor
(FET) is a p-channel device with a width-to-length ratio of 4000 to 4. The feedback capacitance on
the folded cascode charge amplifier is 1.0 pF. Each shaping amplifier string is made up of three
operational amplifiers that have a 0.5 microsecond time constant.
The second circuit developed is a pulse height analyzer (PHA) chip that can accumulate counts in an
external 8K x 8 random access memory (RAM) according to energy and position. A 16-channel stacked
discriminator is used as a flash analog-to-digital converter for log or linear analysis over a
programmable pulse height range with 256 levels. Four pixel identification bits, two mode bits,
and two front-end ID bits are used to route each count to memory. The mode bits are set by the
front-end coincidence logic to specify particle type and energy range. As the system microprocessor
reads the accumulated spectra, a zero is automatically written into each word. Eight-bit D/A
converters control the upper and lower limits of the stacked discriminator. The PHA microchip also
includes eight 17-bit counters that use a common jam signal to transfer their contents to the output
buffers while automatically clearing the scaler counters. The power required for the PHA chip is
only 15 mW.
The third chip features the necessary input/output (I/O) functions to interface with the spacecraft
and the onboard microprocessor. It can serve as the basis of a data processing unit (DPU). Some of
the features of the I/O microcircuit include (l)8-bit D/A converter with a calibrate amplitude
pulse output, (2) 8-bit A/D converter with eight multiplexed inputs, (3) vectored interrupt
controller, (4) 16-bit counter timer, (5) address decoding, (6) dual 8-bit parallel port, (7) serial
port, and (8) microprocessor clock generator processing. The power required for the I/O microchip
is also 15 mW using a 4-MHz clock circuit.
These three modular and programmable KRXD microchips have been found to significantly reduce
instrument development and cost, improve performance, and reduce spacecraft resources by one to two
orders of magnitude compared to conventional designs and are essential for imaging systems
that require many parallel circuits. These most recent combined analog and digital microcircuits
have approximately 60 operational amplifiers that can be used as low-noise amplifiers, filters,
A/D and D/A converters, sample/holdcircuits, or voltage references. About 55% of the array is
available for digital circuit functions.
Global geospace cannot be effectively observed using only multiple in situ measurements. The
dynamics of the ring current are only partially understood.19 The ability to
remotely sense ENA may significantly improve our understanding of the large-scale dynamics of
planetary magnetospheres. Several rocket and many satellite measurements have indicated the
credibility of ENA although the ENA fluxes are low. The recent CRRES IMS-HI data has now made it
possible to compare the ion composition of the ring current with the energetic neutrals and the
resulting ion composition at low altitudes.
Future particle-imaging instrumentation must be suitably designed to maintain a large geometrical
factor and adequate position resolution with a high background rejection capability. Although the
NASA/SEPS instrument geometrical factor is small (0.1 to 10-3 cm2 sr per
detector), its ability to stare at the ring current when over the polar cap for several hours using
the POLAR satellite despun platform will improve the SNR. The implementation of several new
technologies using two-dimensional position sensitive Si detectors and analog/digital microchips
makes ENA imaging practical so that it is an attractive diagnostic tool for mapping the global
The work at Lockheed was supported in part by the Office of Naval Research under contract
N00014-83-C-0476 and by the Lockheed Independent Research Program. The SEPS development and
microcircuits were supported by the NASA ISTP/GGS program. We thank all of our colleagues at the
Lockheed Palo Alto Research Laboratory (LPARL) who have contributed in the development of
instrumentation on the S81-l/SEEP, CRRES, and POLAR satellites. In particular we thank Dr. J. R.
Kilner and Mr. R. A. Baraze for their efforts in the development of the SEPS instrument and ASIC
1. K. C. Hsieh, C. C. Curtis, C. Y. Fan, and M. A. Gruntman. "Techniques for the remote sensing of
space plasma in the heliosphere via energetic neutral aloms: a review," in Solar Wind Seven,
E. Marsch and R. Schwenn, Eds.. p. 357 (1992).
2. E. C. Roelof, "Energetic neutral atom image of a storm-time ring current," Geophys. Res.
Lett. 14, 652-655 (1987).
3. H. D. Voss, W. L. Imhof. and J. Mobilia, "Satellite observations of energetic ions and neutrals
in the equatorial precipitation zone," in Proc. Yosemite '84 Planetary Plasma Environments: A
Comparative View, C. R. Clauer and J. H. Waite. Jr.. Eds., pp. 9.5-96. American Geophysical
4. J. Moritz, "Energetic protons at low altitudes: a newly discovered radiation belt phenomenon
and its explanation." Z. Geophys. 38, 701 (1972).
5. B. A. Tinsley, ''Neutral atom precipitation - a review,'' J. Atmos. Terr. Phys. 43,
6. E. C. Roelof, D. G. Mitchell, and D. J. Williams, "Energetic neutral atoms (e~50 keV) form
the ring current: IMP 7/8 and ISEE 1," J. Gephys. Res. 90, 10,991-11,007 (1985).
7. H. D. Voss and L. G. Smith, "Global zones of energetic particle precipitation," J. Atmos.
Terr. Phys. 42, 227-239 (1980).
8. D. B. Hovestadt. B. Hausler, and M. Scholer, 'Observations of energetic particles at very low
altitudes near the geomagnetic equator," Phys. Rev. Lett. 28, 1340-1344 (1972).
9. P. F. Mizera and J. B. Blake, "Observations of ring current protons at low altitudes." J.
Geophys. Res. 78, 1058-1062 (1973).
