Nov. 22, 2021: The biggest geomagnetic storm in years erupted this month when a Cannibal CME slammed into Earth’s magnetic field. Auroras spread as far south as California and New Mexico. Upon closer inspection, however, not all of those lights were auroras. Some were “SARs.”
SARs are pure red arcs of light that ripple across the sky during strong geomagnetic storms. Here’s an example from Finland in 2018:
“The SAR was visible to the naked eye for nearly 30 minutes and, after fading a bit, remained visible to my camera for another hour and a half,” recalls photographer Matti Helin.
On Nov 4, 2021, Earth experienced a veritable SAR storm. “We photographed SARs as far south as the McDonald Observatory in Texas,” reports Jeff Baumgardner of Boston University’s Center for Space Physics. “The bands of light swept over our cameras near Boston, then headed south. We knew something special was going on.”
SARs may look like auroras, but they not the same. Auroras appear when charged particles rain down from space, hitting the atmosphere and causing it to glow like the picture tube of an old color TV. SARs form differently. They are a sign of heat energy leaking into the upper atmosphere from Earth’s ring current system.
During the storm on Nov. 4th, an all-sky camera in Capital Reef, Utah, caught a really bright one. Play the movie and watch what happens at the 18-second mark:
“It is pretty unusual to see an SAR at this low latitude,” says Asti Bhatt of SRI International. Bhatt operates MANGO, a continent-spanning network of cameras that monitors the sky for unusual phenomena like SARs.
SARs were discovered in 1956 at the beginning of the Space Age. Researchers didn’t know what they were and unwittingly gave them a misleading name: “Stable Auroral Red arcs” or SARs. In fact, SARs are neither stable nor auroras.
“Our group has observed scores of SARs over the last three solar cycles,” says Baumgardner. “In 2015 we published a paper describing them. We found that SARs are ‘stable’ only when compared to very active auroras. When you watch an SAR for an hour or so, it can be quite dynamic.”
Space physicists are keen on SARs because they are linked to Earth’s ring current–a donut-shaped circuit carrying millions of amps around our planet. The ring current skims the orbits of geosynchronous satellites and plays a huge role in determining the severity of geomagnetic storms. Earth is the only rocky planet that has one.
SARs are among the reddest things in the sky, with a monochromatic glow at 6300 Å that comes from atomic oxygen in the upper atmosphere. Unfortunately, the human eye is relatively insensitive to light at this wavelength. SARs are usually so faint that no one notices when they pass overhead. Cameras catch them easily, though. Pro tip for photographers: Use a 6300 Å filter.
“At the peak of a solar cycle we typically see 30 SARs per year near Boston,” says Baumgardner. “We hope this is a start of an active solar cycle with lots more SAR arcs!”