Thanks to a strong geomagnetic storm over the weekend many people saw glowing lights in the sky, including in places where such events are very unusual. Although most assumed what they were witnessing were auroras, experts have said more often than not this was not technically the case. The majority of the red and purple global lights were actually SARs and STEVEs.
SARs stand for Stable Auroral Red arcs, but the name is only half correct. Although the arcs are definitely red, they are not always stable, and it turns out they are not from auroras, although the two phenomena do have something in common.
People have probably been seeing SARs ever since humanity expanded to higher latitudes, but the first attempt to scientifically describe them took place in 1956, which included the name. We have since learned that whereas true auroras involve charged particles from space hitting the air, SARs involve the upper atmosphere getting heated from below.
The confusion that led to the misnaming occurred not only because the two look somewhat alike, but because they both happen during geomagnetic storms. They both occur when bursts of solar activity rain charged particles down upon us.
However, where auroras are caused by the particles directly impacting the atmosphere, with different colors reflecting the gases the particles are striking, SARs are triggered by a more complex process. The geomagnetic storms add so much energy to the Earth’s ring current system that carries electric charge around the planet that some leaks out into the upper atmosphere and produce aurora-like red glows.
As with auroras, modern cameras are able to make them look much brighter than they appear to the naked eye.
"On Nov. 5th, the ring current was pumped up for hours by the geomagnetic storm, with energy dissipating into these SAR arcs," Jeff Baumgardner of Boston University told Space Weather.com. "It was a global event. Our cameras registered SAR activity from Italy to New Zealand."
SARs were the most widespread skylights seen on Sunday, but some people also spotted the purple narrow curtains known as STEVEs. Initially, the name STEVE was given to avoid the same mistake as SAR – not knowing what they were seeing, some amateurs gave it a friendly personal name to avoid getting its nature wrong and having the name stick. The specific name references the film Over the Hedge, where characters do the same thing. However, some scientists managed to turn it into a backronym for Strong Thermal Emission Velocity Enhancement, which is accurate although not all that explanatory.
It's only last year that the connection between SARs and STEVEs was established after analysis of a series of photographs taken in 2015. These showed a SAR over New Zealand’s South Island turning into a STEVE. This transformation has been seen since, in the Northern Hemisphere as well, but how they are related is still poorly understood.
So how do you tell a SAR or STEVE from a true aurora? The further you are from a geomagnetic pole, the more likely it is that what you are witnessing is not a true aurora. (That is unless it is a red equatorial aurora, but currently, there are not many places they can be seen). A SAR will be almost exclusively red, unlike the multi-colored true aurora.
While others were seeing SARs and STEVEs, Finland got true auroras.
Like a suffragette flag, a STEVE can combine purple, green, and white, as well as red, although white most often dominates. It will be a narrow curve across the sky, rather than a large sheet. STEVEs are usually, although not always, seen when green “picket fence” aurora from incoming electrons are visible lower in the sky. If you haven’t got fully away from interfering lights there is a fair chance you’ll be able to see the more intense STEVE, if there is one, but not the SAR or aurora.
The best news is that this event may not be done yet. Sunday was a G3 class geomagnetic storm caused by a major coronal mass ejection (CME). We’re unlikely to see that again soon, but according to NOAA’s update, there are “lingering CME effects” which in combination with “a fast solar wind” means a G2 storm is likely tonight. That probably won’t mean images as spectacular as the ones we just saw, but if skies are clear, and you’re in a position to get away from city lights, there could still be plenty to see at high-medium latitudes.