Technology to create global images of otherwise invisible phenomena
Energetic Neutral Atom (ENA) imaging is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and throughout the heliosphere.
Charged particles— protons, electrons, and various nuclei— emitted from solar wind are the basis of the interstellar medium. These charged particles have the ability to be redirected by magnetic fields such as the magnetic field surrounding the Earth. Occasionally charged particles within the plasma of the solar wind will collide with neutral atoms. This collision results in the previously charged particle becoming a neutrally charged atom. Due to the loss of charge, the atom no longer experiences magnetic attraction while maintaining its gravitational attraction and velocity. These atoms are Energetic Neutral Atoms and the detection of these atoms can be reconstructed to build ENA images.
Earth's magnetosphere preserves its atmosphere and protects life on Earth from cell-damaging radiation. This region of "space weather" is the site of geomagnetic storms that disrupt communications systems and pose radiation hazards to humans traveling in airplanes (at high altitude and latitude) or in orbiting spacecraft. Geomagnetic weather systems have been late to benefit from the satellite imagery taken for granted in weather forecasting and space physics because their origins in magnetospheric plasma frequency present the added problem of invisibility.
The heliosphere shields the Solar System from the majority of cosmic rays, but is so remote that only an imaging technique such as ENA imaging will reveal its properties. The heliosphere's structure is due to the interaction between the solar wind and cold gas from the local interstellar medium.
The creation of ENAs by space plasma was predicted, but their discovery was both deliberate and serendipitous. While some early efforts were made at detection, their signatures also explained inconsistent findings by ion detectors in regions of expected low-ion populations. Ion detectors were co-opted for further ENA detection experiments in other low-ion regions. However, the development of dedicated ENA detectors entailed overcoming significant obstacles in both skepticism and technology.
Although ENAs were observed in space from the 1960s through the 1980s, the first dedicated ENA camera was not flown until 1995 on the Swedish Astrid-1 satellite, to study Earth's magnetosphere wind.
Today, dedicated ENA instruments have provided detailed magnetospheric images from Venus, Mars, Jupiter, and Saturn. Cassini's ENA images of Saturn revealed a unique magnetosphere with complex interactions that have yet to be fully explained. The IMAGE mission's three dedicated ENA cameras observed Earth's magnetosphere from 2000–2005 while the TWINS Mission, launched in 2008, provides stereo ENA imaging of Earth's magnetosphere using simultaneous imaging from two satellites.
The first ever images of the heliosphere boundary, published in October 2009, were made by the ENA instruments aboard the IBEX and Cassini spacecraft, and challenge existing theories about the heliosphere region.
Creation of ENAs.
The most abundant ion in space plasma is the hydrogen ion—a bare proton with no excitable electrons to emit visible photons. The occasional visibility of other plasma ions is not sufficient for imaging purposes. ENAs are created in charge-exchange collisions between hot solar plasma ions and a cold neutral background gas. These charge-exchange processes occur with high frequency in planetary magnetospheres and at the edge of the heliosphere.
Charge exchange.
In a charge-exchange collision between a high energy plasma ion and a cold neutral atom, the ion 'gives' electrons to the neutral atom, producing a cold ion and an energetic neutral atom (ENA). This chemical reaction can be described as:
I1+ + A2 → A1 + I2+
where I1+ is the plasma ion, A2 is a low energy background neutral atom, A1 is the energetic neutral atom (ENA) and I2+ is the lower energy ion.
Species 1 and 2 in this charge-exchange reaction may be the same, such as in proton–hydrogen charge-exchanges:
H+ + H → H + H+Additionally, multiple electrons may be exchanged during the ion / neutral reaction. One example of this is the alpha-helium charge-exchange:He2+ + He → He + He2+
Due to its charge neutrality, the ENA produced in this reaction is only subject to gravitational forces. This is in contrast to charged particles (ions, protons, or electrons) within plasmas that are also subject to electromagnetic forces. Gravitational influences can generally be ignored in space plasmas, so it is common to assume that the ENA preserves the vector momentum of the original pre-interaction plasma ion.
