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Energetic Neutral Atom Imaging (ENA Imaging) of Space Plasmas

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Brief History of Experimental Study of ENAs in Space

This is a chapter from a review paper published in the Review of Scientific Instruments in 1997.The numbers in bracket refer to publications listed in the article 500+ reference list.

Download the full text of the article to see the reference list and figures. Several references to Mike Gruntman's publications are also linked directly to downloadable pdf files.

While there are a number of publications on different aspects of the history of solar-terrestrial and magnetospheric physics[65,66,194–198], the story of ENA experimental study has never been told in detail. The presence of ENAs in the terrestrial environment was reliably established for the first time in 1950 by optical recording of Doppler-shifted hydrogen Balmer Ha emission (6563 A) in an aurora [199,200]. The precipitating hydrogen ENAs are born in the charge exchange between energetic protons and neutrals of the upper atmosphere and exosphere. Balmer Ha emission is in the visible wavelength range. It can be optically detected from the ground, and auroral emissions were extensively used to study the characteristics of energetic particles [194,201]. While hydrogen Balmer lines were observed in the auroral regions since the late 1930s [202], it was not until 1950 that a Doppler-shifted Ha line was unambiguously explained with the presence of hydrogen ENAs [199].

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The importance of ENA production processes for the magnetosphere was understood [203] by noting that the proton–hydrogen atom charge exchange cross section was rather high for collision velocities less than the electron velocity in a Bohr orbit, i.e., for protons with energies <25 keV (Fig. 2). Charge exchange determines many important properties of geomagnetic storms. The "main phase" of a geomagnetic storm may last from 12 to 24 h, and it is characterized by a weakening of the geomagnetic field. The main phase is usually followed by a "recovery phase," when a gradual field recovery toward the initial undisturbed value of the geomagnetic field is observed. The recovery time constant may be 1 day or sometimes longer.

It was suggested for the first time in 1959 that charge exchange between the magnetic storm protons and neutral atmospheric hydrogen atoms provided the mechanism for the recovery phase [77]. The charge exchange process leads to the production of fast hydrogen atoms and observation of such atoms was first proposed in 1961 as a tool to study the proton ring current present during a magnetic storm [78].  A source of ENAs beyond the magnetospheric boundary, viz., charge exchange between the solar wind and the escaping hydrogen geocorona, was also identified for the first time [78]. The concept of imaging the magnetospheric ring current in ENA fluxes from outside [13] and, "in a limited fashion," from inside [14] was introduced much later in 1984.

The presence of atomic hydrogen in interplanetary space was first derived [185] in 1963 from sounding rocket measurements [204] of Doppler-broadened hydrogen Ly-a (1216 A) radiation (see also review of the early study of extraterrestrial Ly-a radiation [205]). It was recognized since the late 1950s that Doppler shift measurements could distinguish between the telluric (geocorona) and interplanetary hydrogen [206]. The emerging concept of the heliosphere [184] was extended in 1963 by the suggestion that about half of the solar wind protons would reenter the solar cavity in the form of hydrogen ENAs (with 3/4 of the initial solar wind velocity) as a result of processes at and beyond the solar wind termination region [185]. It was established later that an interplanetary glow in the hydrogen and helium resonance lines was produced by resonant scattering of the solar radiation by interstellar gas directly entering the solar system [141–144,158–161]. The "returning" neutral solar wind flux [185] is believed to be significantly smaller and highly anisotropic [9].

It was also suggested in the early 1960s that a large number of neutral atoms could be present in the solar wind as transients due to ejection of solar matter in violent events [207,208]. Neutral hydrogen atoms in solar prominences are observed optically, and it was argued that they may reach 1 AU. The follow-on calculations [209,210] showed that most of the neutral atoms would not survive travel to 1 AU because of ionization by solar EUV radiation and electron collisions. The neutral component of the solar wind is born mostly in charge exchange between the solar wind ions and interstellar gas filling the heliosphere [12,142,178].

