“The aurora is the light show after the storm in space,” the National Oceanic and Atmospheric Administration (NOAA) reminded in its latest geomagnetic storm watch, issued before an unusual show is to be observed tonight across much of the northern United States. Triggered by the imminent passage of a symmetric full-halo coronal mass ejection (CME), the display could blanket the aurora borealis’s dancing ribbons over more than 18 states, from Washington and Alaska to Illinois, New York, and Maine.
The CME responsible for bringing on this display erupted Aug. 30 from sunspot AR4199, following a long-duration M2.7-class solar flare. NOAA’s Space Weather Prediction Center and the U.K. Met Office forecast the impact to create a G2 (moderate) to G3 (strong) geomagnetic storm with a chance of reaching G4 (severe) levels. Geomagnetically, it is roughly equivalent to a Kp index of approximately 6.67 strong enough to drive the auroral oval well south of normal polar boundaries. On dark, clear nights, the lights have been visible as far as 1,000 kilometers beyond line of sight.
It’s all physics that begins 93 million miles away from here. CMEs are enormous explosions of magnetized plasma from the corona of the Sun, which periodically accelerate billions of tons of material at rates of up to 3,000 kilometers per second. This is a “cannibal CME,” as the faster-moving ejection catches up with a slower-moving one, acquiring and adding on to the magnetic field before it hits the magnetosphere of our planet. As space weather physicist Dr. Tamitha Skov described, “The larger one catches up with the smaller one just ahead of Earth,” something that can magnify geomagnetic disturbances.
When the CME’s own internal interplanetary magnetic field (IMF) starts to flip southward, it gets nicely coupled with Earth’s northward field, allowing solar wind energy to pour into the magnetosphere. That energy accelerates electrons in the magnetotail, the magnetically compressed nightside tip of the Earth’s magnetic field, down field lines towards the poles. Traveling near a tenth of the speed of light, these electrons strike atmospheric nitrogen and oxygen at altitudes above 100 kilometers, stimulating the molecules and atoms. In returning to their ground states, they release photons: green light by oxygen at 557.7 nanometers between altitudes 120 and 400 kilometers, deep red by oxygen over 300 kilometers, and purples by molecular nitrogen between altitudes 120 and 200 kilometers.
Space satellites like NASA’s Solar Dynamics Observatory and the ESA/NASA Solar and Heliospheric Observatory take pictures of the auroral oval a persistent ring of light around each pole. In the air, the shape depends on the time of day from quiet east–west arcs at early evening to dynamic curtains, curls, and whirls at midnight, typically bursting right across the sky in seconds during geomagnetic substorms. The most severe activity will be from midnight local time to 2 a.m., but the NOAA suggests observers to note from dusk.
Advanced imaging has made the documentation of these events possible. Modern digital sensors are sensitive to softly luminescing auroral radiation invisible to the human eye under night sky conditions, registering hues and forms washed out or absent to the human eye. Long exposure photography can register the full range of color, from the omnipresent greens to less frequent reds and purples, even under depressed atmosphere conditions.
The storm’s approach was initially detected by the Deep Space Climate Observatory satellite in space at the L1 Lagrange point, 1.5 million kilometers in front of the Earth. Sudden increases in solar wind speed, speed, and IMF strength defined the CME’s shock front, presenting forecasters with less than an hour’s notice before the disturbance reached the planet. When it happens, contact with the magnetosphere also will trigger shortwave radio disruptions, satellite failures, and minor power grid oscillations all G3-storm signatures.
For observers in the region of the forecast, the recipe for success is simple but unforgiving: clear sky, minimal light pollution, and perseverance. Even at a distance beyond the naked-eye horizon, the magnitude of the aurora in large storms can span hundreds of miles, giving mid-latitude observers a rare opportunity to observe one of the most brightly colored performances of the Sun–Earth connection.
