Energetic neutral atom
Encyclopedia
Energetic neutral atom (ENA) imaging, often described as "seeing with atoms", is a technology used to create global images of otherwise invisible phenomena in the magnetospheres of planets and at the boundary of the heliosphere
- the far flung outer edge of the solar system
.
ENA images are constructed from the detection of energetic neutral atoms that are created in charge-exchange processes between ions in a hot plasma
, such as the solar wind
, and the atoms of a cold neutral background gas.
Earth's magnetosphere
preserves Earth's atmosphere
and protects us 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 at high polar altitudes or in orbiting spacecraft. A deeper understanding of this region is vitally important. 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 plasmas
present the added problem of invisibility.
The heliosphere
protects the entire 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 invisible interaction between the solar wind and cold gas from the local interstellar medium
.
The creation of ENAs by space plasmas 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 1980s, the first dedicated ENA camera was not flown until 1997 with the NASA/ESA/ASI Cassini mission, to study Saturn's magnetosphere. Cassini's ENA images of Saturn revealed a unique magnetosphere with complex interactions that have yet to be fully explained.
Today, dedicated ENA instruments have provided detailed magnetospheric images from Venus
, Mars
, Jupiter
, and Saturn
. 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 heliospheric boundary, published in October 2009, were made by the ENA instruments aboard the IBEX and Cassini spacecraft. These images are very exciting because they challenge existing theories about the region.
—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.
I1+ + A2 → A1 + I2+
where
Species 1 and 2 may be the same or different and an exchange of two electrons is possible, e.g.
H+ + H → H + H+
Proton–hydrogen charge-exchange
or
He2+ + He → He + He2+
alpha-helium charge-exchange.
Due to its charge
neutrality
, the resulting ENA is subject to gravitation
al forces only. Because gravitation influences can normally be ignored, it is safe 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, 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.
is the most abundant constituent of both plasmas and background gases and hydrogen charge-exchange occurs at very high velocities
involving little exchange of momentum
.
In general, only a few species are important for ENA formation, namely hydrogen
, helium
, oxygen
and sulfur
:
No single particle analyzer can cover the entire energy interval from 10 eV to beyond 1 MeV. ENA instruments are roughly divided into low, medium and high overlapping groups that can be arbitrary and vary from author to author. The low, medium and high energy range 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
clearly higher than 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.
(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.
Although magnetospheric plasmas have 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.
Understanding the magnetosphere increased in importance with the realization of the detrimental impact of geomagnetic storms, caused by solar coronal mass ejections, particularly in years of high solar activity. In addition to long known effects on Earth's cable communication systems, communications, broadcasting, navigation and security applications are increasingly dependent on satellites. Most of these satellites are well within the protective magnetosphere but are vulnerable to space weather systems that affect them adversely. There are also radiation hazards for humans traveling at high polar altitudes or in orbiting spacecraft Many countries, including the U.S., provide a Space Weather Service reporting existing or predicted Geomagnetic Storms, Solar Radiation Storms and Radio Blackouts.
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 on 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. 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.
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 hydroxyl
s in interactions with the regolith. The moon has no magnetosphere.
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.
) 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.
) 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 270 km. above the dayside planetary surface—evidence for solar wind atmospheric erosion.
On its Jupiter flyby in 2000, the NASA/ESA/ASI Cassini's INCA instrument confirmed a neutral gas torus associated with Europa. Cassini's ENA images also showed Jupiter's magnetosphere to be dominated by hydrogen atoms ranging from a few to 100 keV. The atoms are emitted from the planet's atmosphere and from neutral gas tori near the inner Galilean moons. A population of heavier ions was also detected, indicating a significant emission of oxygen and/or sulfur from Jupiter's magnetosphere.
magnetosphere.
Saturn's main radiation belt was measured beginning at an altitude 70,000 km from its surface and reaching out to 783,000 km. Cassini also detected a previously unknown inner belt nearer its surface that is about 6,000 km thick.
