NASA Observatories Take an Unprecedented Look into Superstar Eta Carinae

In this supercomputer simulation, the stars of Eta Carinae are shown as black dots. Lighter colors indicate greater densities in the stellar winds produced by each star. At closest approach, the fast wind of the smaller star carves a tunnel in the thicker wind of the larger star. Image Credit: NASA's Goddard Space Flight Center/T. Madura

Eta Carinae, the most luminous and massive stellar system within 10,000 light-years of Earth, is known for its surprising behavior, erupting twice in the 19th century for reasons scientists still don't understand. A long-term study led by astronomers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, used NASA satellites, ground-based telescopes and theoretical modeling to produce the most comprehensive picture of Eta Carinae to date. New findings include Hubble Space Telescope images that show decade-old shells of ionized gas racing away from the largest star at a million miles an hour, and new 3-D models that reveal never-before-seen features of the stars' interactions.

"We are coming to understand the present state and complex environment of this remarkable object, but we have a long way to go to explain Eta Carinae's past eruptions or to predict its future behavior," said Goddard astrophysicist Ted Gull, who coordinates a research group that has monitored the star for more than a decade.

Located about 7,500 light-years away in the southern constellation of Carina, Eta Carinae comprises two massive stars whose eccentric orbits bring them unusually close every 5.5 years. Both produce powerful gaseous outflows called stellar winds, which enshroud the stars and stymy efforts to directly measure their properties. Astronomers have established that the brighter, cooler primary star has about 90 times the mass of the sun and outshines it by 5 million times. While the properties of its smaller, hotter companion are more contested, Gull and his colleagues think the star has about 30 solar masses and emits a million times the sun's light.

Seen in blue light emitted by doubly ionized iron atoms (4,659 angstroms), these images of Eta Carinae were captured by Hubble's STIS instrument between 2010 and 2014. Gas shells created during the binary's 2003 close approach race outward at about 1 million mph (1.6 million km/h). Image Credit: NASA's Goddard Space Flight Center/T. Gull et al.

Speaking at a press conference at the American Astronomical Society meeting in Seattle on Wednesday, the Goddard researchers discussed recent observations of Eta Carinae and how they fit with the group's current understanding of the system.

At closest approach, or periastron, the stars are 140 million miles (225 million kilometers) apart, or about the average distance between Mars and the sun. Astronomers observe dramatic changes in the system during the months before and after periastron. These include X-ray flares, followed by a sudden decline and eventual recovery of X-ray emission; the disappearance and re-emergence of structures near the stars detected at specific wavelengths of visible light; and even a play of light and shadow as the smaller star swings around the primary.

During the past 11 years, spanning three periastron passages, the Goddard group has developed a model based on routine observations of the stars using ground-based telescopes and multiple NASA satellites. "We used past observations to construct a computer simulation, which helped us predict what we would see during the next cycle, and then we feed new observations back into the model to further refine it," said Thomas Madura, a NASA Postdoctoral Program Fellow at Goddard and a theorist on the Eta Carinae team.

According to this model, the interaction of the two stellar winds accounts for many of the periodic changes observed in the system. The winds from each star have markedly different properties: thick and slow for the primary, lean and fast for the hotter companion. The primary's wind blows at nearly 1 million mph and is especially dense, carrying away the equivalent mass of our sun every thousand years. By contrast, the companion's wind carries off about 100 times less material than the primary's, but it races outward as much as six times faster.

Madura's simulations, which were performed on the Pleiades supercomputer at NASA's Ames Research Center in Moffett Field, California, reveal the complexity of the wind interaction. When the companion star rapidly swings around the primary, its faster wind carves out a spiral cavity in the dense outflow of the larger star. To better visualize this interaction, Madura converted the computer simulations to 3-D digital models and made solid versions using a consumer-grade 3-D printer. This process revealed lengthy spine-like protrusions in the gas flow along the edges of the cavity, features that hadn't been noticed before.    

"We think these structures are real and that they form as a result of instabilities in the flow in the months around closest approach," Madura said. "I wanted to make 3-D prints of the simulations to better visualize them, which turned out to be far more successful than I ever imagined." A paper detailing this research has been submitted to the journal Monthly Notices of the Royal Astronomical Society.

The team detailed a few key observations that expose some of the system's inner workings. For the past three periastron passages, ground-based telescopes in Brazil, Chile, Australia and New Zealand have monitored a single wavelength of blue light emitted by helium atoms that have lost a single electron. According to the model, the helium emission tracks conditions in the primary star's wind. 

