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Chapter 7. Energetic Neutral Atoms (pp. 153ff)

Birth of ENAs

Local, in situ, direct detection of individual neutral atoms in interplanetary space looked unrealistic for decades. Mass spectrometers have been measuring neutral atoms and molecules in the upper atmosphere of the Earth since the 1950s. Such instrumentation could probe, in situ, relatively large number densities in planetary environments. Optical sensors also detected solar ultraviolet and extreme ultraviolet photons scattered by atoms in interplanetary space. These observations were, however, essentially non-local, line-of-sight, and limited in sensitivity.

Charged particles (ions) and neutral particles collide in space plasmas. Then, an exchange of charge sometimes occurs when an electron jumps from one particle to another. The process is known as charge exchange. Whenever an energetic ion undergoes a charge exchange collision with a neutral background atom, the ion becomes an energetic neutral atom (Fig. 7.1). Planetary atmospheres and interstellar gas that fills interplanetary space provide neutral atoms for energetic ions to become neutral. Consequently, such collisions create ENAs everywhere. The number densities of ENAs are exceptionally low and their fluxes are usually weak.

Magnetic fields permeate space, whether near planets (forming magnetospheres) or in interplanetary and interstellar space. Energetic ions cannot travel across magnetic field lines. The field bends ion trajectories by the Lorentz force and causes them to gyrate (Fig. 7.2). Therefore, the magnetic field confines the ions and prevents them from reaching a remote observer. Consequently, one cannot directly examine the properties of ions in, for example, a magnetosphere from a distance.

In contrast to ions, a unique feature of ENAs is that magnetic fields do not exert forces on neutral atoms. Therefore, the newly born ENAs fly across magnetic field lines along straight trajectories (Fig. 7.2) until lost due to ionization. They could cover large distances in space. The gravitational field of the Sun and planets may bend ENA trajectories, but this effect is often small and can be accounted for.

An observer can thus measure ENA fluxes produced by ions in remote plasmas at large distances. Obtaining an image of a space plasma object such as a planetary magnetosphere (Fig. 7.3) or the interstellar boundary of the solar system in emitted ENA fluxes became known as ENA imaging. Scientifically inclined readers can find physics details of the concept and instrumentation in a review article in a physics journal. [1]

ENA imaging of space plasmas is conceptually similar to widely used passive corpuscular diagnostics of fusion plasmas. [2] Already in 1951, ...

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Wandering in wilderness

The presence of energetic ions and neutral gas everywhere in space makes ENAs ubiquitous. The ideas of probing planetary magnetospheres and the interstellar boundary of the solar system in energetic neutral atoms originated in the early days of the space age. Scientific literature occasionally mentioned, as a curiosity, the possibility of the existence of chargeexchange- produced neutral atoms with energies of hundreds and even thousands of electronvolts.

As early as 1961, ...

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Energetic neutral atoms open a way to probe remotely global ion and plasma properties. From a distance, one could obtain an image of a distribution of energetic ions surrounding a planet, its magnetosphere, by detecting and identifying individual energetic neutral atoms originating there. Figure 7.3 shows a spacecraft capturing a series of "enagraphs," as envisioned by us in the mid-1980s, which would reveal "the time evolution of the magnetospheric processes, for instance of the ring current decay." [8] ...

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Detecting neutral atoms directly

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Figure 7.6 shows several basic common techniques and elements of present-day ENA instrumentation. Free-standing ultrathin carbon foils (Fig. 7.6,a), only 10-40 atomic layers thick, had been used in nuclear physics experiments for some time by the late 1970s. Atomic particles with energies of several hundred electronvolts per nucleon and higher could penetrate and fly through them. The first ENA-detection experiments on sounding rockets in 1968 and 1970 relied on stripping neutral atoms (converting to positive ions) passing through such foils (7.6,b), with the subsequent ion analysis and detection. IBEX would later use this approach for mapping the interstellar boundary in ENA fluxes. Other techniques shown in Fig. 7.6 had been used in unrelated applications under different environments and for different energies, species, and wavelengths in nuclear physics experiments (7.6,c), production of high-intensity negative ion beams (7.6,d), and in “superinsulator” shields in the infrared spectral region (7.6,e). [13]

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Fig. 7.1. Charge exchange collision of an energetic (fast) ion and a background (slow) neutral atom. A negatively charged electron e jumps from one colliding particle (neutral atom) to another (ion). The created neutralized ion, the energetic neutral atom or ENA, preserves the magnitude and direction of the original velocity vector, V, of the energetic ion.

Fig. 7.2. Energetic ions gyrate in magnetic field B in space. After a charge exchange collision, the newly created ENA preserves the velocity vector V of the ion at the moment of the collision. It then flies away across magnetic field lines in a straight trajectory as a stone from a sling. Consequently, one can probe the properties of energetic ions remotely from a large distance by detecting ENAs. Recording an image of a plasma object, such as a magnetosphere or the solar system interstellar boundary, formed by emitted energetic neutral atoms became known as ENA imaging.

Fig. 7.5. The Sun’s heliosphere and its galactic neighborhood. A supersonic radial expansion (with a velocity of 500 km/s) of the solar wind ends with a termination shock where the solar wind plasma abruptly slows down and heats up. The heliopause (dashed line) separates the solar and interstellar plasmas and forms the boundary of the heliosphere. The flow of interstellar plasma and neutral gas with a velocity of 25 km/s, or 5 AU/yr, relative to the Sun, the interstellar wind, may form a weak bow shock in front of the heliosphere. The surrounding local interstellar medium, LISM, includes cosmic rays and carries the frozen-in magnetic field B. Interstellar helium atoms fly into the solar system practically unimpeded and reach Earth orbit, where they can be directly detected.
The figure also shows the positions of Voyager 1 (V1) and 2 (V2) spacecraft at the heliocentric distances of 165 AU and 138 AU, respectively, in 2025. These space vehicles fly away from the Sun. Voyager 1 and 2 crossed the termination shock in 2004 and 2007 and the heliopause in 2012 and 2018, respectively.
The gray region between the termination shock and heliopause, the heliosheath, contains the solar wind plasma heated in the shock transition. Charge exchange collisions between hot plasma protons and inflowing interstellar hydrogen atoms produce heliospheric energetic neutral atoms that can reach our planet Earth.
Measuring the directional intensities and energy dependencies of these heliospheric ENAs on a spacecraft near the Earth remotely probes the heliosheath and maps the interstellar boundary of the solar system. Development of the experimental concept and instrumentation for ENA imaging of the heliosphere drove our work at IKI starting in the late 1970s. NASA’s IBEX space mission mapped the interstellar boundary in ENA fluxes for the first time 30 years later in 2009 (Fig. 3.8).

Fig. 7.6. Basic techniques and sensor components for ENA detection. (a) – ultrathin carbon foils for detection of ENAs with energies >400 eV. (b) – ENA stripping in thin foils with subsequent ion analysis. (c) – thin-foil-based time-of-flight ENA detection. (d) – detection of low energy (<40 eV/nucleon) ENAs by converting them into negative ions (left) or sputtering positive and/or negative ions from the surface (right). (e) – filters with small holes or slits allowing passage of ENAs and blocking background EUV and UV photons.

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

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.

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

• ultrathin (2-4 nm or 20-40 A) carbon foils
• ENA stripping in foils
• time-of-flight (TOF) analyzers
• coincidence techniques
• negative ion surface conversion technique
• multi-electron secondary electron emission
• diffraction filtering (ENA filtering by "holes")
• ....

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

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