Study reveals the origin of high-latitude auroras

Posted by ap507 at Dec 19, 2014 10:20 AM |
University of Leicester research helps solve space mystery
Study reveals the origin of high-latitude auroras

Credit: ESA; Labelled diagram of magnetosphere and spacecraft.

Auroras are the most visible manifestation of the Sun’s effect on Earth, but many aspects of these spectacular displays are still poorly understood. Thanks to ESA’s Cluster and NASA’s IMAGE satellites working together, a particular type of very high-latitude aurora has now been explained.

Although separated by some 150 million kilometres, the Sun and Earth are connected by the solar wind. This stream of plasma – electrically charged atomic particles – is launched by the Sun and travels across the Solar System, carrying its own magnetic field with it.

Depending on how this ‘interplanetary magnetic field’ is aligned with Earth’s magnetic field when it arrives, there can be various results.

At the point where the two fields meet, Earth’s magnetic field points north. If the interplanetary field is pointing south, then ‘magnetic reconnection’ can occur, where magnetic field lines pointing in opposite directions spontaneously break and reconnect with other nearby field lines.

But when the interplanetary magnetic field points northward, 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 genesis of the auroral oval emissions is reasonably well understood, the origin of the theta aurora was unclear until now.

“The possibilities have been debated since the first satellite observations of the phenomenon were made in the 1980s,” describes Robert Fear, formerly of the University of Leicester’s Department of Physics and Astronomy, where much of the research took place, now at the University of Southampton, and lead author of the paper reporting the results in Science this week.

“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.

“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.

“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.”