The Space Telescope Imaging Spectrograph (STIS) aboard Hubble captures a different wavelength of blue light emitted by iron atoms that have lost two electrons, which uniquely reveals where gas from the primary star is set aglow by the intense ultraviolet light of its companion. Lastly, X-rays from the system carry information directly from the wind collision zone, where the opposing winds create shock waves that heat the gas to hundreds of millions of degrees.

"Changes in the X-rays are a direct probe of the collision zone and reflect changes in how these stars lose mass," said Michael Corcoran, an astrophysicist with the Universities Space Research Association headquartered in Columbia, Maryland. He and his colleagues compared periastron emission measured over the past 20 years by NASA's Rossi X-ray Timing Explorer, which ceased operation in 2012, and the X-ray Telescope aboard NASA's Swift satellite. In July 2014, as the stars rushed toward each other, Swift observed a series of flares culminating in the brightest X-ray emission yet seen from Eta Carinae. This implies a change in mass loss by one of the stars, but X-rays alone cannot determine which one.

Goddard's Mairan Teodoro led the ground-based campaign tracking the helium emission. "The 2014 emission is nearly identical to what we saw at the previous periastron in 2009, which suggests the primary wind has been constant and that the companion's wind is responsible for the X-ray flares," he explained.

After NASA astronauts repaired the Hubble Space Telescope's STIS instrument in 2009, Gull and his collaborators requested to use it to observe Eta Carinae. By separating the stars' light into a rainbow-like spectrum, STIS reveals the chemical make-up of their environment. But the spectrum also showed wispy structures near the stars that suggested the instrument could be used to map a region close to the binary system in never-before-seen detail.

STIS views its targets through a single narrow slit to limit contamination from other sources. Since December 2010, Gull's team has regularly mapped a region centered on the binary by capturing spectra at 41 different locations, an effort similar to building up a panoramic picture from a series of snapshots. The view spans about 430 billion miles (670 billion km), or about 4,600 times the average Earth-sun distance.

The resulting images, revealed for the first time on Wednesday, show that the doubly ionized iron emission comes from a complex gaseous structure nearly a tenth of a light-year across, which Gull likens to Maryland blue crab. By stepping through the STIS images, vast shells of gas representing the crab's "claws" can be seen racing away from the stars with measured speeds of about 1 million mph (1.6 million km/h). With each close approach, a spiral cavity forms in the larger star's wind and then expands outward along with it, creating the moving shells.
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"These gas shells persist over thousands of times the distance between Earth and the sun," Gull explained. "Backtracking them, we find the shells began moving away from the primary star about 11 years or three periastron passages ago, providing us with an additional way to glimpse what occurred in the recent past."

When the stars approach, the companion becomes immersed in the thickest part of the primary's wind, which absorbs its UV light and prevents the radiation from reaching the distant gas shells. Without this energy to excite it, the doubly ionized iron stops emitting light and the crab structure disappears at this wavelength. Once the companion swings around the primary and clears the densest wind, its UV light escapes, re-energizes iron atoms in the shells, and the crab returns. 

Eta Carinae's great eruption in the 1840s created the billowing Homunculus Nebula, imaged here by Hubble. Now about a light-year long, the expanding cloud contains enough material to make at least 10 copies of our sun. Astronomers cannot yet explain what caused this eruption. Image Credit: NASA, ESA, and the Hubble SM4 ERO Team

Both of the massive stars of Eta Carinae may one day end their lives in supernova explosions. For stars, mass is destiny, and what will determine their ultimate fate is how much matter they can lose -- through stellar winds or as-yet-inexplicable eruptions -- before they run out of fuel and collapse under their own weight.

For now, the researchers say, there is no evidence to suggest an imminent demise of either star. They are exploring the rich dataset from the 2014 periastron passage to make new predictions, which will be tested when the stars again race together in February 2020.

NASA is exploring our solar system and beyond to understand the universe and our place in it. We seek to unravel the secrets of our universe, its origins and evolution, and search for life among the stars.

Source: Nasa

Cassini Catches Titan Naked in the Solar Wind

This diagram depicts conditions observed by NASA's Cassini spacecraft during a flyby in Dec. 2013, when Saturn's magnetosphere was highly compressed, exposing Titan to the full force of the solar wind. Image credit: NASA/JPL-Caltech

Researchers studying data from NASA's Cassini mission have observed that Saturn's largest moon, Titan, behaves much like Venus, Mars or a comet when exposed to the raw power of the solar wind. The observations suggest that unmagnetized bodies like Titan might interact with the solar wind in the same basic ways, regardless of their nature or distance from the sun.

