Weekend auroras

WEEKEND AURORAS

 Auroras are a sign of geomagnetic storms. Sometimes they are a sign of no geomagnetic storms, too. On July 5-6, observers in Canada and upper-tier US states spotted auroras even though no storm was in progress. John Vose sends this picture from Lake Willoughby in northern Vermont:

 

"The clouds were lit by occasional lightning, and the rocks were illuminated with my cell phone," says Vose. It was a lucky shot because "soon after the auroras appeared, the fog rolled in...."

At the time, Earth was passing through a region of interplanetary space with a south-pointing magnetic field. The encounter opened a crack in Earth's magnetosphere; solar wind poured in to fuel the auroras. Despite the colorful lights, geomagnetic activity never crossed the threshold into full-fledged storming.

Actual storms could be in the offing, however. A solar wind stream is approaching Earth, due to arrive on July 7-8. NOAA forecasters estimate a 30% to 35% chance of polar geomagnetic storms.