Energetic Neutral Atom (ENA) imaging is a method used to make images of events that are usually invisible in the magnetospheres of planets and across the heliosphere.

Charged particles, such as protons, electrons, and atomic nuclei, are released by the solar wind and form the interstellar medium. These charged particles can be guided by magnetic fields, like Earth's magnetic field. Sometimes, charged particles in the solar wind's plasma collide with neutral atoms. This collision changes the charged particle into a neutral atom. Because it loses its charge, the atom is no longer affected by magnetic forces but still has gravity and speed.

ENAs are used to image events in the magnetospheres of planets and throughout the heliosphere.

Earth's magnetosphere helps protect its atmosphere and shields life from harmful radiation. This area of space is where geomagnetic storms occur, which can interfere with communication systems and expose people in airplanes or spacecraft to radiation. Geomagnetic weather systems have not benefited as much from satellite images as other weather systems because they are hard to see due to their connection with magnetospheric plasma.

The heliosphere protects the Solar System from most cosmic rays, but it is so far away that only ENA imaging can show its features. The shape of the heliosphere is caused by the interaction between the solar wind and cold gas from the space between stars.

The formation of ENAs by space plasma was predicted, but their discovery happened both by design and by chance. While early attempts were made to detect them, their presence also explained unclear results from ion detectors in areas with few ions. Ion detectors were later used to study ENAs in other low-ion regions. However, creating specialized ENA detectors required solving major challenges related to doubt and technology.

Although ENAs were observed in space from the 1960s through the 1980s, the first ENA camera was not launched until 1995 on the Swedish Astrid-1 satellite to study Earth's magnetosphere.

Specialized ENA instruments have created detailed images of magnetospheres around Venus, Mars, Jupiter, and Saturn. Images of Saturn's magnetosphere taken by Cassini showed complex interactions that are not yet fully understood. The IMAGE mission used three ENA cameras to observe Earth's magnetosphere from 2000 to 2005, while the TWINS Mission, launched in 2008, uses two satellites to take 3D ENA images of Earth's magnetosphere.

The first images of the heliosphere's edge, published in October 2009, were captured by ENA instruments on the IBEX and Cassini spacecraft. These images challenge current theories about the heliosphere.

Creation of ENAs

The most common ion in space plasma is the hydrogen ion, which is a proton without any electrons that can emit visible light. Other plasma ions are sometimes seen, but they are not clear enough to be used for imaging. Energetic neutral atoms (ENAs) are formed when high-energy plasma ions from the Sun collide with cold neutral atoms in space. These collisions happen often in areas like planetary magnetospheres and near the edge of the heliosphere.

During a charge-exchange collision, a high-energy plasma ion takes an electron from a low-energy neutral atom. This creates a cold ion and an energetic neutral atom (ENA). This process can be written as:

I₁ + A₂ → A₁ + I₂

Here, I₁ is the plasma ion, A₂ is the neutral atom, A₁ is the ENA, and I₂ is the ion after the collision. The two species in the reaction can be the same, such as in proton-hydrogen collisions:

H⁺ + H → H + H⁺

Sometimes, more than one electron is exchanged during the reaction. An example is the alpha-helium charge-exchange:

He⁺ + He → He + He⁺

Because ENAs are neutral, they are only affected by gravity. Charged particles, like ions and electrons, are also influenced by electromagnetic forces, which are usually ignored in space because gravity is weaker. This means ENAs often keep the momentum of the original plasma ion before the collision.

Some ENAs are lost due to further collisions or other processes, but many travel long distances without being disturbed. Plasma recombination and solar gravity can also create ENAs in some cases. However, the main exception is interstellar gas, where neutral particles from space enter the heliosphere and are classified as ENAs.

Solar flares and coronal mass ejections (CMEs) can produce ENAs. In 2006, the STEREO spacecraft detected neutral hydrogen atoms with energies between 2–5 MeV from a solar flare or CME. These particles were not seen by an ENA-specific instrument, but other data confirmed their presence. Scientists believe these ENAs formed when solar energetic particles (SEPs) from the flare collided with helium atoms in the solar wind. This created fast hydrogen atoms and slower helium ions. The ENAs traveled through space without following the Parker Spiral, reaching Earth before the helium ions. This was the first observation of ENAs from solar eruptions.

Hydrogen charge-exchange collisions are the most important process in space plasma because hydrogen is the most common element in both plasmas and background gases. These collisions happen at very high speeds with little change in momentum, so the resulting ENAs move quickly.