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Direct ENA measurements promised exceptional scientific return, but the necessary instrument development was only started in the late 1960s by Bernstein and co-workers [119,211–213] at TRW Systems Inc., Redondo Beach, California. Direct, in situ measurement of ENA fluxes in space was first attempted on April 25, 1968 in a pioneer rocket experiment [119].  The first dedicated ENA instrument was launched on a Nike–Tomahawk sounding rocket from Fort Churchill, Manitoba, Canada. This experiment was followed by the launch of a similar instrument on a Javelin sounding rocket on March 7, 1970 to an altitude of 840 km at Wallops Island off the coast of Virginia [214].  The experiments detected hydrogen ENA fluxes in the range 105–109 cm-2 s-1 sr-1 with energies between 1 and 12 keV. The reported ENA fluxes were considered by many as excessively high, however these results have never been directly challenged in the literature.

The first ENA instrument [212] is shown in Fig. 8. It had many features and introduced many components and techniques that would later on become widely used by various modern ENA instruments. The instrument was based on foil-stripping of ENAs and subsequent analysis of the resulting positive ions. Electrostatic deflection plates were used to remove incident protons and electrons with energy <25 keV at the entrance (the instrument also had an additional 100 G magnetic field to remove electrons with energies >50 keV). The deflection plates served to define the solid angle of the instrument. ENAs were stripped passing an ultrathin (2 mg/cm2) carbon foil mounted on an 80% transparent grid. A hemispherical energy analyzer, which focused the ions in one dimension, was used for energy analysis. The stripped ENAs, protons, in a selected energy range, passed through the analyzer and were counted by a channel electron multiplier (CEM). Use of two additional identical instrument sections without deflecting voltage and without an ultrathin foil allowed the simultaneous measurement of proton fluxes and the monitoring of the background count rate during the experiment, respectively.

An attempt to measure ENAs was made in the RIEP experiment (a Russian acronym for "registerer of intensity of electrons and protons") on the Soviet Mars-3 interplanetary mission (launched May 28, 1971; entered low Mars orbit on December 2, 1971). The experiment was designed to measure the energy distribution of plasma ions and electrons in the Mars’ environment as well as in the solar wind during interplanetary coast. The RIEP instrument consisted of eight separate cylindrical electrostatic energy-per-charge analyzers, each followed by a CEM to count particles [215].  Each analyzer unit was designed to measure charged particles of a selected energy-per-charge ratio. Two ultrathin carbon foils (150 A or ~3.5 mg/cm2) were installed in front of two of the eight analyzers. A comparison of count rates from analyzers with and without foil while measuring particles with the same energy-per-charge ratio was expected to provide information on high-intensity neutral atom fluxes. No charged particle deflectors were used in front of the ultrathin foils, and the experiment failed to establish ENA fluxes, which are usually relatively weak.

Another ENA instrument, a slotted-disk velocity selector, was successfully built and mechanically and electrically tested in a rocket flight in 1975 [181]. This narrow (FOV) instrument, which demonstrated efficient rejection of charged particles and photons, was especially suitable for measurement of ENAs with velocities <500 km/s (E<1.3 keV/nucleon). It is interesting (see below) that measurements of the interstellar helium flux and of the neutral component in the solar wind were considered as possible applications [181].  Apparently due to large size, mass, and power consumption as well as the torque exerted on a spacecraft (the instrument included at least two disks 16.24 cm in diameter on a shaft 74 cm long spinning at 4.5´104 rpm) this instrument was never used for ENA measurements, and the technique development was discontinued.

The interest to the concept of a mechanically moving ENA velocity selector was recently revived by suggestion to use unconventional high-frequency mechanical shutters mounted on ceramic piezoelectric crystals [216].  Such an approach seems to be especially promising for the study of low-energy ENAs, but further development is needed to demonstrate its feasibility. The initial ENA measurements [119,214] were not repeated and/or independently verified, and they were largely ignored by the space community. Exceptionally strong EUV/UV radiation background was identified as a major obstacle for reliable ENA measurements in space, and experimental difficulties were perceived as insurmountable by many at that time.