The dynamics of Saturn's magnetosphere are very different from Earth's. Plasma co-rotates with Saturn in its magnetosphere. Saturn's strong magnetic field and rapid rotation create a strong co-rotational electric field that accelerates plasma in its magnetosphere until it reaches rotation speeds near that of the planet. Because Saturn's moons are essentially 'sitting still' in this very high speed flow, a complex interaction between this plasma and the atmosphere of the moon Titan was observed.
took advantage of its orbit to explore Uranus and Neptune, the only spacecraft to ever have done so. In 1986 spacecraft found a Uranian magnetic field that is both large and unusual. More detailed investigations have yet to be carried out.
is a cavity built up by the solar wind
as it presses outward against the pressure of the local interstellar medium
(LISM). As the solar wind is a plasma, it is charged and so carries with it the Sun's magnetic field. So the heliosphere can be conceptualized as the solar system's magnetosphere. The edge of the heliosphere is found far beyond the orbit
of Pluto
where diminishing solar wind pressure is stopped by the pressure from the LISM.
The background neutral gas for ENA production at the heliospheric boundary comes predominantly from interstellar gas penetrating the heliosphere. A tiny amount comes from solar wind neutralization of interplanetary dust near the sun. The heliospheric boundaries are invisible and fluctuating. Although the densities are low, the enormous thickness of the heliosheath
make it a dominant source of ENAs, aside from planetary magnetospheres. Because of the strong dependence of ENA characteristics on heliospheric properties, remote ENA imaging techniques will provide a global view of the structure and dynamics of the heliosphere unattainable by any other means.
The first glimpse of this view was announced in October, 2009, when the NASA IBEX Mission
, returned its first image of the unexpected ENA ribbon at the edge of the heliosphere
. Results revealed a previously unpredicted "very narrow ribbon that is two to three times brighter than anything else in the sky" at the edge of the heliosphere that was not detected by Voyager 1
and Voyager 2
in the region. These results are truly exciting as they do not match any existing theoretical models of this region.
Cassini also ENA-imaged the heliosphere and its results complement and extend the IBEX findings, making it possible for scientists to construct the first comprehensive sky map of the heliosphere. Preliminary Cassini data suggest the heliosphere may not have the comet-like shape predicted by existing models but that its shape may be more like a large, round bubble.
Estimates for size of the heliosphere vary between 150 – 200 AU
. It is believed that Voyager 1
passed the heliosphere's termination shock in 2002 at approx. 85 – 87 AU while Voyager 2
passed the termination shock in 2007 at about 85 AU. Others place the termination shock at a mean distance of ≈100 AU. Because the solar wind
varies by a factor of 2 during the 11 year solar cycle
, there will be variations in the size and shape of the heliosphere, known as heliosphere "breathing."
The huge distances involved mean we will never accumulate a large number of in situ measurements of the various layers of the heliosphere. Voyager 1 and 2 took 27 yrs. and 30 yrs. respectively to arrive at the termination shock. It is worth noting that for large distances to the object, high energy (velocity) and slower ENAs emitted simultaneously would be detected at different times. This time difference varies from 1 - 15 minutes for observing Earth's magnetosphere from a high altitude spacecraft to more than a year for imaging the heliospheric boundary from an Earth orbit.
In the late 1960s, the first direct ENA measurement attempts revealed the difficulties involved. ENA fluxes are very weak, sometimes less than 1 particle per cm2 per second and are typically detected by secondary electron emission upon contact with a solid surface. They exist in regions containing ultraviolet (UV) and extreme ultraviolet (EUV) radiation at fluxes 100 times greater that produce similar emissions.
An ENA instrument ideally would also specifically:
The challenge to remote sensing via ENAs lies in combining mass spectrometry with the imaging of weak particle fluxes within the stringent limitations imposed by an application on a spacecraft.
deflected to a detector to create a start pulse. The ENA arriving at its solid state detector (SSD) creates the end pulse and its impact position yields its trajectory and therefore path length. The start and stop signals enable TOF to be determined.
If the electrons are scattered by incoming photons, no ENA will be detected to create the stop pulse. If no stop pulse is sensed within an established time appropriate to the energy of the expected particles, the start pulse is discarded.
