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Energetic Neutral Atoms (ENAs) in Space

ENA Imaging of Space Plasmas



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invited talk (15 min.) at the Fall Meeting, AGU, December 2012

Energetic Neutral Atom Imaging: The Next Step  –  ENA imaging

Energetic neutral atom imaging of space plasma (review) (pdf)
by Mike Gruntman in Review of Scientific Instruments, 1997


Imaging in Fluxes of Energetic Neutral Atoms (ENA Imaging)

A new field of space experiments and instrumentation has emerged: imaging of space plasmas in fluxes of energetic neutral atoms (ENAs), or ENA imaging. It took over 25 years from the first vague concepts of late 1970s to develop experimental techniques and instrumentation. (See review of the field and development of the concept and experimental techniques and instrumentation in a highly cited article in Review of Scientific Instruments, 1997; also as a pdf file.) The concept of ENA imaging has spectacularly demonstrated its power on the NASA's IMAGE mission (launched in 2000) carrying three ENA instruments for imaging magnetospheric processes in different energy ranges. (See also NASA's IMAGE site.) The Cassini spacecraft used an imaging energetic neutral atom camera (INCA) to study the magnetosphere of Saturn. NASA mission TWINS provided for the first time a spectroscopic view in ENAs of the terrestrial magnetosphere by simultaneous observation from two spacecraft. A few opther space missions carried ENA instruments, e.g., ASTRID.

In 2008, NASA launched the Interstellar Boundary Explorer (IBEX) mission to probe remotely in ENA fluxes the galactic frontier of the solar system. The scientific rationale and experiment concept of IBEX are described in detail in an article (pdf) in Journal of Geophysical Research, 2001.

The concept of ENA imaging the heliosphere and remotely exploring the interstellar frontier of the Solar system first emerged in 1979-1980 (see pp. 28-29 in Preprint-825, IKI, 1983). While the first simple dedicated space experiment (article in Physics of the Outer Heliosphere, 1990) to detect heliospheric ENAs was developed in mid-1980s, it has never flown. (See details in History of ENA study in space.)

It took almost twenty years of physics research and development to refine the exprimental concept of heliosphere ENA imaging and to develop and mature the instrumentation technology.

Energetic Neutral Atoms (ENAs) – Tutorial ena production

The interaction between charged and neutral particles is a common phenomenon in space plasmas. Whenever an energetic ion undergoes a charge exchange process in a collision with a neutral background atom, an energetic neutral atom – ENA – is born. Ion-electron recombination and neutral atom acceleration by the solar gravitation may also contribute to an ENA population under certain conditions. ENAs are ubiquitous in space environment and their study opens a new window on various phenomena in space plasmas with a promise (already partially realized) to qualitatively improve our understanding of global magnetospheric and heliospheric processes.

ena production An ENA is not bound by the magnetic field and, as a stone from a slingshot, leaves the place of its birth along a straight trajectory with a velocity of the energetic ion. ENAs, contrary to charged particles, can travel large distances through space with minimal changes without undergoing further interaction with plasma.

ENA measurements are a powerful tool to remotely study various global plasma objects in space, such as the heliosphere and planetary magnetospheres. By recording ENA fluxes as a function of observational direction, one can reconstruct a global image of the object of interest, thus the term "ENA imaging," first introduced in 1984 for imaging from outside and from inside of the magnetosphere. Plasma ion energy distribution and ion composition can be remotely established by measuring ENA energies and masses. ENA imaging usually means not only determining ENA flux angular distribution but also ENA energies and masses. An ENA imaging experiment ideally produces a set of images of a plasma object in ENAs of different masses and in different energy ranges.

Protons are the most abundant component of space plasma ions. Unlike other space plasma ions (e.g., He+ and O+), protons cannot be imaged optically, which makes ENAs in many cases the only tool to study processes of interest remotely.

