“Six million people lost power in 90 seconds.” That was the picture in Quebec in March 1989, when a Sun geomagnetic storm cast a province into darkness. Solar activity this week won’t match that one for size, but the physics is the same and it might color the skies over the Washington, D.C., area with unusual northern lights.
The National Oceanic and Atmospheric Administration Space Weather Prediction Center has posted a geomagnetic storm watch after an intense coronal mass ejection (CME) on August 30. The CME was propelled by a long-duration M2.7-class flare from sunspot AR4199 and is predicted to arrive at Earth late September 1 into the early morning hours of September 2. NOAA expects a G2 (moderate) storm on September 1, ramping up to a G3 (strong) storm on September 2, with the U.K. Met Office predicting a possibility of G4 (severe) conditions. A G3 storm would have auroras stretching well south of their typical extent visible from Pennsylvania to Iowa to Oregon, and perhaps low on the northern horizon in the D.C. region.
What makes this phenomenon so interesting to space weather researchers is the potential for a “cannibal CME.” Space weather physicist Dr. Tamitha Skov pointed out that two distinct eruptions are potentially involved with the faster catching up to the slower along the way to Earth. The larger one catches up with the smaller one just ahead of Earth, she described, a motion that can amplify the geomagnetic effect. These interactions compress the plasma density and magnetic field, transferring more energy into Earth’s magnetosphere.
The strong colors of the aurora result from electrons from the CME crashing into atmospheric gases. Oxygen at greater altitude gives off green or red light, while nitrogen gives off purples and blues. The shows are most probable between 10 p.m. to 2 a.m. local time, although activity can last through the entire night. For those trying to photograph the spectacle, camera and smartphone digital sensors can record faint auroral light that is not visible to the naked eye. Long exposures of a few seconds, “night sky” modes, and a tripod can greatly enhance outcomes.
Aside from the spectacle, G2–G3 storms present real risks. NOAA cautions against power system voltage irregularities, potential false alarms on grid protection equipment, and added drag on low Earth orbit satellites. Spacecraft orientation problems and occasional GPS accuracy are conceivable, as are interference with high-frequency radio employed in aviation and ship navigation. These results are due to geomagnetically induced currents (GICs) electrical currents driven through infrastructure and the ground by the storm’s changing magnetic fields. In 1989, like currents tripped circuit breakers throughout Hydro-Québec’s network, causing the grid to collapse within less than two minutes.
Sun’s behavior operates on an about 11-year cycle, and it was at solar maximum late last year. During this phase, it produces more common CMEs and flares, both making auroral chances happen more frequently and creating a higher chance of disruptive space weather. The majority of storms are not severe, but history does provide humbling context. The largest geomagnetic storm ever recorded, the 1859 Carrington Event, produced auroras that were seen in the tropics and ignited telegraph hardware. As the National Academies of Sciences indicate, a Carrington-level storm today would cause as much up to $2 trillion in economic damage, disabling telecommunications, banking, GPS, and the electrical grid. Tens of millions of people would experience weeks- to year-long outages.
Monitoring and forecasting have advanced dramatically since Richard Carrington’s time. A fleet of spacecraft including NASA’s Solar Dynamics Observatory and NOAA’s DSCOVR continuously track solar activity, measuring the speed, density, and magnetic orientation of incoming CMEs. These observations feed models that predict arrival times and potential severity, giving grid operators, satellite controllers, and the public critical hours to prepare.
For the time being, the forecast combines beauty with caution. If the skies are partly clear, those living in the D.C. area can get a glimpse of the aurora by facing north from a dark unobstructed spot. And though the lights themselves are temporary, the science behind them reinforces an eternal reality: the Sun, calm from 93 million miles off, is an agitated machine whose tempests can stretch across space to touch every aspect of contemporary existence.