10. V. D. Butenko. O. R. Grigoryan, S. N. Kuznetsov, G. S. Malkiel, and V. G. Stolpvskii. "Proton
currents with Ep>70 keV at low altitudes in the equatorial region." Kosmecheski
Issledovaniya 13, 508-512 (1975).
11. M. Scholer, D. Hovestadt, and G. Mortill, "Energetic He+ ions from the radiation
belt at low altitudes near the geomagnetic equator," J. Geophys. Res. 80, 80-85 (1975).
12. R. R. Meier and C. S. Weller, "Observations of equatorial EUV bands: evidence for low-altitude
precipitation of ring current helium," J. Geophys. Res. 80, 2813-2818 (1975).
13. H. D. Voss. W. L. Imhof, J. Mobilia, E. E. Gaines, and J. B. Reagan. "Energetic particles in
the nighttime middle and low ]atitude ionosphere," Adv. Space Res. 5(4), 175-178 (1984).
14. M. A. Miah, K. Nagata, T. Kohno. H. Murakami, A. Nakamoto, N. Hasebe, J. Kikuchi, and T. Doke,
"Spatial and temporal features of 0.64-35 MeV protons in the space station environment: EXOS-C
observations," J. Geomag. Geoelectr. 44, 591 (1992).
15. H. D. Voss and L. G. Smith. "Rocket observations of energetic ions in the nighttime equatorial
precipitation zone," COSPAR Adv. Space Explor. 8, 131-134 (1979).
16. H. D. Voss, J. B. Reagan, W. L. Imhof, D. O. Murray, D. A. Simpson, D. P. Cauffman, and
J. C. Bakke, "Low temperature characteristics of solid state detectors for energetic x-ray, ion
and electron spectrometers," IEEE Trans. Nucl. Sci. NS-29, 164-168 (1982).
17. H. D. Voss, E. Hertzberg, A. G. Ghielmetri, S. J. Battel, K. A. Appert, B. R. Higgins,
D. O. Murray, and R. R. Vondrak, "The medium energy ion mass and neutral atom spectrometer
(ONR-307-8-3),'' J. Spacecraft Rockets 29(4), 566-569 (1992).
18. H. D. Voss, J. R. Kilner, R. A. Baraze, J. Mobilia, R. B. Kash, A. J. Goodwater, and E. Kwok,
"Analog/digital microcircuits for spaceflight applications," in NASA Small Instrument Workshop,
in press (1993).
19. D. J. Williams, "Dynamics of the eanh's ring current: theory and observations," Space Sci.
Rev. 42, 375 (1985).
Henry D. Voss received the BS degree in 1972 from the Illinois Institute of Technology in
electrical engineering. He received the MS and PhD degrees in 1974 and 1977, respectively, from
the University of Illinois in electrical engineering. Since 1979, Dr. Voss has been with the
Lockheed Space Sciences Laboratory in Palo Alto, California. He has been coinvestigator on eight
instrument payloads that have studied the charged particle environment. Dr. Voss is currently
the principle investigator for the Source/Loss-cone Energetic Particle Spectrometer (SEPS) of the
CEPPAD experiment for flight on the National Aeronautics and Space Administration/lnternational
Solar Terrestrial Physics (NASA/ISTP) POLAR satellite mission. His current areas of scientific
interest are the study of energetic electrons and ions in the magnetosphere plasma, the use of
bremsstrahlung x-rays to remotely image the electron precipitation in the Earth's atmosphere,
advanced instrumentation for spaceflight, and the use of semicustom application-specific
integrated circuit (ASIC) microcircuits for spaceflight.
Joseph Mobilia received the BA degree in 1977 from the University of Massachusetts/Boston in
physics. He received the MS degree in 1979 from Boston College in physics. Mr. Mobilia has been
with the Lockheed Space Sciences Laboratory in Palo Alto, California, since 1980 where he is now a
research scientist. He conducts investigations and performs data analysis in radiation belt
physics using energetic charged particles and bremsstrahlung x-ray data from various satellite
payloads. He is currently program manager of the Source/Loss-cone Energetic Particle Spectrometer
(SEPS) for flight on the National Aeronautics and Space Administration / International Solar
Terrestrial Physics (NASA/ISTP) POLAR satellite as part of the CEPPAD experiment. He is also
program manager for the Lockheed portion of the Space Dust experiment (SPADUS) ONR 502 to be flown
on the Air Force Space Test Program (STP) Advanced Research and Global Observation Satellite
Henry L. Collin received the BSc, MSc, and PhD degrees in 1960, 1963, and 1969,
respectively, from the University of Durham, UK. From 1965 to 1967 he was employed at the Radio
and Space Research Station, UK (now Rutherford Appleton Laboratory), where he assisted with the
development of sounding rocket instrumentation and ionospheric experiments. In 1967 he joined the
Physics Department of Southampton University UK, where he conducted sounding rocket experiments at
polar, middle, and equatorial latitudes and took part in investigations of atmospheric optical
emissions from aurora, airglow, and chemical releases. Since 1981 he has been with Lockheed Space
Sciences Laboratory in Palo Alto, California. His current research interest is the role of
energetic ions in the magnetospheric plasma.
William L. Imhof received his BA, MA, and PhD degrees in physics from the University of
California, Berkeley, in 1951, 1953, and 1956, respectively. He has been employed at the Lockheed
Palo Alto Research Laboratory since 1956 and has held various research and group leader positions.
His scientific activities have involved studies of the Earth's radiation belts, including the
sources and loss mechanisms for trapped particles. In pursuing this activity, he has participated
in the development of particle detectors and x-ray imagers. He is author of more than 100
refereed scientific publications. He is a member of the American Geophysical Union, the American
Astronomical Society, the American Physical Society, Phi Beta Kappa, and Sigma Xi.
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