Some ENAs are lost in further charge-exchange, electron collisions and photoionization and polarization, but a great many travel very long distances in space completely undisturbed.
Although plasma recombination and neutral atom acceleration by the solar gravitation may also contribute to an ENA population under certain conditions, the main exception to this creation scenario is the flux of interstellar gas, where neutral particles from the local interstellar medium penetrate the heliosphere with considerable velocity, which classifies them as ENAs as well.
Solar eruptions.
Solar flares and coronal mass ejections (CMEs) are the result of eruptions on the surface of the Sun, which may provide another source of ENAs. The STEREO spacecraft detected neutral hydrogen atoms with energies in the 2–5 MeV range from the flare/CME SOL2006-12-05. These particles were not detected with an instrument designed to see ENAs, but there was sufficient ancillary data to make the observation quite unambiguous.
Accelerating ENAs without ionizing them would be difficult, so the observed ENAs were interpreted to have resulted from charge exchange between solar energetic particles (SEP) emitted from the flare/CME with helium atoms in the solar wind. Charge exchange then occurred between the extremely fast SEP protons and the slower solar wind helium atoms, to create the highly neutral hydrogen atoms and slower helium ions. The resulting ENAs propagated through space without being bound to follow the Parker Spiral, so were observed near the Earth before the helium ions that were created in this reaction. This event, which occurred in 2006, was the first observation of ENAs produced by solar eruptions.
Species of ENAs.
Proton–hydrogen charge-exchange collisions are often the most important process in space plasma because hydrogen is the most abundant constituent of both plasmas and background gases. Hydrogen charge-exchange occurs at very high velocities involving little exchange of momentum, so the resulting ENAs travel at high speeds.
In general, only a few species are important for ENA formation, namely hydrogen, helium, oxygen and sulfur:
The corresponding neutral gases corresponding to these regions of space are:
Energies.
ENAs are found everywhere in space and are directly observable at energies from 10eV to more than 1 MeV. Their energies are described more with reference to the instruments used for their detection than to their origins.
No single particle analyzer can cover the entire energy interval from 10 eV to beyond 1 MeV. ENA instruments are roughly divided into those that can detect low, medium and high energies in overlapping groups that can be arbitrary and vary from author to author. The low, medium and high energy ranges from one author is shown in the graph along with the energy ranges for the three instruments aboard the IMAGE satellite:
Atoms are usually considered ENAs if they have kinetic energies of gases clearly higher than that can be reached by typical thermodynamic planetary atmospheres, which is usually in excess of 1 eV. This classification is somewhat arbitrary, being driven by the lower limits of ENA measurement instrumentation. The high end limitations are imposed by both measurement techniques and for scientific reasons.
Magnetospheric ENA imaging.
Magnetospheres are formed by the solar wind plasma flow around planets with an intrinsic magnetic field (Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune), although planets and moons lacking magnetic fields may sometimes form magnetosphere-like plasma structures. The ionospheres of weakly magnetized planets such as Venus and Mars set up currents that partially deflect the solar wind flow around the planet. ENAs have been observed in a range of planetary magnetospheres throughout the Solar System.
Although magnetospheric plasma fluctuation has very low densities; e.g. near Jupiter's moon Europa, plasma pressures are about 10−13 bar, compared to 1 bar at Earth's surface, and are responsible for magnetospheric dynamics and emissions. For example, geomagnetic storms create serious disturbances in Earth's cable communications systems, navigational systems and power distribution systems.
The strength and orientation of the magnetic field with respect to solar wind flow determines the shape of the magnetosphere. It is usually compressed on the day side and elongated at the night side.
Earth's magnetosphere.
The Earth's magnetic field forms a magnetic cavity within the solar wind. Energetic particles in this region of space can have major space weather impacts, including damaging satellites that orbit the Earth and presenting hazards for astronauts. ENA imaging is important to understand the dynamics of these energetic particles, which then allows for mitigation of these space weather effects. 
