Posted on by Fraser Cain

Rosetta Can “Smell” a Comet

Image credit: ESA
One of the ingenious instruments on board Rosetta is designed to ?smell? the comet for different substances, analysing samples that have been ?cooked? in a set of miniature ovens.

ESA?s Rosetta will be the first space mission ever to land on a comet. After its lander reaches Comet 67P/Churyumov-Gerasimenko, the main spacecraft will follow the comet for many months as it heads towards the Sun.

Rosetta’s task is to study comets, which are considered the primitive building blocks of the Solar System. This will help us to understand if life on Earth began with the help of ‘comet seeding’.

The Ptolemy instrument is an ?Evolved Gas Analyser?, the first example of a new concept in space instruments, devised to tackle the challenge of analysing substances ?on location? on bodies in our Solar System.

Weighing just 4.5 kilograms and about the size of a shoe box, it was produced by a collaboration of the UK?s Rutherford Appleton Laboratory and Open University.

The analysis of these samples from the surface of the comet will establish what the cometary nucleus is made from, providing valuable information about these most primitive objects.

After the lander touches down on the comet, the Ptolemy instrument will collect comet nucleus material, believed to be a frozen mixture of ices, dust and tar, using the Sampling, Drilling and Distribution system (SD2) supplied by Tecnospazio Milano of Italy. SD2 will drill for small cores of ice and dust from depths of down to 250 millimetres.

Samples collected in this way will be delivered to one of four tiny ?ovens? dedicated to Ptolemy, which are mounted on a circular, rotatable carousel. The German-supplied carousel has 32 of these ovens, with the remainder being used by other Rosetta instruments.

Of the four Ptolemy ovens, three are for solid samples collected and delivered by SD2 while the fourth will be used to collect volatile materials from the near-surface cometary atmosphere.

By heating the solid samples to 800 ?C, the oven converts them into gases which then pass along a pipe into Ptolemy. The gas will then be separated into its constituent chemical species using a gas chromatograph.

Ptolemy can then determine which chemicals are present in the comet sample, and hence help to build up a detailed picture of what the comet is made from.

It does this using the world?s smallest ?ion-trap mass spectrometer?, a small, low-power device built with the latest miniature technology. This device will find out what gases are present in any particular sample and measure stable isotope ratios.

Original Source: ESA News Release

Posted on by Fraser Cain

Eroded Valleys on Mars

This image, taken by the High Resolution Stereo Camera (HRSC) on board ESA’s Mars Express spacecraft, shows the Dao Valles and Niger Valles, a system of outflow channels on Mars.

The image was taken during orbit 528 in June 2004, and shows the Dao Valles and Niger Valles areas at a point where the north-eastern Hellas impact crater basin and the Hesperia Planum volcanic region meet.

The image is centred at Mars longitude 93? East and latitude 32? South. The image resolution is 40 metres per pixel.

The outflow channel system is, in some areas, 40 kilometres wide. The north-eastern ends of the two valleys are almost 200 metres deeper than the south-western regions which are also shown here. The northern Dao Valles, 2400 metres deep, is about 1000 metres deeper than the more southern Niger Valles.

The structure of the valley floor of the Niger Valles is characterised by terraced basins and chaotic fractures. The floor of the Dao Valles is much smoother, but covered with strongly eroded remnants.

These eroded valleys are in a region which is part of the southern flank of the Hadriaca Patera volcano. The surrounding surface is formed by lava streams, probably in a ‘runoff’ process.

Original Source: ESA News Release

Posted on by Fraser Cain

Cassini Discovers Two New Moons

With eyes sharper than any that have peered at Saturn before, the Cassini spacecraft has uncovered two moons, which may be the smallest bodies so far seen around the ringed planet.

The moons are approximately 3 kilometers (2 miles) and 4 kilometers (2.5 miles) across — smaller than the city of Boulder, Colorado. The moons, located 194,000 kilometers (120,000 miles) and 211,000 kilometers (131,000 miles) from the planet’s center, are between the orbits of two other saturnian moons, Mimas and Enceladus. They are provisionally named S/2004 S1 and S/2004 S2. One of them, S/2004 S1, may be an object spotted in a single image taken by NASA’s Voyager spacecraft 23 years ago, called at that time S/1981 S14.