MENA: Medium energy ENAs would lose too much energy penetrating the film used in the HENA instrument. The thinner film required would be vulnerable to damage by incident UV and EUV. Therefore, photons are prevented from entering the instrument by using a gold difraction grating. An ultra thin carbon film is mounted on the back of the grating. ENAs pass through the grating and the film to impact a solid state detector (SSD), scattering electrons and allowing path length and TOF determinations as for the HENA above.
Knowing path length and TOF enables velocity to be determined.
Mass resolution requirements are normally modest, requiring at most distinguishing among hydrogen (1 amu), helium (4 amu), and oxygen (16 amu) atoms with sulfur (32 amu) also expected in Jupiter's magnetosphere.
Because of the need to detect atoms sputtered
from the lunar surface as well lighter ENAs, the ESA LENA on the Chandrayaan-1 incorporated a mass spectrometer designed to resolve heavier masses including sodium
, potassium
, and iron
.
Several improvements are still needed to perfect the technique. Although the angular resolution has now decreased to a few degrees and different species can be separated, one challenge is to expand the energy range upwards to about 500 keV. This high energy range covers most of the plasma pressure of Earth's inner magnetosphere as well as some of the higher-energy radiation belts so is desirable for terrestrial ENA imaging.
For lower energy ENAs, below 1 keV, the imaging techniques are completely different and rely on the spectroscopic analysis of ions stripped from a surface by the impinging ENA. Improvements in sub-keV measurements will be needed to image Mercury's magnetosphere due to the consequences of its smaller magnetic field and it smaller geometry.
The heliosphere is a protective cocoon for the Solar System, just as the Earth's magnetosphere is a protective cocoon for the Earth. The insight provided by ENAs into the behaviour of space plasmas improves our understanding of these protective mechanisms.
Without the magnetosphere, Earth would be subject to direct bombardment by the solar wind and may be unable to retain an atmosphere. This, plus increased exposure to solar radiation means that life on Earth as we know it would not be possible without the magnetosphere. Similarly, the heliosphere protects the Solar System from the majority of otherwise damaging cosmic rays, with the remainder being deflected by the Earth's magnetosphere.
Although most orbiting satellites are protected by the magnetosphere, geomagnetic storms induce currents in conductors that disrupt communications both in space and in cables on the ground. Better understanding of the magnetosphere and the ring current and its interaction with the solar wind during high solar activity will allow us to better protect these assets.
Astronauts on deep space missions will not have Earth's protections so understanding the factors that may affect their exposure to cosmic rays and the solar wind is critical to manned space exploration.
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
- the far flung outer edge of the solar system
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
.
ENA images are constructed from the detection of energetic neutral atoms that are created in charge-exchange processes between ions in a hot plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
, such as the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, and the atoms of a cold neutral background gas.
Earth's magnetosphere
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
preserves Earth's atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...
and protects us from cell damaging radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
. This region of "space weather
Space weather
Space weather is the concept of changing environmental conditions in near-Earth space or thespace from the Sun's atmosphere to the Earth's atmosphere. It is distinct from the concept ofweather within the Earth's planetary atmosphere...
" is the site of geomagnetic storms that disrupt communications systems and pose radiation
Radiation
In physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
hazards to humans traveling at high polar altitudes or in orbiting spacecraft. A deeper understanding of this region is vitally important. Geomagnetic weather systems have been late to benefit from the satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
imagery taken for granted in weather forecasting, and space physics because their origins in magnetospheric plasmas
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
present the added problem of invisibility.
The heliosphere
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
protects the entire 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 invisible interaction between the solar wind and cold gas from the local interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
.
The creation of ENAs by space plasmas 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 1980s, the first dedicated ENA camera was not flown until 1997 with the NASA/ESA/ASI Cassini mission, to study Saturn's magnetosphere. Cassini's ENA images of Saturn revealed a unique magnetosphere with complex interactions that have yet to be fully explained.
Today, dedicated ENA instruments have provided detailed magnetospheric images from Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
, Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
. 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 heliospheric boundary, published in October 2009, were made by the ENA instruments aboard the IBEX and Cassini spacecraft. These images are very exciting because they challenge existing theories about the region.
