Fast solar wind moving at up to 430 miles per second could spark geomagnetic storms tonight and tomorrow, raising the chance that the northern lights will appear across parts of the northern United States on April 17 and April 18.
NOAA's Space Weather Prediction Center issued a moderate G2 geomagnetic storm watch on April 17, and forecasters at the U.K. Met Office said there is a chance of strong G3 bursts if the activity intensifies. If that happens, the aurora borealis could reach farther south than usual, with possible visibility as far as Illinois and Oregon.
The burst of solar wind, measured at about 700 km/s, matters because stronger flow can drive more energetic geomagnetic storms and make the lights more widespread. Auroras form when solar wind interacts with Earth's magnetic field and sends charged particles into the upper atmosphere, where they produce the glow seen from the ground.
Geomagnetic storms are ranked on a G-scale from G1, meaning minor, to G5, meaning extreme, and a G2 watch signals a moderate event with room to grow. The tension in this forecast is whether the activity stays at the moderate level or builds quickly enough to trigger the stronger G3 bursts that would widen the viewing area across mid-latitudes.
For skywatchers, the practical answer is straightforward: the best chance to see the northern lights is tonight and tomorrow, April 17 and April 18, especially in northern U.S. states, with a farther-south display possible if the solar wind strengthens.
