Earth received a direct hit from a mass of charged particles from the sun last night, which cause displays of the Northern Lights.
Although forecasters had warned of a mild display of aurora, they hadn't foreseen what materialized -- a coronal mass ejection clashing with Earth's atmosphere to cause a G1 geomagnetic storm.
A CME is a cloud of charged particles that explodes from the sun, typically in the wake of solar flares, an intense burst of electromagnetic radiation from a sunspot on our star's surface. The Northern Lights are caused by the solar wind, a stream of charged particles from the sun interacting with Earth's magnetic field.
Solar flares travel at light speed and supercharge the solar wind but do not cause northern lights. The most recent solar flares on the sun were an M2-class event on Dec. 13 and an M3.1-class on Dec. 15. In their wake, a CME was hurled into space, but space weather forecasters had predicted that it would narrowly miss Earth today.
Instead, SpaceWeather.com reported a direct hit, with the National Oceanic and Atmospheric Administration's Space Weather Prediction Center recording the impact at 05:19 UTC (00:19 a.m. EST) on Tuesday, Dec. 17.
Shortly after, the Kp index -- which provides a rough guide to their intensity -- reached 5, which equates to a G1-class geomagnetic storm.
That data came when charged particles struck the sensors of NOAA's DSCOVR satellite, which orbits Earth. It measures the solar wind's speed and magnetic intensity, which is critical in calculating how it is about to change.
Previous data had forecast aurora four hours later, from around 09:00-15:00 UTC on Tuesday, Dec. 17 (from 4:00 a.m.EST, 03:00 CST, 02:00 MST and 01:00 PST).
The exact times of displays of the aurora are difficult to forecast more than a few minutes in advance. DSCOVR gives about 15-30 minutes warning of a space weather event and any resulting aurora displays. Observers are advised to keep an eye on NOAA's 30-minute forecast, where the latest updates are posted, and on apps like Aurora Now, which gives real-time alerts as geomagnetic conditions develop.
Aurora may have been visible from some northern-tier and midwest U.S. states, including Washington, northern Idaho, Montana, North Dakota, South Dakota, Minnesota, Wisconsin, Michigan and Maine in the U.S.
Aurora is typically seen in polar regions at around latitudes of 70 degrees north and south, but during intense periods of solar activity, the auroral oval can bulge, with displays then seen as low as 25 degrees north and south of the equator.