ENA instrumentation

ENAs remained poorly explored experimentally for many years due to enormous instrumental difficulties. The reason is the following. The energies of ENAs (100 eV and higher) are sufficient to produce secondary electrons on surface impact. Therefore, ENAs can be conveniently registered by secondary electron multipliers, such as channel electron multipliers and microchannel plate detectors. The problem is that space is filled with fluxes of H Lyman-α photons (λ = 121.6 nm = 1216 A) that efficiently produce photoelectrons from surfaces and consequently trigger secondary electron multipliers. (The other important background emission line is at λ = 58.4 nm = 584 A.) The count rate of a typical detector due to background photons would be 4-7 orders of magnitude higher than the count rate due to fluxes of ENAs. Another experimental challenge is that fluxes of ENAs are very weak and one needs to develop instruments with large geometrical factors. The experimental difficulties were perceived as insurmountable by many – except very few brave souls – at that time.

Various experimental techniques and instrument components and designs (with imaging and analyzing capabilities) have been developed to enable ENA imaging across wide range of ENA energies, from a few eV to 100 keV. We mention here

ENA instrumentation today is mature, with excellent performance characteristics. See review article on ENA space instrumentation.


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Selected Mike Gruntman's publications

that advanced the ENA imaging concept, physics of instrumentation,

and instrument components.

  • Charge exchange on interstellar helium dominates properties of the neutral solar wind in the downwind (interstellar wind) hemisphere.

M. Gruntman, The neutral component of the solar wind at Earth's orbit, Cosmic Research, 18, 649-651, 1980
  • Dynamics properties of the flux of interstellar helium atoms at 1 AU

  • Evaluation of the possibility of a space experiment to directly detect interstellar helium atoms at 1 AU
M. Gruntman, Interstellar helium at Earth orbit, Preprint-543 (Report-543), Space Research Institute (IKI), The USSR Academy of Sciences, 1980
  • Motivation for instrumentation development for study of hot plasmas in space (neutral solar wind component) and laboratory (fusion) 

  • Experimental demonstration of ultrathin-foil TOF ENA analysis and detection at ENA energies as low as 600 eV (TOF was applied at that time for energies >25 keV)
  • Demonstration of three TOF "direct-exposure" detector designs, with some features strikingly similar to those of the current space ENA imaging cameras
  • Measurement of absolute detection efficiencies of detectors (and instruments) without detector absolute calibration using features of coincidence techniques
  • New sensor design to directly detect flux of interstellar helium in a coincidence mode
M. Gruntman and V.A. Morozov, Study of performance characteristics of detector-energy analyzer of fast H atoms based on foil, Preprint-667 (Report-667), Space Research Institute (IKI), The USSR Academy of Sciences, 1981
  • Experimental demonstration of ultrathin-foil TOF ENA analysis and detection at ENA energies as low as 600 eV (TOF was applied at that time for energies >25 keV)

  • Demonstration of three TOF "direct-exposure" detector designs, with some features strikingly similar to those of the current space ENA imaging cameras
  • Measurement of absolute detection efficiencies of detectors (and instruments) without detector absolute calibration using features of coincidence techniques
  • New sensor design to directly detect flux of interstellar helium in a coincidence mode
M. Gruntman and V.A. Morozov, H atom detection and energy analysis by use of thin foils and TOF technique, Journal of Physics E: Scientific Instruments, 15,1356-1358, 1982 
  • ENA fluxes disturb the approaching supersonic interstellar wind far beyond the bow shock (how far one needs to go to reach pristine interstellar medium). One of the explanations of the IBEX-discovered "ribbon" in 2009 is based on refinement of this concept.

M. Gruntman, Effect of neutral component of the solar wind on the interaction of the solar system with the interstellar gas flow, Soviet Astronomy Letters, 8, 24-26, 1982
  • Feasibility study and comprehensive proposal for a space experiment to detect ENAs in space

  • Primary focus: neutral component of the solar wind and heliospheric ENAs originating at the solar system heliospheric interface (termination shock)

  • Instrumentation basis: TOF foil-based detectors
  • Reference to internal IKI reports that first had proposed the concept:
    M. Gruntman, Report 9-182, Laboratory 182, IKI, 1979
    M. Gruntman, Report 14-182, Laboratory, IKI, 1980
  • New concept of diffraction filtering for improving signal-to-noise ratio in ENA instruments (as on IMAGE-MENA)
  • New concept of conversion of low-energy ENAs into negative ions on specially prepared surfaces (as on IMAGE-LENA) 
M. Gruntman and V.B. Leonas, Neutral Solar Wind. Possibilities of experimental study,  Preprint-825 (Report-825), Space Research Institute (IKI), The USSR Academy of Sciences, 1983
  • Description of the built prototype of the flight space instrument to detect ENAs in the solar wind (neutral solar wind), from the heliospheric interface (heliosphere ENA imaging), and ENAs from magnetospheres of the Earth, Jupiter, and Saturn 