Three groups accepted the experimental challenge and continued independent development of dedicated ENA instrumentation in the late 1970s. A group at the Max-Planck-Institute for Aeronomy (MPAe), Lindau, Germany targeted direct in situ detection of interstellar helium flowing into the solar system [174,175]. (The experiment was suggested for the first time in 1972 in a proposal by H. Rosenbauer, H. Fahr, and W. Feldman.)  Another group at the Space Research Institute (IKI), Moscow, USSR, planned to measure the neutral component in the solar wind and heliospheric ENAs [177,182,217] It is interesting that the Moscow group initially considered also direct, in situ detection of the interstellar helium flux [218] but ultimately decided to concentrate on the neutral solar wind (NSW) and heliospheric ENAs. The NSW experiment was actively supported by a group at Space Research Center in Warsaw, Poland, which also theoretically studied heliospheric ENAs and ENAs produced in the giant-planet magnetospheres [219–221]. A third group at the University of Arizona focused on the possibilities of measuring ENAs in geospace [222].

The GAS instrument [175] to directly detect fluxes of interstellar helium (E = 30–120 eV) had a dramatic history of being completely redesigned and built within a record 3 month period in an "almost super-human effort," [176] which had become necessary to realize the original experimental concept on the European-built Ulysses probe after cancellation of the U.S. spacecraft. The GAS was then successfully flown [153,154,176] on Ulysses which was launched in 1990 after many delays [223], including the one caused by the Space Shuttle Challenger explosion. The neutral helium instrument is based on secondary ion emission from a specially prepared surface [175,176]. The experiment has produced unique data on interstellar helium characteristics and ENAs emitted from Jupiter’s Io torus and continues operating successfully [154].

A new ultrathin-foil based "direct-exposure" technique, which does not require stripping of incoming ENAs, was developed by the Moscow group to detect the neutral solar wind [177,182,217]. NSW measurements can be performed from an interplanetary or high-apogee earth-orbiting spacecraft by pointing the instrument several degrees off the sun. The possibility of taking advantage of the aberration caused by the earth’s motion around the sun was first suggested in 1975 [181] and was independently "rediscovered" later [177,182]. The NSW instrument was built for the Soviet Relikt-2 mission which was originally planned to be launched in 1987 [182]. The initial experiment also included measurements of ENAs from the heliospheric interface and ENAs emitted from the terrestrial magnetosphere. Detection of ENAs escaping Jupiter and Saturn was also expected [182,220,224]. The Relikt-2 mission was postponed many times, and is still awaiting launch (now scheduled for 1999).

The NSW instrument included a diffraction filter [182,225] to significantly increase the ENA-to-EUV/UV photon ratio in the sensor. The filter was optional for the planned measurements [182], and it was not fully developed, when the instrument was built. A new diffraction filter technology is emerging [226,227], and a new generation of diffraction filters for ENA instruments is currently being developed and evaluated (see Sec. VI F) [228,229,230,231]. An introduction of the diffraction filters opens the way to take full advantage of a highly efficient direct-exposure technique.

ENA instrument development in the late 1970s and early 1980s did not attract much attention in the space community, and did not enjoy enthusiastic support of the funding agencies. However, the importance of ENAs for mass, energy, and momentum transport in space was established and new opportunities offered by remote ENA imaging in separating spatial variations from temporal ones in space plasmas were gradually recognized.

Several measurements of large fluxes of ions (E<10 keV) near the equator at altitudes below 600 km had been reported since the early 1960s [232]. Fluxes of high-energy ions (0.25<E<1.5 MeV) were measured later at low altitudes by the German AZUR spacecraft [124,233], and the ions with energies down to 10 keV were detected there in 1973 [234]. Theoretical calculations predicted a short lifetime for such low altitude protons near the equator due to collisions with atmospheric particles. The proton loss thus required an injection of protons in a limited region below 600 km. The required proton source to compensate proton loss due to interaction with atmosphere must also be atmosphere dependent. The explanation, found in 1972 [124], suggested that trapped energetic ions in the ring current at much higher altitudes produce ENAs in charge exchange, and a fraction of these ENAs reaches low altitudes where they are re-ionized by charge exchange and are consequently trapped by magnetic field (Fig. 6).