Creation of ENAs
The most abundant ion in space plasmas is the hydrogen ionHydrogen ion
Hydrogen ion is recommended by IUPAC as a general term for all ions of hydrogen and its isotopes.Depending on the charge of the ion, two different classes can be distinguished: positively charged ions and negatively charged ions....
—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 'steals' electrons from the neutral atom, producing a cold ion and an energetic neutral atom(ENA).I1+ + A2 → A1 + I2+
where
- I1+ plasma ion
- A2 background neutral atom (lower energy)
- A1 energetic neutral atom (ENA)
- I2+ lower energy ion
Species 1 and 2 may be the same or different and an exchange of two electrons is possible, e.g.
H+ + H → H + H+
Proton–hydrogen charge-exchange
or
He2+ + He → He + He2+
alpha-helium charge-exchange.
Due to its charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
neutrality
Neutral particle
In physics, a neutral particle is a particle with no electric charge. This is not to be confused with a real neutral particle, a neutral particle that is also identical to its own antiparticle.-Stable or long-lived neutral particles:...
, the resulting ENA is subject to gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
al forces only. Because gravitation influences can normally be ignored, it is safe to assume that the ENA preserves the vector momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
of the original pre-interaction plasma ion.
Some ENAs are lost in further charge-exchange, electron collisions and photoionization, but a great many travel very long distances in space completely undisturbed.
Although plasma recombination
Plasma recombination
Plasma recombination is a process by which ions of a plasma capture the free energetic electrons to form new neutral atoms.Recombination usually take place in the whole volume of a plasma , although in some cases it is confined to some special region of it...
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
Flux
In the various subfields of physics, there exist two common usages of the term flux, both with rigorous mathematical frameworks.* In the study of transport phenomena , flux is defined as flow per unit area, where flow is the movement of some quantity per time...
of interstellar gas, where neutral particles from the local interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
penetrate the heliosphere with considerable velocity, which classifies them as ENAs as well.
Species of ENAs
Proton–hydrogen charge-exchange collisions are often the most important process in space plasma because HydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
is the most abundant constituent of both plasmas and background gases and hydrogen charge-exchange occurs at very high velocities
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
involving little exchange of momentum
Momentum
In classical mechanics, linear momentum or translational momentum is the product of the mass and velocity of an object...
.
In general, only a few species are important for ENA formation, namely hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
, helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
:
- Atomic hydrogen dominates Earth's neutral particle environment from altitudes of 600 km to 1000 km (solar minimumSolar minimumSolar minimum is the period of least solar activity in the solar cycle of the sun. During this time, sunspot and solar flare activity diminishes, and often does not occur for days at a time...
- maximum.) - The interstellar and solar winds are mainly protons with the solar wind also containing ~5% alpha particles (He2+ )
- Helium and oxygen are also important Earth species.
- Planetary magnetospheric plasma consists mostly of protons with some helium and oxygen.
- Jupiter's magnetosphere contains sulfur ions as well, due to volcanic activity on the moon Io.
Background gases
The corresponding neutral gases are:- the geocoronaGeocoronaThe geocorona is the luminous part of the outermost region of the Earth's atmosphere, the exosphere. It is seen primarily via far-ultraviolet light from the Sun that is scattered from neutral hydrogen. It extends to at least 15.5 Earth radii...
for the Earth's magnetosphere - a planetary exosphereExosphereThe exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...
for a planetary magnetosphere - the local interstellar mediumInterstellar mediumIn astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
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 low, medium and high overlapping groups that can be arbitrary and vary from author to author. The low, medium and high energy range 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 currentRing currentA ring current is an electric current carried by charged particles trapped in a planet's magnetosphere. It is caused by the longitudinal drift of energetic particles.-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
Kinetic energy
The kinetic energy of an object is the energy which it possesses due to its motion.It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its acceleration, the body maintains this kinetic energy unless its speed changes...
clearly higher than can be reached by typical thermodynamic planetary atmospheres
Atmosphères
Atmosphères is a piece for full orchestra, composed by György Ligeti in 1961. It is noted for eschewing conventional melody and metre in favor of dense sound textures...