M. Gruntman and V.B. Leonas, Possibility of experimental study of energetic neutral atoms in interplanetary space, Preprint-1109 (Report-1109), Space Research Institute (IKI), The USSR Academy of Sciences, 1986 (in English)
  • Feasibility study of ENA imaging of planetary magnetospheres

  • Feasibility study of coded-aperture imaging technique as applied for ENA imaging
M. Gruntman and V.B. Leonas, Experimental opportunity of planetary magnetosphere imaging in energetic neutral atoms, Preprint-1181 (Report-1181), Space Research Institute (IKI), The USSR Academy of Sciences, 1986 (in English)
  • Elastic collisions of interstellar helium atoms with solar wind protons, leading to the "growing wings" of atom angular (as in detection by Ulysses and IBEX) distributions

M. Gruntman, Concerning the problem of collisional heating of the interstellar helium flow by solar wind protons, Planetary and Space Science, 34, 387-389, 1986 (followed by M. Gruntman, Elastic Collisions of Interstellar Helium Atoms with Solar Wind Protons, Journal of Geophysical Research, 118, 1366–1378, 2013 and M. Gruntman, Collisional Heating of Interstellar Helium Flux at 1 AU, Journal of Geophysical Research, 123, 3291-3298, 2018)
  • Experimental demonstration of a TOF instrument arrangement to directly detect low-energy He atoms (as interstellar helium at 1 AU) in a noise-suppression mode

M. Gruntman, MASTIF: mass analysis of secondaries by time-of-flight technique. New approach to secondary ion mass spectrometry, review of Scientific Instruments, 60, 3188-3196, 1989 
  • Description of the space experiment and instrument (developed and built in mid-1980s and never flown) to measure a neutral component in the solar wind (neutral solar wind) and heliospheric ENAs from the heliospheric interface (heliosphere ENA imaging)

M. Gruntman, V.B. Leonas, and S. Grzedzielski, Neutral solar wind experiment, in Physics of the Outer Heliosphere, Pergamon Press, 1990
  • Physics of multi-electron secondary emission (emission statistics) from thin foils for ENA mass identification (through detector amplitude analysis) in TOF instruments

M. Gruntman, A.A. Kozochkina, V.B. Leonas, Multielectron secondary emission from thin foils bombarded by accelerated beams of atoms, JETP Letters, 51, 22-25, 1990
  • Evaluation of various available technologies for application as diffraction filters (ENA filters) for ENA imaging of space plasmas

M. Gruntman, Submicron structures: promising filters in EUV - a review, EUV, X-Ray, and Gamma-Ray Instrumentation for Astronomy, Proc. SPIE 1549, 385-394, 1991
  • Prediction of highly anisotropic fluxes of ENAs from the heliospheric interface

  • Hence, properties of ENA fluxes point to heliosphere ENA imaging as a powerful technique (sensitive to properties of the interaction) to study the heliospheric boundary
M. Gruntman, Anisotropy of the energetic neutral atom flux in the heliosphere, Planetary and Space Sciences, 40, 439-445, 1992
  • Comprehensive feasibility study introduces a concept of surface conversion of ENAs into negative ions for ENA imaging of space plasmas (as on IMAGE-LENA)

  • The proposed technique is specifically evaluated for detecting interstellar gas (H, D, O, etc.) inflowing the solar system (as it will be done by surface-conversion instrument on IBEX - IBEX-Lo) 
M. Gruntman, A new technique for in situ measurement of the composition of neutral gas in interplanetary space, Planetary and Space Science, 41, 307-319, 1993
  • Evaluation of coded-aperture technique for imaging of planetary magnetospheres in ENA fluxes

  • Tutorial on coded aperture imaging
M. Gruntman, Coded-aperture technique for magnetosphere imaging: advantages and limitations, Instrumentation for Magnetospheric Imagery II, Proc. SPIE 2008, 58-73, 1993
  • ENAs in the solar wind - neutral solar wind properties

