CHICAGO – Radiation from the largest solar flare in four years is expected to reach Earth late on Thursday or Friday.

Such events can cause radio blackouts and interfere with communication satellites, but the most likely outcome this time will be brilliant Northern Lights displays, U.S. scientists said.

NASA scientists on Monday reported an X-class solar flare, the first in more than four years. X-class flares are the most powerful of all solar events that can trigger radio blackouts and long-lasting radiation storms.

It was one of a series of three solar flares and prompted speculation that a new solar cycle may be ramping up.

"This is one of the first real solar events of the next solar maximum -- that is when you would see the highest number of solar flares," said Brady O'Hanlon, a doctoral student at Cornell University in Ithaca, New York.

O'Hanlon said solar activity typically peaks in 11-year cycles. "We've been at the depths of one of the quietest ever 11-year periods," he said in a telephone interview.

Solar flares are intense, short-lived releases of energy. They show up as bright areas on the sun, producing high levels of radiation and charged particles that can intensify solar winds -- electrically charged particles continuously spewing outward from the sun.

The Earth's magnetic field largely protects the planet from space weather. But massive solar flares can disrupt power grids, interfere with high-frequency airline and military communications, disrupt Global Positioning System signals and interrupt civilian communications, according to the National Oceanic and Atmospheric Administration, which tracks solar flares.

NASA says the particle cloud produced by February 14 event is relatively weak, and most likely will only result in some beautiful sightings of the aurora borealis -- shimmery displays of red, green and purple that are expected to light up the northern sky this week.

But O'Hanlon, who conducts research on space weather and its effects on GPS software receivers, says people who have come to rely on their GPS technology during the period of quiet solar activity may see more interference with their navigation systems as solar activity picks up.

"It's been minimum activity, and we haven't had to really worry about GPS. That may not be quite the case over the next few years," he said.