“One of our major objectives in returning to Saturn was to survey the entire system for new bodies,” said Dr. Carolyn Porco, imaging team leader, Space Science Institute, Boulder, Colo. Porco planned the imaging sequences. “So, it’s really gratifying to know that among all the other fantastic discoveries we will make over the next four years, we can now add the confirmation of two new moons, skipping unnoticed around Saturn for billions of years until just now.”

The moons were first seen by Dr. Sebastien Charnoz, a planetary dynamicist working with Dr. Andre Brahic, imaging team member at the University of Paris. “Discovering these faint satellites was an exciting experience, especially the feeling of being the first person to see a new body of our solar system,” said Charnoz. “I had looked for such objects for weeks while at my office in Paris, but it was only once on holiday, using my laptop, that my code eventually detected them. This tells me I should take more holidays.”

The smallest previously known moons around Saturn are about 20 kilometers (12 miles) across. Scientists expected that moons as small as S/2004 S1 and S/2004 S2 might be found within gaps in the rings and perhaps near the F ring, so they were surprised these small bodies are between two major moons. Small comets careening around the outer solar system would be expected to collide with small moons and break them to bits. The fact that these moons exist where they do might provide limits on the number of small comets in the outer solar system, a quantity essential for understanding the Kuiper Belt of comets beyond Neptune, and the cratering histories of the moons of the giant planets.

“A comet striking an inner moon of Saturn moves many times faster than a speeding bullet,” said Dr. Luke Dones, an imaging team member from the Southwest Research Institute in Boulder, Colo. “If small, house-sized comets are common, these moons should have been blown apart many times by cometary impacts during the history of the solar system. The disrupted moon would form a ring, and then most of the material would eventually gather back together into a moon. However, if small comets are rare, as they seem to be in the Jupiter system, the new moons might have survived since the early days of the solar system.”

Moons surrounding the giant planets generally are not found where they originally formed because tidal forces from the planet can cause them to drift from their original locations. In drifting, they may sweep through locations where other moons disturb them, making their orbits eccentric or inclined relative to the planet’s equator. One of the new moons might have undergone such an evolution.

Upcoming imaging sequences will scour the gaps in Saturn’s rings in search of moons believed to be there. Meanwhile, Cassini scientists are eager to get a closer look, if at all possible, at their new finds. Porco said, “We are at this very moment looking to see what the best times are for retargeting. Hopefully, we haven’t seen the last of them.”

The Cassini-Huygens mission is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages the Cassini-Huygens mission for NASA’s Science Mission Directorate, Washington, D.C. The Cassini orbiter and its two onboard cameras were designed, developed and assembled at JPL. The imaging team is based at the Space Science Institute, Boulder, Colo.

For more information about the Cassini-Huygens mission, visit http://saturn.jpl.nasa.gov and the Cassini imaging team home page, http://ciclops.org.

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Chandra Sees Clouds Coming Together

A NASA Chandra X-ray Observatory image has revealed a complex of several intergalactic hot gas clouds in the process of merging. The superb Chandra spatial resolution made it possible to distinguish individual galaxies from the massive clouds of hot gas. One of the clouds, which that envelopes hundreds of galaxies, has an extraordinarily low concentration of iron atoms, indicating that it is in the very early stages of cluster evolution.

“We may be seeing hot intergalactic gas in a relatively pristine state before it has been polluted by gas from galaxies,” said Q. Daniel Wang of the University of Massachusetts in Amherst, and lead author on an upcoming Astrophysical Journal article describing the study. “This discovery should provide valuable insight into how the most massive structures in the universe are assembled.”