Titan is large enough that it could be considered a planet if it orbited the sun on its own, and a flyby of the giant moon in Dec. 2013 simulated that scenario, from Cassini's vantage point. The encounter was unique within Cassini's mission, as it was the only time the spacecraft has observed Titan in a pristine state, outside the region of space dominated by Saturn's magnetic field, called its magnetosphere.

"We observed that Titan interacts with the solar wind very much like Mars, if you moved it to the distance of Saturn," said Cesar Bertucci of the Institute of Astronomy and Space Physics in Buenos Aires, who led the research with colleagues from the Cassini mission. "We thought Titan in this state would look different. We certainly were surprised," he said.

The solar wind is a fast-flowing gale of charged particles that continually streams outward from the sun, flowing around the planets like islands in a river. Studying the effects of the solar wind at other planets helps scientists understand how the sun's activity affects their atmospheres. These effects can include modification of an atmosphere's chemistry as well as its gradual loss to space.

Titan spends about 95 percent of the time within Saturn's magnetosphere. But during a Cassini flyby on Dec. 1, 2013, the giant moon happened to be on the sunward side of Saturn when a powerful outburst of solar activity reached the planet. The strong surge in the solar wind so compressed the sun-facing side of Saturn's magnetosphere that the bubble's outer edge was pushed inside the orbit of Titan. This left the moon exposed to, and unprotected from, the raging stream of energetic solar particles.

Using its magnetometer instrument, which is akin to an equisitely sensitive compass, Cassini has observed Titan many times during the mission's decade in the Saturn system, but always within Saturn's magnetosphere. The spacecraft has not been able to detect a magnetic field coming from Titan itself. In its usual state, Titan is cloaked in Saturn's magnetic field.

This time the influence of Saturn was not present, allowing Cassini's magnetometer to observe Titan as it interacted directly with the solar wind. The special circumstance allowed Bertucci and colleagues to study the shockwave that formed around Titan where the full-force solar wind rammed into the moon's atmosphere.

At Earth, our planet's powerful magnetic field acts as a shield against the solar wind, helping to protect our atmosphere from being stripped away. In the case of Venus, Mars and comets -- none of which is protected by a global magnetic field -- the solar wind drapes around the objects themselves, interacting directly with their atmospheres (or in the comet's case, its coma). Cassini saw the same thing at Titan.

Researchers thought they would have to treat Titan's response to the solar wind with a unique approach because the chemistry of the hazy moon's dense atmosphere is highly complex. But Cassini's observations of a naked Titan hinted at a more elegant solution. "This could mean we can use the same tools to study how vastly different worlds, in different parts of the solar system, interact with the wind from the sun," Bertucci said.

Bertucci noted that the list of similarly unmagnetized bodies might include the dwarf planet Pluto, to be visited this year for the first time by NASA's New Horizons spacecraft.

"After nearly a decade in orbit, the Cassini mission has revealed once again that the Saturn system is full of surprises," said Michele Dougherty, principal investigator of the Cassini magnetometer at Imperial College, London. "After more than a hundred flybys, we have finally encountered Titan out in the solar wind, which will allow us to better understand how such moons maintain or lose their atmospheres."

The new research is published today in the journal Geophysical Review Letters.

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. NASA's Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the mission for NASA's Science Mission Directorate, Washington, D.C. JPL designed, developed and assembled the Cassini orbiter. The magnetometer team is based at Imperial College, London, U.K.

Source: Nasa

The Ancient exoplanet discovery boosts chances of finding alien life

An artist's impression of the oldest known system of terrestrial-sized planets, Kepler-444. Tiago Campante/Peter Devine, University of Birmingham, Author provided Credit: By Daniel Huber, Astronomer at University of Sydney

One of the crucial variables in calculating the likelihood that alien life exists elsewhere in our galaxy is the number of stars that possess planetary systems, and the proportion of those planets that might be suitable for life.

So the discovery of no less than five sub-Earth-sized exoplanets orbiting an ancient star, Kepler-444, which is not too distant from our own solar system, has significant ramifications for the possibility we might one day run into ET.

Formed over 11-billion years ago, the Kepler-444 system proves that such small planets have existed through most of the history of our universe. And the more small planets that exist, the higher the chances that one of them (or one of their moons) might sit in the so-called “Goldilocks zone” that enables life to exist.

