Theta Auroras are bizarre and dynamic, but scientists were uncertain how they formed - until now. These displays form high in the atmosphere, at far greater altitudes than normal auroras. They are named after the Greek letter theta - an oval bisected by a line - which they resemble, when viewed from below.
Auroras are normally formed when charged particles from the solar wind are diverted into the atmosphere by the magnetic field of the Earth. These displays are commonly known as northern and southern lights, and can be visible throughout most temperature latitudes.
Particles in two lobes of the magnetosphere were studied by astronomers. Normally, plasma in these lobes is cool, but in the case of theta auroras, this ionized gas appears hot.
"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. 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," Robert Fear of the University of Southampton said.
The auroral oval, located at latitudes around 65 degrees north, or roughly 65 degrees south, are the locations where auroras generally form. Theta auroras take place at higher latitudes, closer to the poles.
Theta auroras were studied through data collected by the Image satellite, managed by NASA, as well as the European Space Agency's (ESA) Cluster observatory. As high-energy plasma in a southern lobe was observed by one of the cluster satellites, the Image observatory recorded theta aurora forming. Energetic plasma develops over magnetic field lines near the poles, combining into closed magnetic loops. This can drive energy, heating the formations, researchers discovered.
Magnetic reconnection takes place when the solar wind is aligned opposite to the magnetic field of Earth where the two intersect. When solar wind, aligned south, runs into a portion of the terrestrial magnetic field facing north, other particles in the wind quickly align to the south. This opens up a "funnel" for solar wind to pour into the magnetosphere, fueling theta aurora.
"This is the first time that the origin of the theta aurora phenomenon has been revealed, and it is thanks to localised measurements from Cluster combined with the wide-field view of IMAGE that we can better understand another aspect of the Sun-Earth connection," Philippe Escoubet, ESA's Cluster project scientist, told the press.
The solar wind, particles of charged particles, constantly stream outwards from the Sun, occasionally causing dazzling auroral displays.
Investigation of theta aurora was published in the journal Science.