The complex, known as Abell 2125,is about 3 billion light years from Earth, and is seen at a time about 11 billion years after the Big Bang, when many galaxy clusters are believed to have formed. The Chandra Abell 2125 image shows several huge elongated clouds of multimillion degree gas coming together from different directions. These hot gas clouds, each of which contains hundreds of galaxies, appear to be in the process of merging to form a single massive galaxy cluster.

Chandra, Hubble Space Telescope, and Very Large Array radio telescope data show that several galaxies in the Abell 2125 core cluster are being stripped of their gas as they fall through surrounding high-pressure hot gas. This stripping process has enriched the core cluster’s gas in heavy elements such as iron.

The gas in the pristine cloud, which is still several million light years away from the core cluster, is conspicuous for its lack of iron atoms. This anemic cloud must be in a very early evolutionary stage. The iron atoms produced by supernovas in the embedded galaxies must still be contained in and around the galaxies, perhaps in grains of dust not well mixed with the observed X-ray-emitting gas. Over time, as the cluster merges with the other clusters and the hot gas pressure increases, the dust grains will be driven from the galaxies, mixed with the hot gas, and destroyed, liberating the iron atoms.

Building a massive galaxy cluster is a step-by-step enterprise that takes billions of years. Exactly how long it takes for such a cluster to form depends on many factors, such as the density of subclusters in the vicinity, the rate of the expansion of the universe, and the relative amounts of dark energy and dark matter.

Cluster formation also involves complex interactions between the galaxies and the hot gas that may determine how large the galaxies in the cluster can ultimately become. These interactions determine how the galaxies maintain their gas content, the fuel for star formation. The observations of Abell 2125 provide a rare glimpse into the early steps in this process.

NASA’s Marshall Space Flight Center, Huntsville, Ala., manages the Chandra program for NASA’s Office of Space Science, Washington. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor for the observatory. The Smithsonian Astrophysical Observatory controls science and flight operations from the Chandra X-ray Center in Cambridge, Mass.

Additional information and images are available at:

http://chandra.harvard.edu
and
http://chandra.nasa.gov

Original Source: Chandra News Release

Posted on by Fraser Cain

Evidence for Planets Around a Young Star

The sharpest image ever taken of a dust disk around another star has revealed structures in the disk which are signs of unseen planets.

Dr. Michael Liu, an astronomer at the University of Hawaii’s Institute for Astronomy, has acquired high resolution images of the nearby star AU Microscopii (AU Mic) using the Keck Telescope, the world’s largest infrared telescope. At a distance of only 33 light years, AU Mic is the nearest star possessing a visible disk of dust. Such disks are believed to be the birthplaces of planets.

“We cannot yet directly image young planets around AU Mic, but they cannot completely hide from us either. They reveal themselves through their gravitational influence, forming patterns in the sea of dust grains orbiting the star,” said Dr. Liu.

The results will be published in the August 12th online Science Express and in the September print edition of Science.

A dust disk ordinarily would appear relatively featureless and symmetric, because any disturbances would be smoothed out as the material orbits the star. However, this is not observed in the case of AU Mic. Instead, Dr. Liu has found its disk is uneven and possesses clumps. These structures arise and are maintained due to the gravitational influence of unseen planetary companions.

The clumps in AU Mic’s disk lie at separations of 25 to 40 Astronomical Units away from the central star (where one Astronomical Unit is the distance from the Earth to the Sun), or about 2 to 4 billion miles. In our own solar system, this corresponds to the regions where Neptune and Pluto reside.

AU Mic is a dim red star, with only half the mass and one-tenth the energy output as the Sun. Previous studies have shown that AU Mic is about 12 million years old, an epoch believed to be an active phase of planet formation. In comparison, our Sun is about 4.6 billion years old, or about 400 times older, and planet formation has long since ended.

“By studying very young stars like AU Mic, we gain insight into the planet formation process as it is occurring. As a result, we learn about the birth of our own solar system and its planets,” said Liu.

The images alone cannot yet tell us what kinds of planets are present, only that the planets are massive enough to gravitationally alter the distribution of the dust. However, many structures in the AU Mic disk are observed to be elliptical (non-circular), indicating that the planetary orbits are elliptical. This is different than in our own solar system, where most planets follow circular orbits.

