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:
“It was visible for two whole hours,” recalls photographer Matti Helen. “Really amazing!”
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 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.
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 reaching this low latitude,” says Asti Bhat 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. In those days, researchers didn’t know what they were and accidentally 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 their varying morphologies. 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 connect 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 you will ever see, almost monochromatic in the 6300 Å atomic oxygen line. And therein lies the difficulty in seeing them. The human eye is relatively insensitive to light at this frequency. SARs are often overlooked even though they frequently sweep over heavily populated areas of Earth. Pro tip for photographers: Use a 6300 Å filter. This will make an SAR pop out even when it is barely visible to the human eye.
“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!”
via Spaceweather.com
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