Origin of high-latitude auroras discovered

Chuck Bednar for redOrbit.com – Your Universe Online
Spectacular to look at, but poorly understood, auroras have been somewhat of a conundrum for scientists. But new data from NASA and ESA satellites has finally shed, ehem, light on one particular type of very high-latitude aurora.
Auroras, which the ESA refers to as “the most visible manifestation of the Sun’s effect on Earth,” are a natural light display predominantly seen in the skies above the Arctic and Antarctic regions. One type is known as a “theta aurora” because, when viewed from above, it resembles the Greek letter theta (an oval with a line crossing through its center).
Auroras are typically formed by a stream of plasma (electrically charged atomic particles) known as the solar wind. The solar wind originates from the Sun and travels across the Solar System, the ESA explained, bringing its own magnetic field with it on the journey. Depending upon how that magnetic field is aligned with that of Earth’s, there can be several different results.
When the two fields meet, Earth’s magnetic field always points north. If the solar wind’s magnetic field is pointing south, than a phenomenon known as “magnetic reconnection” can occur, causing the opposite-facing lines to break and reconnect with other nearby field lines.
This allows solar wind plasma to enter the magnetosphere, causing the auroras typically known as the Northern or Southern Lights to be produced as the particles are carried along the planet’s magnetic field lines, striking atoms high in the atmosphere. When they interact with oxygen or nitrogen atoms, they produce various different colors, including red, green, blue and purple.
In most cases, the this display takes place in what its known as the “auroral oval,” with encircles the polar caps starting at 65 to 70 degrees north or south of the equator. However, when the solar wind’s magnetic field points in the opposite direction, to the north, auroras – including theta auroras – can take place at even higher latitudes.
“While the genesis of the auroral oval emissions is reasonably well understood, the origin of the theta aurora was unclear until now,” the ESA said. “A clue comes from the particles observed 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, though quite how was unclear.”
In research published online Friday in the journal Science, Robert Fear, formerly of the University of Leicester’s Department of Physics and Astronomy and now with the University of Southampton, and his colleagues reviewed data collected simultaneously by the ESA’s Cluster and NASA’s Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) satellites on September 15 to learn more about this plasma.
“The possibilities have been debated since the first satellite observations of the phenomenon were made in the 1980s. 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,” he explained.
“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,” Fear added.
On September 15, the four Cluster satellites were located in the southern hemisphere magnetic lobe while IMAGE had a wide-field view of that region’s aurora. As one of the ESA probes saw uncharacteristically energetic plasma in the lobe, the NASA satellite observed the arc of the theta aurora as it crossed the magnetic footprint of Cluster, the agency explained.
“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,” Fear said. “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 behavior of the theta aurora, our observations provide strong evidence that the plasma trapping mechanism is responsible for the theta aurora.”
“The study highlights the intriguing process that can occur in the magnetosphere when the interplanetary magnetic field of the solar wind points northwards,” adds ESA Cluster project scientist Philippe Escoubet. “This is the first time that the origin of the theta aurora phenomenon has been revealed, and it is thanks to localized measurements from Cluster combined with the wide-field view of IMAGE that we can better understand another aspect of the Sun–Earth connection.”
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