Only a few elements are important for creating ENAs: hydrogen, helium, oxygen, and sulfur. The neutral gases in space that match these elements include:

ENAs are found throughout space and can be observed with energies from 10 eV to over 1 MeV. Their energy levels depend more on the instruments used to detect them than on their origins.

No single instrument can detect all ENA energy levels. ENA detectors are usually grouped into low, medium, and high energy ranges, though these groups may vary between scientists. The energy ranges used by the IMAGE satellite’s instruments are shown in a graph.

Atoms are considered ENAs if their energy is much higher than what is typical in planetary atmospheres, usually more than 1 eV. This classification depends on the limits of measurement tools. The highest energy limits are set by both measurement methods and scientific needs.

Magnetospheric ENA imaging

Magnetospheres form when the solar wind moves around planets that have their own magnetic fields (Mercury, Earth, Jupiter, Saturn, Uranus, and Neptune). Some planets and moons without magnetic fields can still create structures that act like magnetospheres. The ionospheres of planets with weak magnetic fields, such as Venus and Mars, create electric currents that help redirect the solar wind around the planet. Energetic Neutral Atoms (ENAs) have been observed in magnetospheres across the Solar System.

Even though magnetospheric plasma has very low density, the pressure can be high. For example, near Jupiter’s moon Europa, the pressure is about 10 bar, compared to 1 bar at Earth’s surface. These pressures influence magnetospheric activity and emissions. For instance, geomagnetic storms can cause problems for Earth’s communication systems, navigation systems, and power grids.

The strength and direction of a planet’s magnetic field compared to the solar wind determine the shape of its magnetosphere. It is usually squeezed on the side facing the Sun and stretched on the opposite side.

Earth’s magnetic field creates a cavity in the solar wind. High-energy particles in this region can affect space weather, damaging satellites and posing risks to astronauts. ENA imaging helps scientists study these particles, which can help reduce the effects of space weather.

The first ENA instrument was launched on a Nike–Tomahawk sounding rocket from Fort Churchill, Manitoba, Canada. A similar instrument was later sent on a Javelin rocket to an altitude of 840 kilometers (520 miles) at Wallops Island, Virginia, in 1970. In 1972 and 1973, ENA signatures helped explain unusual data from the IMP-7 and 8 satellites.

In 1982, data from the NASA/ESA ISEE 1 satellite allowed scientists to create the first global image of Earth’s storm-time ring current. This was a major discovery that showed how ENAs could be used as an imaging tool. ENAs were also detected during a 1982 magnetic storm by the SEEP instrument on the NASA S81-1 spacecraft. In 1989, the NASA Dynamic Explorer (DE-1) satellite studied hydrogen atoms in Earth’s exosphere.

A dedicated high-energy ENA detection instrument was sent on the 1991 NASA CRRES satellite. A more advanced instrument was launched on the 1992 NASA/ISAS GEOTAIL spacecraft to study Earth’s magnetosphere. Precipitating ENAs can be observed from low Earth orbit, as measured by CRRES and the 1995 Swedish ASTRID satellites.

In the new millennium, ENA imaging became more advanced. The NASA IMAGE Mission (2000–2005) used three ENA instruments to study Earth’s magnetosphere in detail. In July 2000, ENA images of Earth’s ring current were captured during a geomagnetic storm caused by a coronal mass ejection from the Sun.

The NASA TWINS Mission (launched in 2008) uses two identical ENA instruments on spacecraft to create 3D images of Earth’s magnetosphere. This helps scientists study large-scale structures and movements within the magnetosphere.

Magnetospheres of other planets have been studied using spacecraft, orbiters, landers, and Earth-based observations.

In February 2009, the ESA SARA LENA instrument on India’s Chandrayaan-1 detected hydrogen ENAs on the Moon’s surface, produced by solar wind protons. Scientists predicted that all protons would be absorbed by the Moon’s surface, but 20% are reflected as low-energy hydrogen ENAs. This may be related to water or hydroxyls forming in the Moon’s regolith. The Moon has no magnetosphere.

The ESA BepiColombo mission (launched in 2018) includes ENA instruments to study Mercury’s magnetic field. The LENA instrument will be similar to the one used on the Moon. It will also study sputtering from Mercury’s surface.

