I signed up for the regular National Oceanic and Atmospheric Administration's Space Weather Prediction Center space weather bulletins a couple of years ago, and there followed an unprecedentedly low level of solar activity. Finally, this morning, I've received a bulletin warning of a "strong" geomagnetic storm that began just over an hour ago. Here's the text of the warning:
A geomagnetic storm began at 05:55 AM EST Monday, April 5, 2010. Space weather storm levels reached Strong (G3) levels on the Geomagnetic Storms Space Weather Scale. The source of the storming is an Earth-directed Coronal Mass Ejection associated with a weak solar flare that occurred in Active Region 1059 on April 3 at 05:54 AM EST. This is expected to be an isolated storm that should subside quickly. Other than the flare and CME erupting on April 3, this active region has not produced any significant activity. Systems that can be affected include electric power systems, spacecraft operations, high-frequency communications, GPS, and other navigation systems.
Their scale for the severity of solar storms goes from G1 ("Minor") to G5 ("Extreme"). What can happen during a G3 ("Strong") geomagnetic storm? NOAA has a handy page that explains the scale:
Power systems: voltage corrections may be required, false alarms triggered on some protection devices.
Spacecraft operations: surface charging may occur on satellite components, drag may increase on low-Earth-orbit satellites, and corrections may be needed for orientation problems.
Other systems: intermittent satellite navigation and low-frequency radio navigation problems may occur, HF radio may be intermittent, and aurora has been seen as low as Illinois and Oregon (typically 50° geomagnetic lat.)**.
Thanks to this morning's successful launch of Discovery, there is a boatload of astronauts in space right now -- I don't actually know how or if this affects them. Commenters, feel free to explain!