Images of disks around nearby stars are very rare. Earlier this year, Dr. Liu and his colleagues announced the discovery of the large dusty disk around AU Mic. The light from AU Mic’s disk arises from small dust particles which reflect the light of the central star. The new images are 30 times sharper than the earlier ones, enabling discovery of the clumps in the inner disk of AU Mic.

Dr. Liu used the Keck II Telescope located on Mauna Kea, Hawaii for this research. The two Keck Telescopes are the largest infrared telescopes in the world, each with a primary mirror of 10-meter (33 feet) in diameter. The telescopes are equipped with adaptive optics, a powerful technology which corrects astronomical images for the blurring caused by the Earth’s turbulent atmosphere.

The resulting infrared images are the sharpest ever obtained of a circumstellar disk, with an angular resolution of 1/25 of an arcsecond, about 1/500,000 the diameter of the full moon. If a person’s vision were as sharp as the Keck adaptive optics system, he would be able to read a magazine that was one mile away. In the case of AU Mic, the Keck images can see features as small as 0.4 Astronomical Units, less than half the distance from the Earth to the Sun.

“It is remarkable how quickly Adaptive Optics at Keck has come from being an exotic demonstration technology to producing scientific results of unprecedented quality,” said Dr. Frederic H. Chaffee, the director of the W. M. Keck Observatory. “We are entering a new age of high resolution imaging in astronomy. Dr. Liu’s breathtaking images of possible planets in formation around AU Mic would have been unimaginable from any telescope — space-based or on Earth — a few short years ago. This is an exciting time for us all.”

A preprint of the paper may be found at the astro-ph website

http://arxiv.org/abs/astro-ph/0408164

This work was supported in part by the National Science Foundation.

Original Source: IfA News Release

Posted on by Fraser Cain

Star Clusters Could Be Galaxy Remnants

Globular star clusters are like spherical cathedrals of light – collections of millions of stars lumped into a space only a few dozen light-years across. If the Earth resided within a globular cluster, our night sky would be alight with thousands of stars more brilliant than Sirius.

Our own Milky Way Galaxy currently holds about 200 globular clusters, but once possessed many more. According to the hierarchical theory of galaxy formation, galaxies have grown larger over time by consuming smaller dwarf galaxies and star clusters. And sometimes, it seems that the unfortunate prey is not swallowed whole but instead is munched like a peach, stripped of its outer layers to leave behind only the pit. New research by Paul Martini (Harvard-Smithsonian Center for Astrophysics) and Luis Ho (Observatories of the Carnegie Institution of Washington) shows that some globular clusters may be remnants of dwarf galaxies that were stripped of their outer stars, leaving only the galaxy’s nucleus behind.

Martini and Ho reported their results in the July 20, 2004, issue of The Astrophysical Journal.

Their findings hint at an important yet puzzling connection between the largest globular clusters and the smallest dwarf galaxies. “Star clusters and galaxies are quite different from a structural standpoint – star clusters are much more centrally concentrated, for example – and so the mechanisms that form them must be quite different. Identification of star clusters in the same mass range as galaxies is a very important step toward understanding how both types of objects form,” says Martini.

For their investigation, the team studied 14 globular clusters in the large elliptical galaxy Centaurus A (NGC 5128) using the 6.5-meter-diameter Magellan Clay telescope at Carnegie’s Las Campanas Observatory, Chile. The clusters were selected for their brightness, and since brighter clusters tend to contain more stars and more mass, were expected to be massive. Yet their results surprised even Martini and Ho, showing that the globular clusters of Centaurus A are much more massive than most globulars in the Local Group of galaxies (which includes the Milky Way and the Andromeda Galaxy).

“The essence of our findings is that these 14 globulars are 10 times more massive than the smaller globulars in our neighborhood, and whatever process makes them can produce some really huge objects – they begin to overlap with the smallest galaxies,” says Martini.