This remarkable discovery was made possible not only by the space-based NASA Kepler telescope but also a technique called asteroseismology.

Kepler continuously measured the brightness of more than 150,000 stars for four years. As planets orbit in front of the stellar disc they cause small dips in the brightness of the star, yielding information on the planet’s orbital period and size relative to the size of their host star.

More than 1,800 exoplanets have been discovered to date, including some Earth-sized planets in the habitable zone. Such discoveries have demonstrated that planets with favourable conditions for life may actually be common.

But the age of the host stars – and therefore the age of the planets – was often unknown. This is because the clues that give a hint to the age of a star tend to be hidden beneath its visible surface.

Using asteroseismology to date a star

An artist’s impression of Kepler-10, illustrating the paths of sound waves in the stellar interior which can be used to determine the fundamental properties – including age – of planet host stars. Gabriel Perez Diaz, Instituto de Astrofisica de Canarias

Fortunately, the variability in the brightness of stars offers a way to resolve this problem using asteroseismology.

Stars with similar and cooler temperatures than our sun transport energy to their surface through the up-flow and down-flow of gas that flows due to the interplay of buoyancy and gravity. The turbulent motion of the gas excites pressure waves to travel through the stellar interior.

The frequency of these waves – also referred to as oscillations – are determined by the sound speed, which in turn depends on the stellar interior structure and composition.

These oscillations also travel to different depths within the star, thereby offering a way to probe the structure by observing the oscillations. As the core properties of the star change with time, such changes are imprinted in the oscillation frequency patterns.

Conveniently, we can measure stellar oscillations using the same data we use to discover transiting planets. Thus we were able to use asteroseismology to study a fascinating planetary system in exquisite detail and to determine the age of the host star.


Kepler-444: An ancient laboratory for planetary and stellar astrophysics

Comparison of the sizes of inner solar system planets to the planets discovered in the Kepler-444 system. Daniel Huber & NASA

Unlike our solar system, however, the Kepler-444 planets orbit their host star in less than 10 days. Even taking into account the cooler temperature of Kepler-444 compared to our sun, this places these ancient planets well outside the habitable zone.

Despite the rather hostile environment, Kepler-444 marks an important milestone to understand whether life may be common outside the solar system. While the Kepler mission has previously demonstrated that small planets are abundant, Kepler-444 proves that such planets have formed for most of the history of our universe.

If life can form on Earth-sized planets in the habitable zone of other stars, this implies that it may have formed on distant planets long before life emerged here on Earth.

Source: University of Sydney

Sun sizzles in high-energy X-rays

X-rays stream off the sun in this image showing observations from by NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, overlaid on a picture taken by NASA's Solar Dynamics Observatory (SDO).
Credit: NASA/JPL-Caltech/GSFC

For the first time, a mission designed to set its eyes on black holes and other objects far from our solar system has turned its gaze back closer to home, capturing images of our sun. NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, has taken its first picture of the sun, producing the most sensitive solar portrait ever taken in high-energy X-rays.

NuSTAR will give us a unique look at the sun, from the deepest to the highest parts of its atmosphere," said David Smith, a solar physicist and member of the NuSTAR team at University of California, Santa Cruz.

Solar scientists first thought of using NuSTAR to study the sun about seven years ago, after the space telescope's design and construction was already underway (the telescope launched into space in 2012). Smith had contacted the principal investigator, Fiona Harrison of the California Institute of Technology in Pasadena, who mulled it over and became excited by the idea.

"At first I thought the whole idea was crazy," says Harrison. "Why would we have the most sensitive high energy X-ray telescope ever built, designed to peer deep into the universe, look at something in our own back yard?" Smith eventually convinced Harrison, explaining that faint X-ray flashes predicted by theorists could only be seen by NuSTAR.

While the sun is too bright for other telescopes such as NASA's Chandra X-ray Observatory, NuSTAR can safely look at it without the risk of damaging its detectors. The sun is not as bright in the higher-energy X-rays detected by NuSTAR, a factor that depends on the temperature of the sun's atmosphere.
This first solar image from NuSTAR demonstrates that the telescope can in fact gather data about sun. And it gives insight into questions about the remarkably high temperatures that are found above sunspots -- cool, dark patches on the sun. Future images will provide even better data as the sun winds down in its solar cycle.

"We will come into our own when the sun gets quiet," said Smith, explaining that the sun's activity will dwindle over the next few years.

