March 24, 2023: Forecasters did not see this one coming. On March 23-24, auroras spread into the United States as far south as New Mexico (+32.8N) during a severe (category G4) geomagnetic storm–the most intense in nearly 6 years. The cause of the storm is still unclear; it may have been the ripple effect of a near-miss CME on March 23rd.
“Aurora pillars were visible from Shenandoah National Park in Central Virginia,” says Peter Forister, who photographed the light show at latitude +38.7 degrees:
“Beautiful red and green colors were visible to the naked eye around 11 pm local time,” he says. Other notable low-latitude sightings were made in Colorado (+38.7N), Missouri (+40.2N), Colorado again (+38.3N), Nebraska (+41N) and North Carolina (+36.2N). More than half of all US states were in range of the display.
Not every light in the sky was the aurora borealis, however. There was also STEVE:
Joseph Shaw photographed the luminous ribbon over Bozeman, Montana. It also appeared over South Dakota, Washington State, Idaho, Montana again, and Scotland.
STEVE (Strong Thermal Emission Velocity Enhancement) looks like an aurora, but it is not. The phenomenon is caused by hot (3000°C) ribbons of gas flowing through Earth’s magnetosphere at speeds exceeding 6 km/s (13,000 mph). These ribbons appear during strong geomagnetic storms, revealing themselves by their soft purple glow.
This remarkable and surprising storm began on March 23rd when magnetic fields in the space around Earth suddenly shifted. In the jargon of space weather forecasting “BsubZ tipped south.” South-pointing magnetic fields can open a crack in Earth’s magnetosphere and, indeed, that’s what happened. Earth’s “shields were down” for almost 24 hours, allowing solar wind to penetrate and the storm to build to category G4.
These developments may have been caused the close passage of an unexpected CME. The storm cloud could have left the sun on March 20-21 when SOHO coronagraph data were unusually sparse. We didn’t know it was coming. For aurora watchers, it was a welcome surprise. Aurora alerts: SMS Text.