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 fieldMagnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
(Mercury
Mercury (planet)
Mercury is the innermost and smallest planet in the Solar System, orbiting the Sun once every 87.969 Earth days. The orbit of Mercury has the highest eccentricity of all the Solar System planets, and it has the smallest axial tilt. It completes three rotations about its axis for every two orbits...
, Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
, Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
, Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
, and Neptune
Neptune
Neptune is the eighth and farthest planet from the Sun in the Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times...
), although planets and moons lacking magnetic fields may sometimes form magnetosphere-like plasma structures. The ionospheres of weakly magnetized planets such as Venus
Venus
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. The planet is named after Venus, the Roman goddess of love and beauty. After the Moon, it is the brightest natural object in the night sky, reaching an apparent magnitude of −4.6, bright enough to cast shadows...
and Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
set up currents that partially deflect the solar wind flow around the planet.
Although magnetospheric plasmas have 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
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
. It is usually compressed on the day side and elongated at the night side.
Earth's magnetosphere
Earth's magnetic field dominates the terrestrial magnetosphere and prevents the solar wind from hitting us head on. Lacking a large protective magnetosphere, Mars is thought to have lost much of its former oceans and atmosphere to space in part due to the direct impact of the solar wind. Venus with its thick atmosphere is thought to have lost most of its water to space in large part owing to solar wind ablation.Understanding the magnetosphere increased in importance with the realization of the detrimental impact of geomagnetic storms, caused by solar coronal mass ejections, particularly in years of high solar activity. In addition to long known effects on Earth's cable communication systems, communications, broadcasting, navigation and security applications are increasingly dependent on satellites. Most of these satellites are well within the protective magnetosphere but are vulnerable to space weather systems that affect them adversely. There are also radiation hazards for humans traveling at high polar altitudes or in orbiting spacecraft Many countries, including the U.S., provide a Space Weather Service reporting existing or predicted Geomagnetic Storms, Solar Radiation Storms and Radio Blackouts.
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 840 km 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
Dynamics Explorer
Dynamics Explorer was a NASA mission, launched on August 3, 1981 and terminated on February 28, 1991. It consisted of two unmanned satellites, DE-1 and DE-2, whose purpose was to investigate the interractions between plasmas in the magnetosphere and those in the ionosphere...
(DE-1) satellite.
An instrument with a dedicated high-energy ENA detection channel was flown on the 1991 NASA CRRES
CRRES
The Combined Release and Radiation Effects Satellite was launched on July 25, 1990 into a geosynchronous transfer orbit for a nominal three-year mission to investigate fields, plasmas, and energetic particles inside the Earth's magnetosphere...
satellite. A more sophisticated high energy particle Instrument was launched on the 1992 NASA/ISAS GEOTAIL
GEOTAIL
GEOTAIL is a satellite observing the Earth's magnetosphere. It was developed by ISAS in association with NASA, and launched by a Delta II rocket on July 24, 1992....
spacecraft dedicated to observing Earth's magnetosphere. Precipitating ENAs can be studied from a low-earth orbit and were measured "looking out" by CRRES and the 1995 Swedish ASTRID
Astrid (satellites)
Astrid-1 and Astrid-2 were two microsatellites designed and developed by Swedish Space Corporation on behalf of the Swedish National Space Board. They were piggyback launched on a Cosmos-3M launch vehicle from Plesetsk, Russia...
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
IMAGE
IMAGE , or Explorer 78, was a NASA MIDEX mission that studied the global response of the Earth's magnetosphere to changes in the solar wind...
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 sputteredSputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
from the lunar surface by solar wind protons. Predictions had been that all impacting protons would be absorbed by the lunar regolith
Lunar soil
Lunar soil is the fine fraction of the regolith found on the surface of the Moon. Its properties can differ significantly from those of terrestrial soil...
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 hydroxyl
Hydroxyl
A hydroxyl is a chemical group containing an oxygen atom covalently bonded with a hydrogen atom. In inorganic chemistry, the hydroxyl group is known as the hydroxide ion, and scientists and reference works generally use these different terms though they refer to the same chemical structure in...
s in interactions with the regolith. The moon has no magnetosphere.