With NuSTAR's high-energy views, it has the potential to capture hypothesized nanoflares -- smaller versions of the sun's giant flares that erupt with charged particles and high-energy radiation. Nanoflares, should they exist, may explain why the sun's outer atmosphere, called the corona, is sizzling hot, a mystery called the "coronal heating problem." The corona is, on average, 1.8 million degrees Fahrenheit (1 million degrees Celsius), while the surface of the sun is relatively cooler at 10,800 Fahrenheit (6,000 degrees Celsius). It is like a flame coming out of an ice cube. Nanoflares, in combination with flares, may be sources of the intense heat.

If NuSTAR can catch nanoflares in action, it may help solve this decades-old puzzle.

"NuSTAR will be exquisitely sensitive to the faintest X-ray activity happening in the solar atmosphere, and that includes possible nanoflares," said Smith.
What's more, the X-ray observatory can search for hypothesized dark matter particles called axions. Dark matter is five times more abundant than regular matter in the universe. Everyday matter familiar to us, for example in tables and chairs, planets and stars, is only a sliver of what's out there. While dark matter has been indirectly detected through its gravitational pull, its composition remains unknown.

It's a long shot, say scientists, but NuSTAR may be able spot axions, one of the leading candidates for dark matter, should they exist. The axions would appear as a spot of X-rays in the center of the sun.

Meanwhile, as the sun awaits future NuSTAR observations, the telescope is continuing with its galactic pursuits, probing black holes, supernova remnants and other extreme objects beyond our solar system.

NuSTAR is a Small Explorer mission led by Caltech and managed by NASA's Jet Propulsion Laboratory, also in Pasadena, for NASA's Science Mission Directorate in Washington. The spacecraft was built by Orbital Sciences Corporation, Dulles, Virginia. Its instrument was built by a consortium including Caltech; JPL; the University of California, Berkeley; Columbia University, New York; NASA's Goddard Space Flight Center, Greenbelt, Maryland; the Danish Technical University in Denmark; Lawrence Livermore National Laboratory, Livermore, California; ATK Aerospace Systems, Goleta, California; and with support from the Italian Space Agency (ASI) Science Data Center.

NuSTAR's mission operations center is at UC Berkeley, with the ASI providing its equatorial ground station located at Malindi, Kenya. The mission's outreach program is based at Sonoma State University, Rohnert Park, California. NASA's Explorer Program is managed by Goddard. JPL is managed by Caltech for NASA.

Source: NASA

Sun's rotating 'magnet' pulls lightning towards UK

Big Ben and Houses of Parliament, London, UK. The Sun may be playing a part in the generation of lightning strikes on Earth by temporarily 'bending' the Earth's magnetic field and allowing a shower of energetic particles to enter the upper atmosphere. Credit: © TTstudio / Fotolia

The Sun may be playing a part in the generation of lightning strikes on Earth by temporarily 'bending' the Earth's magnetic field and allowing a shower of energetic particles to enter the upper atmosphere.

This is according to researchers at the University of Reading who have found that over a five year period the UK experienced around 50% more lightning strikes when the Earth's magnetic field was skewed by the Sun's own magnetic field.

The Earth's magnetic field usually functions as an in-built force-field to shield against a bombardment of particles from space, known as galactic cosmic rays, which have previously been found to prompt a chain-reaction of events in thunderclouds that trigger lightning bolts.

It is hoped these new insights, which have been published today, 19 November, in IOP Publishing's journal Environmental Research Letters, could lead to a reliable lightning forecast system that could provide warnings of hazardous events many weeks in advance.

To do so, weather forecasters would need to combine conventional forecasts with accurate predictions of the Sun's spiral-shaped magnetic field known as the heliospheric magnetic field (HMF), which is spewed out as the Sun rotates and is dragged through the solar system by the solar wind.

Lead author of the research Dr Matt Owens said: "We've discovered that the Sun's powerful magnetic field is having a big influence on UK lightning rates.

"The Sun's magnetic field is like a bar magnet, so as the Sun rotates its magnetic field alternately points toward and away from the Earth, pulling the Earth's own magnetic field one way and then another."

In their study, the researchers used satellite and Met Office data to show that between 2001 and 2006, the UK experienced a 50% increase in thunderstorms when the HMF pointed towards the Sun and away from Earth.

This change of direction can skew or 'bend' the Earth's own magnetic field and the researchers believe that this could expose some regions of the upper atmosphere to more galactic cosmic rays--tiny particles from across the Universe accelerated to close to the speed of light by exploding stars.

