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Researchers have solved a long standing space mystery about the origin of an unusual type of aurora.

Auroras—the most visible manifestation of the sun’s effect on Earth—are seen as colorful 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.

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.

But they 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 center.

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

Hot plasma

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.

“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,” says Robert Fear from University of Southampton and formerly of University of Leicester, where much of the research took place. Fear is lead author of the study published in the journal Science.

“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.”

Solar winds

The mystery was finally solved by studying data collected simultaneously by the European Space Agency’s (ESA) Cluster and NASA’s IMAGE satellites. While the four Cluster satellites were located in the southern hemisphere magnetic lobe, IMAGE had a wide-field view of the southern hemisphere aurora.

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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,” Fear says.

“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,” says 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 localized measurements from Cluster combined with the wide-field view of IMAGE that we can better understand another aspect of the Sun–Earth connection.”

Source: University of Southampton