The ESA VEX (Venus Express) mission (launched in 2003) used the ASPERA instrument to study interactions between the solar wind and Venus’s atmosphere. In 2006, ENA images showed oxygen ions escaping from Venus’s atmosphere.

The ESA MEX (Mars Express) mission (launched in 2003) used ASPERA to study the solar wind’s interaction with Mars’s atmosphere. Observations in 2004 showed solar wind plasma and accelerated ions deep in Mars’s ionosphere, indicating atmospheric erosion.

The GAS instrument on the ESA/NASA Ulysses spacecraft (launched in 1990) studied interstellar helium and ENAs from Jupiter’s Io torus. The Cassini spacecraft (launched in 1997) confirmed a neutral gas torus near Jupiter’s moon Europa and found hydrogen atoms in Jupiter’s magnetosphere.

The first dedicated ENA camera was on the Cassini mission to study Saturn’s magnetosphere. Saturn’s main radiation belt extends from 70,000 kilometers (43,000 miles) to 783,000 kilometers (487,000 miles) above its surface. Cassini also found a previously unknown inner belt near Saturn.

Saturn’s magnetosphere behaves differently from Earth’s. Plasma in Saturn’s magnetosphere moves with the planet’s rotation. Saturn’s strong magnetic field and fast spin create an electric field that accelerates plasma. This leads to complex interactions with the atmosphere of its moon Titan.

Cassini’s MIMI-INCA instrument studied Titan’s magnetospheric interactions with its dense atmosphere. Scientists have studied Titan’s ENA emissions extensively.

NASA’s Voyager 2 explored Uranus and Neptune, the only spacecraft to do so. In 1986, Voyager 2 discovered a large and unusual magnetic field around Uranus. More detailed studies of Uranus and Neptune are still needed.

Heliosphere ENA imaging

The heliosphere is a large area of space created when the solar wind pushes outward against the pressure of gas found between stars, called the local interstellar medium (LISM). The solar wind is made of charged particles, which carry the Sun's magnetic field with them. This means the heliosphere can be thought of as the Solar System's magnetosphere. The edge of the heliosphere is far beyond the orbit of Pluto, where the weakening solar wind is stopped by the pressure of the LISM.

The neutral gas used to create energetic neutral atoms (ENAs) at the edge of the heliosphere mainly comes from gas from space that enters the heliosphere. A very small amount of this gas comes from solar wind particles that neutralize dust near the Sun. The boundaries of the heliosphere cannot be seen and change over time. Even though the gas density is low, the large size of the heliosphere makes it a major source of ENAs, along with planetary magnetospheres. Because the properties of ENAs depend strongly on the heliosphere, imaging techniques that use ENAs can provide a complete view of the heliosphere’s structure and movement, which cannot be achieved in other ways.

In October 2009, NASA’s IBEX Mission shared the first image of an unexpected ENA ribbon at the edge of the heliosphere. This ribbon was very narrow and two to three times brighter than other areas in space, but it was not seen by the Voyager 1 or Voyager 2 spacecraft in that region.

The Cassini mission also used ENA imaging to study the heliosphere. Its findings support and add to the IBEX results, helping scientists create the first full map of the heliosphere. Early Cassini data suggest the heliosphere may not look like a comet, as some models predicted, but instead may appear more like a large, round bubble.

Estimates of the heliosphere’s size range from 150 to 200 astronomical units (AU). It is believed that Voyager 1 crossed the boundary called the termination shock in 2002 at about 85 to 87 AU, while Voyager 2 crossed it in 2007 at around 85 AU. Some scientists think the termination shock is about 100 AU away on average. The solar wind changes strength over an 11-year cycle, causing the heliosphere’s size and shape to change slightly, a process called "heliosphere breathing."

The great distances involved make it difficult to measure the layers of the heliosphere directly. Voyager 1 and 2 took 27 and 30 years, respectively, to reach the termination shock. Also, when observing from far away, high-energy and slower ENAs emitted at the same time would be detected at different times. This time difference ranges from 1 to 15 minutes when observing Earth’s magnetosphere from a spacecraft to more than a year when imaging the heliosphere from Earth orbit.

ENA instruments

The study of ENAs (Energetic Neutral Atoms) was expected to improve understanding of space processes, but progress was slow because of big experimental challenges. In the late 1960s, early attempts to measure ENAs showed how difficult this was. ENA fluxes are very weak, sometimes less than 1 particle per square centimeter each second. They are usually detected when they hit a solid surface, causing electrons to be released. These ENAs exist in areas with strong ultraviolet (UV) and extreme ultraviolet (EUV) radiation, which are 100 times more intense than what causes similar emissions.