Mercury
The proposed 2014 ESA BepiColomboBepiColombo
BepiColombo is a joint mission of the European Space Agency and the Japan Aerospace Exploration Agency to the planet Mercury, due to launch in 2014. The mission is still in the planning stages so changes to the current description are likely over the next few years...
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 ExpressVenus Express
Venus Express is the first Venus exploration mission of the European Space Agency. Launched in November 2005, it arrived at Venus in April 2006 and has been continuously sending back science data from its polar orbit around Venus. Equipped with seven science instruments, the main objective of the...
) 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 ExpressMars Express
Mars Express is a space exploration mission being conducted by the European Space Agency . The Mars Express mission is exploring the planet Mars, and is the first planetary mission attempted by the agency. "Express" originally referred to the speed and efficiency with which the spacecraft was...
) 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 270 km. 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 torus associated with Europa. Cassini's ENA images also showed Jupiter's magnetosphere to be dominated by hydrogen atoms ranging from a few to 100 keV. The atoms are emitted from the planet's atmosphere and from neutral gas tori near the inner Galilean moons. A population of heavier ions was also detected, indicating a significant emission of oxygen and/or sulfur from Jupiter's magnetosphere.
Saturn
The first dedicated ENA camera was flown on the NASA/ESA/ASI Cassini mission, launched in 1997 to study Saturn'sSaturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
magnetosphere.
Saturn's main radiation belt was measured beginning at an altitude 70,000 km from its surface and reaching out to 783,000 km. Cassini also detected a previously unknown inner belt nearer its surface that is about 6,000 km thick.
The dynamics of Saturn's magnetosphere are very different from Earth's. Plasma co-rotates with Saturn in its magnetosphere. Saturn's strong magnetic field and rapid rotation create a strong co-rotational electric field that accelerates plasma in its magnetosphere until it reaches rotation speeds near that of the planet. Because Saturn's moons are essentially 'sitting still' in this very high speed flow, a complex interaction between this plasma and the atmosphere of the moon Titan was observed.
Uranus and Neptune
NASA's Voyager 2Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
took advantage of its orbit to explore Uranus and Neptune, the only spacecraft to ever have done so. In 1986 spacecraft found a Uranian magnetic field that is both large and unusual. More detailed investigations have yet to be carried out.
Heliospheric ENA imaging
The heliosphereHeliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
is a cavity built up by the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
as it presses outward against the pressure of the local interstellar medium
Interstellar medium
In astronomy, the interstellar medium is the matter that exists in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, dust, and cosmic rays. It fills interstellar space and blends smoothly into the surrounding intergalactic space...
(LISM). As the solar wind is a plasma, it is charged and so carries with it the Sun's magnetic field. So the heliosphere can be conceptualized as the solar system's magnetosphere. The edge of the heliosphere is found far beyond the orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
of Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
where diminishing solar wind pressure is stopped by the pressure from the LISM.
The background neutral gas for ENA production at the heliospheric boundary comes predominantly from interstellar gas penetrating the heliosphere. A tiny amount comes from solar wind neutralization of interplanetary dust near the sun. The heliospheric boundaries are invisible and fluctuating. Although the densities are low, the enormous thickness of the heliosheath
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
make it a dominant source of ENAs, aside from planetary magnetospheres. Because of the strong dependence of ENA characteristics on heliospheric properties, remote ENA imaging techniques will provide a global view of the structure and dynamics of the heliosphere unattainable by any other means.
The first glimpse of this view was announced in October, 2009, when the NASA IBEX Mission
Interstellar Boundary Explorer
Interstellar Boundary Explorer is a NASA satellite that will make the first map of the boundary between the Solar System and interstellar space. The mission is part of NASA's Small Explorer program. The IBEX satellite was launched with a Pegasus-XL rocket on October 19, 2008, at 17:47:23 UTC...
