Next to be hit will be power plants, transformers and power lines. They, too, will respond as if to a power surge and may shut down. That happened in Quebec in 1989 during a powerful magnetic storm.
Since then, power companies have responded by reducing power slightly before a magnetic storm to absorb surges. Still, this weekend may be a good time to hit the computer's save button more often and to be sure the machine is turned off when not in use.
Norman Cohen, a space weather forecaster at the federal Space Environment Center in Boulder, Colo., said in an interview that he expected "a medium-grade storm" today, tomorrow and Sunday.
The northern lights, or aurora borealis, a colorful sparkling event usually seen only in Alaska and northern Canada, will move farther south as a consequence. Weather permitting, people in Michigan, Minnesota, North Dakota, Wisconsin, Washington, Idaho, Montana, New York and New England may get a good view this weekend, Cohen said.
The storms could continue into next week because the part of the sun where the spot is, just a bit above and to the right of the center, does not turn away from Earth until April 5, he said.
The sunspot is so large that if someone had proper eye protection - such as glasses approved for use during a solar eclipse - it could be seen with the naked eye. Do not try it without such protection, Cohen advised.
Seth Borenstein's e-mail address is firstname.lastname@example.org.