Martini also points out the recent discovery of a suspected intermediate-mass black hole in the Andromeda Galaxy globular cluster known as G1, which offers further evidence linking globular clusters to dwarf galaxies. The presence of a moderate-sized black hole is more understandable if it once occupied the center of a dwarf galaxy – a galaxy that lost its outer stars to the pull of Andromeda, leaving it only a shadow of its former self.

Ho, a co-discoverer of the intermediate-mass black hole in G1, adds, “One of the most surprising findings is that the black hole in G1 obeys the same tight correlation between black hole mass and host galaxy mass that has been well established for supermassive black holes in the centers of big galaxies. This puzzling result is more understandable if G1 was once the nucleus of a larger galaxy. A very interesting question is whether some of the massive clusters in Centaurus A also contain central black holes.”

Although most of our Galaxy’s globular clusters are much smaller than those of Centaurus A, the largest Milky Way globulars (such as the omega Centauri star cluster) rival those of the elliptical galaxy. The similarities between massive globulars in both galaxies may point to similar formation mechanisms. Future studies of these most massive globular clusters will explore connections between the formation processes for star clusters and galaxies.

Centaurus A is located approximately 12.5 million light-years away. It is about 65,000 light-years across and is more massive than the Milky Way and Andromeda galaxies put together. Centaurus A possesses a total of about 2000 globular clusters – more than all of the galaxies in the Local Group combined. Recent Spitzer Space Telescope observations of Centaurus A uncovered evidence that it merged with a spiral galaxy about 200 million years ago.

Original Source: Harvard-Smithsonian CfA News Release

Posted on by Fraser Cain

Genesis Heads for Home

Thirty days before its historic return to Earth with NASA’s first samples from space since the Apollo missions, the Genesis spacecraft successfully completed its twentieth trajectory maneuver.

At 12:00 Universal Time (5:00 a.m. Pacific Daylight Time), Mon., August 9, Genesis fired its 90 gram (.2 pound) thrusters for a grand total of 50 minutes, changing the solar sampler’s speed by 1.4 meters per second (about 3.1 miles per hour). The maneuver required half a kilogram (1.1 pounds) of hydrazine monopropellant to complete.

“It was a textbook maneuver,” said Ed Hirst, Genesis’s mission manager at NASA’s Jet Propulsion Laboratory, Pasadena, Calif. “After sifting through all the post-burn data, I expect we will find ourselves right on the money.”

The Genesis mission was launched in August of 2001 on a journey to capture samples from the storehouse of 99 percent of all the material in our solar system — the Sun. The samples of solar wind particles, collected on ultra-pure wafers of gold, sapphire, silicon and diamond, will be returned for analysis by Earth-bound scientists. The samples Genesis provides will supply scientists with vital information on the composition of the Sun, and will shed light on the origins of our solar system.

Helicopter flight crews, navigators and mission engineers continue to prepare for the return of the Genesis spacecraft on September 8. On that date, Genesis will dispatch a sample return capsule that will re-enter Earth’s atmosphere for a planned mid-air capture at the U.S. Air Force Utah Test and Training Range. To preserve the delicate particles of the Sun in their prisons of silicon, gold, sapphire and diamond, specially trained helicopter pilots will snag the return capsule from mid-air using the space-age equivalent of a fisherman’s rod and reel. The flight crews for the two helicopters assigned for Genesis capture and return are comprised of former military aviators and Hollywood stunt pilots.

JPL manages the Genesis mission for NASA’s Space Mission Directorate, Washington, DC. Lockheed Martin Space Systems, Denver, developed and operates the spacecraft. JPL is a division of the California Institute of Technology, the home institute of Genesis’s principal investigator Dr. Don Burnett.

More information about Genesis is available at http://genesismission.jpl.nasa.gov/. More information about the actual capture and return process is available at http://www.genesismission.org/mission/recgallery.html.

