British researchers say they've solved the mystery of a particular kind of aurora, one of the spectacularly colorful displays in the night sky created when the solar plasma wind interacts with the Earth's magnetic.
Most of these displays occur in a region dubbed the "auroral oval," at about 65 degrees to 75 degrees north and south from the equator, but there's another type -- called a theta aurora for its resemblance to the Greek letter theta, an oval containing a straight line across its center -- that can occur at higher latitudes.
The causes of auroras in their usual "oval" region have been reasonably understood, but what generates theta auroras has been unknown -- until now, scientists at the University of Southampton report in the journal Science.
Researchers observing particles from the sun in two "lobe" areas of the Earth's magnetosphere say plasma seen there is normally cold, but some previous observations suggested theta auroras could be linked with unexpectedly hot lobe plasma.
The new study has confirmed that, the researchers say, showing that these unique auroras originate from hot plasma being funneled into near-Earth space from the Sun.
A question has been how that plasma becomes heated.
"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 study lead author Robert Fear.
The origin of theta auroras has been a subject of debate since the first satellite observations of the phenomenon in the 1980s, he says.
"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."
Confirmation of that hypothesis came from analysis of data gathered by satellites operated by NASA and the European Space Agency.
Four ESA Cluster satellites were within the magnetic lobe of the southern hemisphere while NASA's IMAGE satellite was capturing a wide-field view of southern hemisphere auroras.
As one of the Cluster satellites detected plasma of unusually high energy in the lobe, IMAGE saw a theta aurora "arc" across 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.
"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," he explains.