, returned its first image of the unexpected ENA ribbon at the edge of the heliosphere
Heliosphere
The heliosphere is a bubble in space "blown" into the interstellar medium by the solar wind. Although electrically neutral atoms from interstellar volume can penetrate this bubble, virtually all of the material in the heliosphere emanates from the Sun itself...
. Results revealed a previously unpredicted "very narrow ribbon that is two to three times brighter than anything else in the sky" at the edge of the heliosphere that was not detected by Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
and Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
in the region. These results are truly exciting as they do not match any existing theoretical models of this region.
Cassini also ENA-imaged the heliosphere and its results complement and extend the IBEX findings, making it possible for scientists to construct the first comprehensive sky map of the heliosphere. Preliminary Cassini data suggest the heliosphere may not have the comet-like shape predicted by existing models but that its shape may be more like a large, round bubble.
Estimates for size of the heliosphere vary between 150 – 200 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
. It is believed that Voyager 1
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
passed the heliosphere's termination shock in 2002 at approx. 85 – 87 AU while Voyager 2
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
passed the termination shock in 2007 at about 85 AU. Others place the termination shock at a mean distance of ≈100 AU. Because the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
varies by a factor of 2 during the 11 year solar cycle
Solar cycle
The solar cycle, or the solar magnetic activity cycle, is a periodic change in the amount of irradiation from the Sun that is experienced on Earth. It has a period of about 11 years, and is one component of solar variation, the other being aperiodic fluctuations. Solar variation causes changes in...
, there will be variations in the size and shape of the heliosphere, known as heliosphere "breathing."
The huge distances involved mean we will never accumulate a large number of in situ measurements of the various layers of the heliosphere. Voyager 1 and 2 took 27 yrs. and 30 yrs. respectively to arrive at the termination shock. It is worth noting that for large distances to the object, high energy (velocity) and slower ENAs emitted simultaneously would be detected at different times. This time difference varies from 1 - 15 minutes for observing Earth's magnetosphere from a high altitude spacecraft to more than a year for imaging the heliospheric boundary from an Earth orbit.
ENA instruments
Although the study of ENAs promised improvements in the understanding of global magnetospheric and heliospheric processes, its progress was hindered due to initially enormous experimental difficulties.In the late 1960s, the first direct ENA measurement attempts revealed the difficulties involved. ENA fluxes are very weak, sometimes less than 1 particle per cm2 per second and are typically detected by secondary electron emission upon contact with a solid surface. They exist in regions containing ultraviolet (UV) and extreme ultraviolet (EUV) radiation at fluxes 100 times greater that produce similar emissions.
An ENA instrument ideally would also specifically:
- prevent the entrance of charged particles
- suppress background light (photons), particularly UV and EUV radiation
- measure mass and energy of incoming ENAs
- determine trajectories of incoming ENAs
- measure ENA fluxes from 10−3 to 105 per cm2 per steradian per second
- measure ENAs ranging in energy from a few eV up to >100 keV
The challenge to remote sensing via ENAs lies in combining mass spectrometry with the imaging of weak particle fluxes within the stringent limitations imposed by an application on a spacecraft.
Medium and high energy ENA cameras
It became clear very early that to succeed, instruments would have to specialize in specific ENA energies. The following describes, in very simplified terms, a typical instrument function for high (HENA) or medium (MENA) energy instrument, with differences noted. The accompanying illustration is of the HENA camera flown on the NASA IMAGE mission and the description that follows most closely resembles IMAGE mission instruments.Collimator
A set of electrostatic plates deflect charged particles away from the instrument and collimates the beam of incoming neutral atoms to a few degrees.Photon rejection & time of flight (TOF)
HENA: TOF is determined by a coincidence detection requirement that turns out to be efficient at eliminating photon background noise as well. An ENA passes through a thin film to a particle energy detector with its energy nearly completely preserved. At the same time, electrons forward scattered from the film are electrostaticallyElectrostatics
Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....
deflected to a detector to create a start pulse. The ENA arriving at its solid state detector (SSD) creates the end pulse and its impact position yields its trajectory and therefore path length. The start and stop signals enable TOF to be determined.