Original Source: NASA/JPL News Release

Posted on by Fraser Cain

Hubble Sees a Gas Cavity in Space

In this unusual image, NASA’s Hubble Space Telescope captures a rare view of the celestial equivalent of a geode ? a gas cavity carved by the stellar wind and intense ultraviolet radiation from a hot young star.

Real geodes are baseball-sized, hollow rocks that start out as bubbles in volcanic or sedimentary rock. Only when these inconspicuous round rocks are split in half by a geologist, do we get a chance to appreciate the inside of the rock cavity that is lined with crystals. In the case of Hubble’s 35 light-year diameter “celestial geode” the transparency of its bubble-like cavity of interstellar gas and dust reveals the treasures of its interior.

The object, called N44F, is being inflated by a torrent of fast-moving particles (called a “stellar wind”) from an exceptionally hot star once buried inside a cold dense cloud. Compared with our Sun (which is losing mass through the so-called “solar wind”), the central star in N44F is ejecting more than a 100 million times more mass per second. The hurricane of particles moves much faster at about 4 million miles per hour (7 million kilometers per hour), as opposed to about 0.9 million miles per hour (1.5 million kilometers per hour) for our Sun. Because the bright central star does not exist in empty space but is surrounded by an envelope of gas, the stellar wind collides with this gas, pushing it out, like a snowplow. This forms a bubble, whose striking structure is clearly visible in the crisp Hubble image.

The nebula N44F is one of a handful of known interstellar bubbles. Bubbles like these have been seen around evolved massive stars (Wolf-Rayet stars), and also around clusters of stars (where they are called “super-bubbles”). But they have rarely been viewed around isolated stars, as is the case here.

On closer inspection N44F harbors additional surprises. The interior wall of its gaseous cavity is lined with several four- to eight-light-year-high finger-like columns of cool dust and gas. (The structure of these “columns” is similar to the Eagle Nebula’s iconic “pillars of creation” photographed by Hubble a decade ago, and is seen in a few other nebulae as well). The fingers are created by a blistering ultraviolet radiation from the central star. Like windsocks caught in a gale, they point in the direction of the energy flow. These pillars look small in this image only because they are much farther away from us than the Eagle Nebula’s pillars.

N44F is located about 160,000 light-years in our neighboring dwarf galaxy the Large Magellanic Cloud, in the direction of the southern constellation Dorado. N44F is part of the larger N44 complex, which is a large super-bubble, blown out by the combined action of stellar winds and multiple supernova explosions. N44 itself is roughly 1,000 light-years across. Several compact star-forming regions, including N44F, are found along the rim of the central super-bubble.

This image was taken with Hubble’s Wide Field Planetary Camera 2 in March 2002, using filters that isolate light emitted by sulfur (shown in blue, a 1,200-second exposure) and hydrogen gas (shown in red, a 1,000-second exposure).

Original Source: Hubble News Release

Posted on by Fraser Cain

Wallpaper: Little Ghost Nebula

Known to amateur astronomers as the ‘Little Ghost Nebula’, because it appears as a small, ghostly cloud surrounding a faint dying star, NGC 6369 lies in the direction of the constellation Ophiuchus.

The NASA/ESA Hubble Space Telescope has took this image of the planetary nebula NGC 6369, at a distance estimated to be between about 2000 and 5000 light-years from Earth.

When a star with a mass similar to that of our own Sun nears the end of its lifetime, it expands in size to become a ‘red giant’. The red-giant stage ends when the star expels its outer layers into space, producing a faintly glowing nebula.

Astronomers call such an object a planetary nebula, because its round shape resembles that of a planet when viewed with a small telescope.

The Hubble photograph of NGC 6369, captured with the Wide Field Planetary Camera 2 (WFPC2) in 2002, reveals remarkable details of the ejection process that are not visible from ground-based telescopes because of the blurring produced by the Earth’s atmosphere.

The remnant stellar core in the centre is now sending out a flood of ultraviolet (UV) light into the surrounding gas. The prominent blue-green ring, nearly a light-year in diameter, marks the location where the energetic UV light has stripped electrons off of atoms in the gas. This process is called ionisation.

