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Origin of long-standing space mystery revealed

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Posted December 19, 2014

A 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 the earth’s magnetic field.

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

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

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 theta aurora as seen by NASA's IMAGE satellite on Sept. 15, 2005. The green lines show latitude and longitude lines and the outlines of the continents; Australia is to the right, South America is to the left and Antarctica is in the middle. The theta aurora is seen slightly off-center, above the right-hand side of Antarctica in this orientation, its characteristic shape defined by the 'bar' connecting the auroral oval. The bright region to the left is 'day glow' (the sunlit atmosphere). Credit: NASA/R. Fear et al (2014)

The theta aurora as seen by NASA’s IMAGE satellite on Sept. 15, 2005. The green lines show latitude and longitude lines and the outlines of the continents; Australia is to the right, South America is to the left and Antarctica is in the middle. The theta aurora is seen slightly off-center, above the right-hand side of Antarctica in this orientation, its characteristic shape defined by the ‘bar’ connecting the auroral oval. The bright region to the left is ‘day glow’ (the sunlit atmosphere). Credit: NASA/R. Fear et al (2014)

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 via EurekAlert!

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