If the electrons are scattered by incoming photons, no ENA will be detected to create the stop pulse. If no stop pulse is sensed within an established time appropriate to the energy of the expected particles, the start pulse is discarded.
MENA: Medium energy ENAs would lose too much energy penetrating the film used in the HENA instrument. The thinner film required would be vulnerable to damage by incident UV and EUV. Therefore, photons are prevented from entering the instrument by using a gold difraction grating. An ultra thin carbon film is mounted on the back of the grating. ENAs pass through the grating and the film to impact a solid state detector (SSD), scattering electrons and allowing path length and TOF determinations as for the HENA above.
Knowing path length and TOF enables velocity to be determined.
Energy
The solid state detector (SSD) impacted by the ENA after it passes through the foil registers its energy. The small energy loss due to passing through the foil is handled by instrument calibration.Mass
Knowing the energy and velocity, the mass of the particle can be calculated from energy = mv2/2. Alternatively, the number of scattered electrons detected can also serve to measure the mass of the ENA.Mass resolution requirements are normally modest, requiring at most distinguishing among hydrogen (1 amu), helium (4 amu), and oxygen (16 amu) atoms with sulfur (32 amu) also expected in Jupiter's magnetosphere.
2D and 3D imaging
Usually, obtaining images from a spinning spacecraft provides the second dimension of direction identification. By combining synchronized observations from two different satellites, stereo imaging becomes possible. Results from the TWINS Mission are eagerly awaited, as two viewing points will provide substantially more information about the 3-D nature of Earth's magnetosphere.Low energy ENA cameras
While the collimator is similar, low-energy instruments such as the NASA GSFC LENA use a foil-stripping technique. Incident ENAs interact with a surface such as tungsten to generate ions that are then analyzed by an ion spectrometer.Because of the need to detect atoms sputtered
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
from the lunar surface as well lighter ENAs, the ESA LENA on the Chandrayaan-1 incorporated a mass spectrometer designed to resolve heavier masses including sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, potassium
Potassium
Potassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
, and iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
.
Future
As of 2005, a total of only six dedicated ENA detectors had been flown. The launch of instruments aboard in the TWINS and IBEX missions brings the total to nine in 2009 - a 50% increase in only 4 years. Space plasma observation using ENA imaging is an emerging technology that is finally coming into its own.Several improvements are still needed to perfect the technique. Although the angular resolution has now decreased to a few degrees and different species can be separated, one challenge is to expand the energy range upwards to about 500 keV. This high energy range covers most of the plasma pressure of Earth's inner magnetosphere as well as some of the higher-energy radiation belts so is desirable for terrestrial ENA imaging.
For lower energy ENAs, below 1 keV, the imaging techniques are completely different and rely on the spectroscopic analysis of ions stripped from a surface by the impinging ENA. Improvements in sub-keV measurements will be needed to image Mercury's magnetosphere due to the consequences of its smaller magnetic field and it smaller geometry.
Importance for Earth
In addition to the obvious intellectual benefits brought by increased understanding of our space environment, there are many practical motivations for enhancing our knowledge of space plasmas.The heliosphere is a protective cocoon for the Solar System, just as the Earth's magnetosphere is a protective cocoon for the Earth. The insight provided by ENAs into the behaviour of space plasmas improves our understanding of these protective mechanisms.
Without the magnetosphere, Earth would be subject to direct bombardment by the solar wind and may be unable to retain an atmosphere. This, plus increased exposure to solar radiation means that life on Earth as we know it would not be possible without the magnetosphere. Similarly, the heliosphere protects the Solar System from the majority of otherwise damaging cosmic rays, with the remainder being deflected by the Earth's magnetosphere.
Although most orbiting satellites are protected by the magnetosphere, geomagnetic storms induce currents in conductors that disrupt communications both in space and in cables on the ground. Better understanding of the magnetosphere and the ring current and its interaction with the solar wind during high solar activity will allow us to better protect these assets.
Astronauts on deep space missions will not have Earth's protections so understanding the factors that may affect their exposure to cosmic rays and the solar wind is critical to manned space exploration.
External links
- List of Neutral Atom References
- List of LENA Team Publications