"From our results, we propose that galactic cosmic rays are channelled to different locations around the globe, which can trigger lightning in already charged-up thunderclouds. The changes to our magnetic field could also make thunderstorms more likely by acting like an extra battery in the atmospheric electric circuit, helping to further 'charge up' clouds," Dr Owens continued.

The results build on a previous study by University of Reading researchers, also published in Environmental Research Letters, which found an unexpected link between energetic particles from the Sun and lightning rates on Earth.

Professor Giles Harrison, head of Reading's Department of Meteorology and co-author of both studies, said: "This latest finding is an important step forward in our knowledge of how the weather on Earth is influenced by what goes on in space. The University of Reading's continuing success in this area shows that new insights follow from atmospheric and space scientists working together."

Dr Owens continued: "Scientists have been reliably predicting the solar magnetic field polarity since the 1970s by watching the surface of the Sun. We just never knew it had any implications on the weather on Earth. We now plan to combine regular weather forecasts, which predict when and where thunderclouds will form, with solar magnetic field predictions. This means a reliable lightning forecast could now be a genuine possibility."

Source:  Institute of Physics

Origin of long-standing space mystery revealed: Origin of the 'theta aurora'

The night side of the terrestrial magnetosphere forms a structured magnetotail, consisting of a plasma sheet at low latitudes that is sandwiched between two regions called the magnetotail lobes. The lobes consist of the regions in which Earth's magnetic field lines are directly connected to the magnetic field carried by the solar wind. Different plasma populations are observed in these regions -- plasma in the lobes is very cool, whereas the plasma sheet is more energetic. The diagram labels by two red dots the location of an ESA Cluster satellite and NASA's IMAGE satellite on 15 September 2005, when particular conditions of the magnetic field configuration gave rise to a phenomenon known as 'theta aurora.'  Credit: ESA/NASA/SOHO/LASCO/EIT

University of Southampton researcher has helped solve a long-standing space mystery -- the origin of the 'theta aurora'.

Auroras are the most visible manifestation of the Sun's effect on Earth. They are seen as colourful displays in the night sky, known as the Northern or Southern Lights. They are caused by the solar wind, a stream of plasma -- electrically charged atomic particles -- carrying its own magnetic field, interacting with Earth's magnetic field.

Normally, the main region for this impressive display is the 'auroral oval', which lies at around 65-70 degrees north or south of the equator, encircling the polar caps.
However, auroras can occur at even higher latitudes. One type is known as a 'theta aurora' because seen from above it looks like the Greek letter theta -- an oval with a line crossing through the centre.

While the cause of the auroral oval emissions is reasonably well understood, the origin of the theta aurora was unclear until now.

Researchers observed particles in the two 'lobe' regions of the magnetosphere. The plasma in the lobes is normally cold, but previous observations suggested that theta auroras are linked with unusually hot lobe plasma.

Dr Robert Fear from the University of Southampton (formerly at the University of Leicester, where much of the research took place), and lead author of the paper published in Science this week, says: "Previously it was unclear whether this hot plasma was a result of direct solar wind entry through the lobes of the magnetosphere, or if the plasma is somehow related to the plasma sheet on the night side of Earth.

"One idea is that the process of magnetic reconnection on the night side of Earth causes a build-up of 'trapped' hot plasma in the higher latitude lobes."

The mystery was finally solved by studying data collected simultaneously by the European Space Agency's (ESA) Cluster and NASA's IMAGE satellites on 15 September 2005. While the four Cluster satellites were located in the southern hemisphere magnetic lobe, IMAGE had a wide-field view of the southern hemisphere aurora. As one Cluster satellite observed uncharacteristically energetic plasma in the lobe, IMAGE saw the 'arc' of the theta aurora cross the magnetic footprint of Cluster.

"We found that the energetic plasma signatures occur on high-latitude magnetic field lines that have been 'closed' by the process of magnetic reconnection, which then causes the plasma to become relatively hot," says Dr Fear.

"Because the field lines are closed, the observations are incompatible with direct entry from the solar wind. By testing this and other predictions about the behaviour of the theta aurora, our observations provide strong evidence that the plasma trapping mechanism is responsible for the theta aurora," he adds.

"The study highlights the intriguing process that can occur in the magnetosphere when the interplanetary magnetic field of the solar wind points northwards," adds Philippe Escoubet, ESA's Cluster project scientist.

"This is the first time that the origin of the theta aurora phenomenon has been revealed, and it is thanks to localised measurements from Cluster combined with the wide-field view of IMAGE that we can better understand another aspect of the Sun-Earth connection," he adds.

Source: University of Southampton