Energetic neutral atom

Energetic neutral atoms are charged particles—protons, electrons, and various atomic nuclei—emitted from solar wind that 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.
ENAs are used for imaging phenomena in the magnetospheres of planets and throughout the heliosphere.
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 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.
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 neutral atom gives electrons to the ion, producing a cold ion and an energetic neutral atom. This 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 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 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

s and coronal mass ejections 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 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:

Atomic hydrogen dominates Earth's neutral particle environment from altitudes of to. This altitude variation occurs as the solar cycle varies from solar minimum to solar maximum.
The interstellar and solar winds are mainly protons, with the solar wind also containing ~5% alpha particles.
Planetary magnetospheric plasma consists mostly of protons with some helium and oxygen.
Helium and oxygen are also important species in the Earth's inner magnetosphere, particularly in regions of ionospheric outflow.
Jupiter's magnetosphere additionally contains sulfur ions, due to the volcanic activity of its moon Io.

The corresponding neutral gases corresponding to these regions of space are:

the geocorona for the Earth's magnetosphere wind
a planetary exosphere for a planetary magnetosphere
the local interstellar medium in the boundary region of the heliosphere at the termination shock and the heliopause.
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:

a high energy instrument, HENA measuring 10–500 Kev energy to study Earth's ring current;
a medium ENA instrument, MENA measuring 1–30 Kev to study the plasma sheet; and
a low ENA instrument measuring between 10 eV and 500 eV to study the ionospheric source of ions flowing from the polar cap.

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, 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.