ENA detection in Earth's magnetosphere.
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 in 1970 to an altitude of at Wallops Island off the coast of Virginia. In 1972 and 1973, the presence of ENA signatures explained inconsistencies in measurements by the IMP-7 and 8 satellites.
ENA data from the NASA/ESA ISEE 1 satellite enabled the construction of the first global image of the storm time ring current in 1982. This was a breakthrough that paved the way for the use of ENAs as a powerful imaging technique. ENAs were also detected during the 1982 magnetic storm by SEEP instrument on the NASA S81-1 spacecraft. In 1989, the exospheric hydrogen atom population around Earth was extensively studied by the NASA Dynamic Explorer (DE-1) satellite.
An instrument with a dedicated high-energy ENA detection channel was flown aboard the 1991 NASA CRRES satellite. A more sophisticated high energy particle instrument was launched on the 1992 NASA/ISAS GEOTAIL spacecraft dedicated to observing Earth's magnetosphere astronomy. Precipitating ENAs can be studied from a low Earth orbit and were measured "looking out" by CRRES and the 1995 Swedish ASTRID satellites.
The new millennium saw ENA Imaging coming into its own. Extensive and detailed observations of the Earth's magnetosphere were made with three ENA instruments aboard the NASA IMAGE Mission from 2000 – 2005. In July 2000, a set of ENA images of the Earth's ring current were made during a geomagnetic storm. (See image at the top of the page.) The storm was triggered by a fast coronal mass ejection that erupted from the Sun on July 14, 2000 and arrived at Earth the next day.
Launched in 2008, the NASA TWINS Mission (two wide-angle Imaging Neutral-atom Spectrometers) provides the capability for stereoscopically imaging the magnetosphere. By imaging ENAs over a broad energy range (~1–100 KeV) using identical instruments on two widely spaced high-altitude, high-inclination spacecraft, TWINS enables 3-dimensional visualization and the resolution of large-scale structures and dynamics within the magnetosphere.
Planetary and other magnetospheres.
Magnetospheres of other planets have been studied by flyby spacecraft by orbiters, landers and by Earth-based observations.
Earth's Moon.
In February 2009, the ESA SARA LENA instrument aboard India's Chandrayaan-1 detected hydrogen ENAs sputtered from the lunar surface by solar wind protons. Predictions had been that all impacting protons would be absorbed by the lunar regolith but, for an as yet unknown reason, 20% of them are bounced back as low energy hydrogen ENAs. It is hypothesized that the absorbed protons may produce water and hydroxyls in interactions with the regolith. The Moon has no magnetosphere.
Mercury.
Launched in 2018, the ESA BepiColombo mission includes ENA instruments to further its objective to study the origin, structure and dynamics of Mercury's magnetic field. The LENA instrument will resemble the SARA instrument sent to Earth's Moon. In addition to magnetospheric ENAs, sputtering from Mercury's surface is also expected.
Venus.
Launched in 2005, the ESA VEX (Venus Express) mission's ASPERA (Energetic Neutral Atoms Analyser) consists of two dedicated ENA detectors. In 2006 ENA images were obtained of the interaction between the solar wind and the Venusian upper atmosphere, showing massive escape of planetary oxygen ions.
Mars.
Launched in 2003, the ESA MEX (Mars Express) mission's ASPERA instrument has obtained images of the solar wind interacting with the upper Martian atmosphere. The 2004 observations show solar wind plasma and accelerated ions very deep in the ionosphere, down to . above the dayside planetary surface—evidence for solar wind atmospheric erosion.
Jupiter.
The GAS instrument on the ESA/NASA Ulysses, launched in 1990, produced unique data on interstellar helium characteristics and ENAs emitted from Jupiter's Io torus. On its Jupiter flyby in 2000, the NASA/ESA/ASI Cassini's INCA instrument confirmed a neutral gas