In the heliosphere, the neutral solar wind is believed to provide a transport mechanism similar to that of magnetospheric ENAs [180]. As the solar wind expands toward the boundaries of the heliosphere, the neutral fraction in the solar wind gradually increases to 10%–20% at the termination shock (Fig. 7). While the solar wind plasma flow is terminated by the shock, the solar wind ENAs easily penetrate the region of the heliospheric interface and enter the LISM, sometimes called "very local" interstellar medium (VLISM). The solar wind ENAs thus interact with the approaching local interstellar plasma via charge exchange with plasma protons. Hence, the boundary of the region of the sun’s influence, the solar system "frontier," extends further into "pristine" interstellar medium. The significance of this neutral solar wind effect [180] on LISM was recently confirmed by detailed computer simulations of the heliospheric interface [149,150].

The interest in ENA characteristics and instrument development began to grow after the serendipitous discovery of ENAs made by energetic particle instruments on several spacecraft. Energetic particle instruments are usually based on solid state detectors and often capable of discriminating against electrons. However, such instruments generally cannot distinguish between a charged particle and an ENA. Therefore an energetic particle detector would efficiently serve as an ENA detector only in the absence of the normally abundant ions. Only high-energy ENAs (E>10–20 keV/nucleon) can be detected by such instruments.

Analysis [235] of inconsistencies in interpretation of energetic particle measurements by the IMP 7 and 8 satellites (at ~ 30–35 RE from the Earth; 1 RE = 6378 km is the Earth radius) led to the conclusion that a certain fraction of counts during periods of very low fluxes [236] was caused by radiation belt-produced ENAs with energies 0.3–0.5 MeV.

Both energetic ions and neutrals were detected during the magnetic storm in 1982 by the SEEP instrument on the S81-1 spacecraft at low altitude. A double charge-exchange mechanism (Fig. 6) was invoked to explain the observations, and it was also suggested that measurements of energetic neutral atoms at low altitude "might be able to image, in a limited fashion, the integral ion intensities of the ring current as a function of latitude and longitude" [14].

ENA fluxes of nonterrestrial origin were detected on a Voyager 1 spacecraft during flybys of Jupiter [237] and Saturn [238] n March 1979 and November 1980, respectively. The low energy charged particle (LECP) instrument on Voyager-1 included a silicon detector to accumulate counts from eight separate directions in the ecliptic plane [239]. The detector was designed to measure ions (electrons were swept away by magnetic field) with energies >40 keV.

During the approach to Jupiter, when Voyager 1 was still outside the gigantic Jovian magnetosphere, an excess count rate was measured in the sector containing the planet in its FOV. The detector characteristics limit the possible sources of the excess counts to energetic ions, x rays, and ENAs. No energetic ion fluxes with required intensity and energy were expected at the location of measurements since there were no magnetic field lines connecting to Jupiter [237].  It is known that x rays can be generated in planetary magnetospheres by precipitating energetic electrons, usually in polar regions. Consideration of x-ray generation showed that the required fluxes of electrons were several orders of magnitude higher than those found in the Jovian magnetosphere. The conclusion of data analysis was that "the only remaining possibility of explaining the excess counts ... is energetic neutral atoms" [237]. The energy dependence of the observed ENA spectra was similar to the one established for energetic ions in the magnetosphere during the close flyby.

A similar excess count rate was measured one and a half years later during the flyby of Saturn [238]. Here again the observed count rate, if interpreted in terms of x rays, cannot be reasonably related to precipitating magnetospheric electrons. It was concluded that "charge exchange of energetic ions with satellite tori is an important loss mechanism at Saturn as well as at Jupiter" [238].

