London: Researchers from University of Southampton have 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.
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.
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.
"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," explained lead author Robert Fear from the University of Southampton.
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 added.
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.
Caused by the solar wind, a stream of plasma - electrically charged atomic particles - "theta aurora" carry their own magnetic field, interacting with the earth's magnetic field.
The study appeared in the journal Science.
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