M. Gruntman, Neutral solar wind properties: advance warning of major geomagnetic storms, Journal of Geophysical Research, 99, 19213-19227, 1994
  • New high-throughput collimator design for ENA instruments

M. Gruntman, A new collimator design for energetic neutral atom instruments, Review of Scientific Instruments, 65, 758-759, 1994
  • Evaluation of free-standing transmission gratings for application as diffraction filters for ENA imaging of space plasmas (used on IMAGE-MENA and TWINS)

M. Gruntman, EUV radiation filtering by free-standing transmission gratings, Applied Optics, 34, 5732-5737, 1995.
  • Evaluation of free-standing transmission gratings for application as diffraction filters for ENA imaging of space plasmas (used on IMAGE-MENA and TWINS)

M. Gruntman, Transmission grating filtering and polarization characteristics in EUV, X-ray and Extreme Ultraviolet Optics, Proc. SPIE 2515, 231-239, 1995 
  • Outgassing of interplanetary dust under solar wind bombardment

M. Gruntman, H2+ pickup ions in the solar wind. Outgassing of interplanetary dust, Journal of Geophysical Research, 101, 15555-15568, 1996
  • Evaluation of free-standing transmission gratings for application as diffraction filters for ENA imaging of space plasmas (used on IMAGE-MENA and TWINS)

M. Gruntman, Transmission grating filtering of 52-140-nm radiation, Applied Optics, 36, 2203-2205, 1997
  • Review (> 100 citations) of the experimental techniques and instrumentation for space plasma imaging in ENA fluxes; 505 references

M. Gruntman, Energetic neutral atom imaging of space plasmas, Review of Scientific Instruments, 68, 3617-3656, 1997
  • Modern physical concept of heliosphere imaging in ENA fluxes (> 100 citations), a key publications, per competition debrief, to IBEX selection by NASA

  • Heliosphere ENA maps highly sensitive to physically distinct scenarios of the the interaction between the solar wind and galactic medium

M. Gruntman, E.C. Roelof, D.G. Mitchell, H.J. Fahr, H.O. Funsten, and D.J. McComas, Energetic neutral atom imaging of the heliospheric boundary region, Journal of Geophysical Research, 106, 15767-15781, 2001
  • Includes a concept of an ENA experiment on Interstellar Probe

M. Gruntman, Instrumentation for interstellar exploration, Advances in Space research, 34, 204-212, 2004
  • New effect: importance of mass transport in the heliosphere by ENAs for interplanetary neutral atom properties in the sun's vicinity (< 1 AU)

M. Gruntman and V. Izmodenov, Mass transport in the heliosphere by energetic neutral atoms, Journal of Geophysical Research, 109, A12108, doi:10.1029/2004JA010727, 2004
  • Comprehensive study of the solar system frontier by ENA imaging and EUV mapping

M. Gruntman, Solar system frontier: exploring the heliospheric interface from 1 AU, Missions to the Outer System and Beyond, 4-th IAA Symposium on realistic and Advanced Scientific Space Missions, IAA, 2005
   
Selected publications on mapping the heliopause in EUV  
  • Concept of mapping the heliopause in extreme ultraviolet (EUV)

M. Gruntman and H.J. Fahr, Access to the heliospheric boundary: EUV-echoes from the heliopause, Geophysical Research Letters, 25, 1261-1264, 1998.
  • Concept of mapping the heliopause in extreme ultraviolet (EUV)

  • Detailed evaluation of heliopause mapping at O+ line at 83.4 nm

M. Gruntman and H.J. Fahr, Heliopause imaging in EUV: Oxygen O+ ion 83.4-nm resonance line emission, Journal of Geophysical Research, 105, 5189-5200, 2000
  • Detailed evaluation of solar wind emissions at He+ line at 30.4 nm

  • Concept of imaging of the three-dimensional solar wind and mapping the heliopause at 30.4 nm

M. Gruntman, Imaging the three-dimensional solar wind, Journal of Geophysical Research, 106, 8205-8216, 2001
  • Concept of imaging of the three-dimensional solar wind and mapping the heliopause at 30.4 nm and 83.4 nm

M. Gruntman, Mapping the heliopause in EUV, in The Outer Heliosphere: The Next Frontiers, Pergamon, 263-271, 2001

 



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