In the redder gas at larger distances from the star, where the UV light is less intense, the ioniszation process is less advanced. Even farther outside the main body of the nebula, one can see fainter wisps of gas that were lost from the star at the beginning of the ejection process.

This colour image has been produced by combining WFPC2 pictures taken through filters that isolate light emitted by three different chemical elements with different degrees of ionisation.

The doughnut-shaped blue-green ring represents light from ionised oxygen atoms that have lost two electrons (blue) and from hydrogen atoms that have lost their single electrons (green). Red marks emission from nitrogen atoms that have lost only one electron. Our own Sun may eject a similar nebula, but not for another 5000 million years.

The gas will expand away from the star at about 15 miles per second, dissipating into interstellar space after some 10 000 years. After that, the remnant stellar member in the centre will gradually cool off for millions of years as a tiny white dwarf star, and eventually wink out.

Original Source: ESA News Release

Posted on by Fraser Cain

How the Solar Wind Gets Past the Earth’s Shield

ESA?s quartet of space-weather watchers, Cluster, has discovered vortices of ejected solar material high above the Earth. The superheated gases trapped in these structures are probably tunnelling their way into the Earth?s magnetic ?bubble?, the magnetosphere. This discovery possibly solves a 17-year-mystery of how the magnetosphere is constantly topped up with electrified gases when it should be acting as a barrier.

The Earth?s magnetic field is our planet?s first line of defence against the bombardment of the solar wind. The solar wind itself is launched from the Sun and carries the Sun?s magnetic field throughout the Solar System. Sometimes this magnetic field is aligned with Earth?s and sometimes it points in the opposite direction.

When the two fields point in opposite directions, scientists understand how ?doors? in Earth?s field can open. This phenomenon, called ?magnetic reconnection?, allows the solar wind to flow in and collect in the reservoir known as the boundary layer. On the contrary, when the fields are aligned they should present an impenetrable barrier to the flow. However, spacecraft measurements of the boundary layer, dating back to 1987, present a puzzle because they clearly show that the boundary layer is fuller when the fields are aligned than when they are not. So how is the solar wind getting in?

Thanks to the data from the four formation-flying spacecraft of ESA?s Cluster mission, scientists have made a breakthrough. On 20 November 2001, the Cluster flotilla was heading around from behind Earth and had just arrived at the dusk side of the planet, where the solar wind slides past Earth?s magnetosphere. There it began to encounter gigantic vortices of gas at the magnetopause, the outer ?edge? of the magnetosphere.

?These vortices were really huge structures, about six Earth radii across,? says Hiroshi Hasegawa, Dartmouth College, New Hampshire who has been analysing the data with help from an international team of colleagues. Their results place the size of the vortices at almost 40 000 kilometres each, and this is the first time such structures have been detected.

These vortices are known as products of Kelvin-Helmholtz instabilities (KHI). They can occur when two adjacent flows are travelling with different speeds, so one is slipping past the other. Good examples of such instabilities are the waves whipped up by the wind slipping across the surface of the ocean. Although KHI-waves had been observed before, this is the first time that vortices are actually detected.

When a KHI-wave rolls up into a vortex, it becomes known as a ?Kelvin Cat?s eye?. The data collected by Cluster have shown density variations of the electrified gas, right at the magnetopause, precisely like those expected when travelling through a ?Kelvin Cat?s eye?.

Scientists had postulated that, if these structures were to form at the magnetopause, they might be able to pull large quantities of the solar wind inside the boundary layer as they collapse. Once the solar wind particles are carried into the inner part of the magnetosphere, they can be excited strongly, allowing them to smash into Earth?s atmosphere and give rise to the aurorae.

Cluster?s discovery strengthens this scenario but does not show the precise mechanism by which the gas is transported into Earth?s magnetic bubble. Thus, scientists still do not know whether this is the only process to fill up the boundary layer when the magnetic fields are aligned. For those measurements, Hasegawa says, scientists will have to wait for a future generation of magnetospheric satellites.

Original Source: ESA News Release