Rare Mystery Aerial Phenomenon Seen after Surprise Solar Storm

A strange line of light appeared in the sky over southern Canada as a result of a huge, unexpected solar storm that hit Earth on August 7.

The bizarre sight appeared as the northern lights subsided, and lasted for around 40 minutes, according to a tweet from astronomy author and photographer Alan Dyer.

This aerial phenomenon is known as STEVE, which stands for Strong Thermal Emission Velocity Enhancement, and was first identified in 2016 by Canadian citizen scientists.

STEVE usually appears as a long, mauve-colored line in the night sky, and occasionally forms a characteristic "picket fence" pattern, forming several parallel green stripes. It is thought to be caused by similar atmospheric disturbances that lead to the northern and southern lights, also known as the Aurora Borealis and Australis, respectively, and also appears to coincide with solar storms.

The aurora are believed to be caused by high-energy electrons—fired out from the sun during solar storms and coronal mass ejections (CMEs)—hitting the ionosphere of our atmosphere. Here, they are accelerated along the magnetic field lines towards the Earth's poles and collide with the plentiful oxygen and nitrogen atoms and molecules in Earth's upper atmosphere.

These atoms are excited to a higher energy state by the collision, after which they release their excess energy as a blue-green glow.

The solar storm that hit Earth on Sunday was classed as a G2 geomagnetic storm and caused aurora as far south as New York. SpaceWeather.com said the storm had hit by surprise, with the solar wind picking up speed over the course of the day.

According to the National Oceanic and Atmospheric Administration, rather than super-fast electrons, STEVE may be caused by a stream of hot gas that is also ejected from the sun during solar flares and CMEs.

Research published in the journal Geophysical Research Letters in 2019 found that STEVE is a result of a flowing "river" of charged plasma particles colliding in Earth's ionosphere, which creates friction, heating the particles and causing them to emit STEVE's purple light. This is also how old filament light bulbs work: the tungsten atoms in the filament are heated by the electric current to the point that it glows.

To explain the green patterning that STEVE often occurs in, the researchers used imaging and satellite observation to find that something called "energetic particle precipitation" drives the pattern. This is when high-frequency waves move from Earth's magnetosphere to its ionosphere and energize electrons, pushing them out of the magnetosphere.

While still a relative mystery to the scientific community, the prevalence of camera phones and social media is helping scientists to study STEVE in more detail.

"As commercial cameras become more sensitive and increased excitement about the aurora spreads via social media, citizen scientists can act as a 'mobile sensor network,' and we are grateful to them for giving us data to analyze," Toshi Nishimura, a space physicist at Boston University and lead author of the 2019 study said in a statement.