When is an aurora not an aurora?

The glowing green, red and purple colours of northern and southern lights, called the auroras, are one of the best-known phenomena which are on many people's must-see lists. 

Sometimes, these are accompanied by mysterious streaks of mauve and white called ‘Steve’ and a glowing green ‘picket fence.’ A new study reveals that although these streaks might look like auroras, they are caused by completely different processes.

Steve, which was named after a scary hedge in 2006’s children’s movie, and picket fence were first recognised as distinct from the auroras in 2018. Now, a new study led by Claire Gasque, a University of California, Berkeley graduate student in physics, has described the physics behind Steve in a paper published in the journal Geophysical Research Letters.

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Gasque has calculated that in a region in the upper atmosphere farther south than that in which auroras form, electric fields parallel to Earth's magnetic field could produce the colour spectrum of the picket fence, the university's statement explains. If this estimation is correct, then this could impact how physicists understand energy flow between Earth's magnetosphere, which surrounds and protects Earth from solar winds, and the ionosphere.

"This would upend our modelling of what creates light and the energy in the aurora in some cases," Gasque said in the statement. Brian Harding, the study's co-author, added that physicists were aware that the Steve spectrum is indicating that there is exotic physics happening, but they didn’t know what could be. Now, the paper shows parallel electric fields which could explain the phenomena.

The common auroras that people often see are caused by solar wind energising particles in Earth’s magnetosphere, triggering oxygen and nitrogen molecules in the upper atmosphere to emit specific frequencies of light, Earth.com explains.

However, Steve exhibits a broad range of frequencies centered around purple or mauve, without the blue light found in the most energetic particle interactions in auroras. Notably, ‘Steve’ and the picket fence occur at lower latitudes than common auroras and this could even be near the equator.

In the statement, Gasque explains that "the light from the picket fence is being created by particles that have to be energised right there in space by a parallel electric field, which is a completely different mechanism than any of the aurora that we've studied or known before."

Gasque and her team of researchers have proposed that Nasa launch a rocket into the heart of the aurora to find out if she's correct.

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