Possible ENA signatures in the experimental data obtained by energetic particle instruments [240] on IMP 7,8 and ISEE 1 were analyzed in 1982 [241,242]. Detection of ENAs (E~50 keV) was unambiguously established, and their source was identified as the ring current in the terrestrial magnetosphere [107].  Coarse spatial information on ENA-producing regions was derived and ENA energy distribution and mass composition were determined. The ENA measurements were made from positions where magnetic field lines allowed only negligible fluxes of energetic ions, so that the detectors counted only ENAs. The analysis [107] of these measurements was a major milestone in validating the idea that the global magnetospheric processes can be efficiently studied remotely by measuring ENAs. It was also suggested that ENA imaging could be used to study the magnetospheres of Jupiter and Saturn [107].

The follow-on analysis of the ISEE 1 data demonstrated the powerful potential of the ENA detection as an imaging technique by reconstructing the first ENA global image of the storm-time ring current (at E~50 keV) [79].  The instrument [240] on ISEE 1 was capable of measuring the incoming ENA flux, and imaging was performed by a combination of spacecraft spin and instrument axis scanning by a moving motorized platform. A procedure for computer simulation of all-sky ENA images was established and the theoretically predicted images were compared with the ring current image obtained by ISEE 1 at a radial distance of 2.6 RE during a 5 min observation.

The analysis of excessive count rates [237,238] detected by Voyager 1 during Jupiter and Saturn flybys allowed one to determine some ENA characteristics in the vicinity of the giant planets [108]. Further computer simulations of expected ENA emissions demonstrated the efficiency of ENA imaging as a tool to study the magnetospheres of Jupiter and Saturn from flyby spacecraft and orbiters [109,110,224,243].

Use of energetic particle instruments for detection of high-energy ENA fluxes in the absence of ions became an established experimental technique. The storm-time ENA images of the polar cap [244] were recently obtained by the CEPPAD experiment [245] on the POLAR spacecraft. The instrument cannot distinguish between ions and neutrals, and the ENA images were recorded during the portions of the spacecraft orbit where the fluxes of the charged particles were very low.

An instrument with a dedicated high-energy ENA (20<E<1500 keV) detection channel was flown on the CRRES satellite in 1991 [21,125]. The neutral channel that consisted of a magnetic deflector followed by a solid-state detector measured ENA fluxes precipitating to low altitudes at the equatorial regions. A small geometrical factor put limit to the imaging capabilities of the instrument. A conceptually similar ENA instrument was recently flown on ASTRID [22,23]. A more sophisticated high energy particle (HEP) Instrument (10–100 keV ENAs) was launched on the GEOTAIL spacecraft in 1992 [20].

The current phase in the study of ENAs in space plasmas is characterized by extensive computer simulations of ENA images, novel instrument development, and the preparation and planning of a number of dedicated ENA space experiments. (MG: remember that this was written and published in 1997, before IMAGE, Cassini, TWINS, and IBEX.) The concept of global ENA imaging of the magnetosphere is firmly established [1–5,100,111,112].  Computer simulations of low-energy [113–115] and high-energy [111,116,118,246,247] ENA imaging emphasized the importance of ENA measurements throughout a wide energy range from few eV up to several hundred keV. Various aspects of heliospheric ENA global imaging were theoretically studied for various species and energy ranges [6–8,9,10,11,12,157,192,248]. The first ENA detections obtained from planetary magnetospheres in the early 1980s triggered new interest in developing dedicated ENA instrumentation: various techniques for detection and imaging of low- and high-energy ENAs were proposed and their laboratory evaluation was begun [243,249,250–252,253,254].  Several new research groups have entered the ENA-imaging field since then and significant improvements of the known experimental techniques as well as a number of innovative approaches were proposed, especially for low-energy ENAs [4,10,125,216,225,228,229,230,254–258].

NASA recently selected an IMAGE mission to perform comprehensive imaging of the terrestrial magnetosphere in EUV, FUV, and ENA fluxes. A sophisticated first large size ENA camera INCA [24] on Cassini will soon perform ENA imaging of the Saturnian magnetosphere and the exosphere of the Saturnian moon Titan [259].

ENA Imaging

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