An ideal ENA instrument would need to:

The challenge of using ENAs for remote sensing is combining mass spectrometry with imaging weak particle streams, while meeting strict spacecraft design limits. Early efforts showed that instruments must focus on specific ENA energy levels. The following describes, in simple terms, how high-energy (HENA) and medium-energy (MENA) ENA instruments work, with differences noted. The HENA camera used on NASA’s IMAGE mission is an example of such an instrument.

A set of electrostatic plates uses electric forces to direct charged particles away from the instrument and narrow the direction of incoming neutral atoms to a few degrees.

For HENA: Time of flight (TOF) is measured using a method that also helps reduce background noise from photons. An ENA passes through a thin film to a particle energy detector, where most of its energy is preserved. At the same time, electrons scattered by the film are redirected to a detector to create a start signal. When the ENA hits a solid state detector (SSD), it creates an end signal. The position of the impact helps determine the ENA’s path length. The start and end signals together allow TOF to be calculated.

If electrons are scattered by photons, no end signal will be created. If no end signal is detected within a time expected for the particle’s energy, the start signal is ignored.

For MENA: Medium-energy ENAs would lose too much energy passing through the thin film used in HENA instruments. A thinner film would be damaged by UV and EUV radiation, so a gold diffraction grating is used to block photons. An ultra-thin carbon film is placed behind the grating. ENAs pass through the grating and film to hit an SSD, scattering electrons and allowing path length and TOF measurements, similar to the HENA method.

Knowing path length and TOF allows velocity to be calculated. The SSD records the energy of the ENA after it passes through the film. A small energy loss from the film is corrected through instrument calibration.

Using energy and velocity, the particle’s mass can be calculated with the formula energy = (mass × velocity²)/2. Alternatively, the number of scattered electrons can also help determine the ENA’s mass.

Most mass resolution requirements are simple, requiring the ability to distinguish between hydrogen (1 AMU), helium (4 AMU), oxygen (16 AMU), and sulfur (32 AMU) atoms, as sulfur is found in Jupiter’s magnetosphere.

Images from a spinning spacecraft provide the second dimension of direction. Combining data from two satellites allows stereo imaging. Results from the TWINS Mission are expected to provide more information about Earth’s magnetosphere in three dimensions.

While the collimator is similar, low-energy instruments like NASA’s GSFC LENA use a foil-stripping technique. Incident ENAs hit a surface like tungsten, creating ions analyzed by an ion spectrometer.

To detect atoms from the lunar surface and lighter ENAs, the ESA LENA on Chandrayaan-1 used a mass spectrometer capable of identifying heavier elements like sodium, potassium, and iron.

By 2005, only six ENA detectors had been launched. The TWINS and IBEX missions added three more by 2009, increasing the total by 50% in four years. ENA imaging is becoming a useful tool for studying space plasma.

Improvements are still needed. Angular resolution is now a few degrees, and different particle types can be separated. However, expanding the energy range to about 500 keV is a challenge. This range covers much of Earth’s inner magnetosphere and some high-energy radiation belts, making it important for imaging.

For lower-energy ENAs below 1 keV, imaging relies on analyzing ions stripped from a surface by the ENA. Better sub-keV measurements are needed to image Mercury’s magnetosphere, due to its smaller magnetic field and size.

Importance for Earth

The heliosphere acts as a protective shield for the Solar System, similar to how Earth's magnetosphere protects Earth. The information gained from ENAs about how space plasma behaves has been important in helping scientists understand the space environment.

Without the magnetosphere, Earth would be hit directly by the solar wind and might not be able to hold onto its atmosphere. In addition to greater exposure to solar radiation, life on Earth would likely not exist without the magnetosphere. Likewise, the heliosphere shields the Solar System from most harmful cosmic rays, with the remaining rays being redirected by Earth's magnetosphere.

Even though many satellites are protected by the magnetosphere, geomagnetic storms can create electrical currents in conductors that interfere with communication systems in space and on Earth. Understanding the magnetosphere, the ring current, and how they interact with the solar wind during periods of high solar activity helps scientists better protect satellites and space missions.

Astronauts on deep space missions will not have Earth's natural protections. Therefore, understanding the factors that influence their exposure to cosmic rays and the solar wind is very important for future crewed space exploration.