Thanks to data collected by ESA’s Cluster spacecraft and a NASA mission called the Imager for Magnetopause-to-Aurora Global Exploration (IMAGE), scientists have solved a long-standing space mystery – the origin of a colorful display in the night sky known as the theta aurora.

Auroras are the most visible manifestation of the Sun’s effect on Earth.

They are caused by the solar wind, a stream of plasma – electrically charged atomic particles – carrying its own magnetic field, interacting with the magnetic field of our planet.

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.

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 Θ (theta) – an oval with a line crossing through the center.

While the genesis of the auroral oval emissions is reasonably well understood, the origin of the theta aurora was unclear until now.

The new study, published in the journal Science, shows that hot plasma funneled into near-Earth space from the Sun helps cause these unique aurora.

“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,” said study lead author Dr Robert Fear of the University of Southampton.

“The possibilities have been debated since the first satellite observations of the phenomenon were made in the 1980s.”

The mystery was finally solved by studying data collected simultaneously by the IMAGE satellite and Cluster’s PEACE electron spectrometer instruments on September 15, 2005.

While the four Cluster satellites were located in the southern hemisphere magnetic lobe, IMAGE had a wide-field view of the southern hemisphere aurora.

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,” Dr Fear explained.

“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.”

Prof Andrew Fazakerley of the University College London’s Mullard Space Science Laboratory, a co-author on the study, added: “the PEACE data were used to show that the hot plasma in the lobes is on magnetic field lines that are connected at both ends to the Earth, so theta aurora are not caused by particles directly entering from the solar wind but by trapped hot plasma.”

“The study highlights the intriguing process that can occur in the magnetosphere when the interplanetary magnetic field of the solar wind points northwards,” said Dr Philippe Escoubet of ESA’s Cluster project, who was not involved in the study.

“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.”

Dr Melvyn Goldstein of NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said: “solving the question of the origin of the theta aurora required Cluster’s high inclination orbit that sweeps over the region where the aurora are generated together with the imaging capability of IMAGE, which is no longer functioning.”

“Hopefully, future missions will give comparable capabilities to view the polar regions of the magnetosphere.”

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R.C. Fear et al. 2014. Direct observation of closed magnetic flux trapped in the high-latitude magnetosphere. Science, vol. 346, no. 6216, pp. 1506-1510; doi: 10.1126/science.1257377