Steve and the Milky Way at Childs Lake, Manitoba, Canada. Credit: © Krista Trinder.

Data provided by the European Space Agency’s (ESA) Swarm magnetic field satellites has helped a Nasa-led team to understand the mystery of Steve, a shining streak of purple light in the night sky discovered in 2016.

The team was led by Nasa space physicist Elizabeth MacDonald. It has found that Steve comprises a fast-moving stream of extremely hot atomic particles called a sub-auroral ion drift.

While scientists have been aware of the drift for some time, its visual effect was not known to them.

Swarm’s data have also revealed that though Steve can appear at the same time as an aurora, it is very different.

“Steve can help us understand how the chemical and physical processes in our upper atmosphere can sometimes have local noticeable effects in lower parts of the atmosphere.”

Auroras are usually hues of green, blue and red that can last hours, but Steve is a purple streak and remains in the sky for a short duration.

Formation of Steve follows the same process as a normal aurora. However, it travels along multiple magnetic fields and can appear at much lower latitudes where the alignment of the global electric and magnetic fields makes ions and electrons flow rapidly in the east-west direction, heating them in the process.

MacDonald said: “Steve can help us understand how the chemical and physical processes in our upper atmosphere can sometimes have local noticeable effects in lower parts of the atmosphere.

“This provides good insight on how Earth’s system works as a whole.”

The Swarm satellite constellation comprises three identical satellites that have been in orbit since 2013.

The satellites are designed to provide a range of information regarding the origin of Earth’s magnetic field and how it protects the planet’s habitats from dangerous electrically charged atomic particles in the solar wind.