Solar Flares: X-Class

As reported by The Christian Science Monitor: Don't be distracted by Star Trek's M-class planets or X-Men's First Class of students – while the terms have been appropriated by science fiction, M-class and X-class solar flares are very real.

Two enormous solar flares – an X1.7 and X2.1 class, respectively – erupted from the surface of the sun on Friday morning. The smaller flare peaked at 4:01 a.m. EDT, while the X2.1 solar flare peaked seven hours later, at 11:03 a.m. EDT.

The last time the sun released an X-class flare was on May 14, 2013, but there were two weaker M-class flares on Thursday.

The steady stream of solar flares, coronal mass ejections (CMEs), and other solar storms are associated with the solar maximum. Solar storms occur on a roughly 11-year cycle, but the "maximum" isn't a one-day peak, but a prolonged period of more than a year.

The minimum-maximum pattern is more of a gentle sine wave, like a system of rolling hills and valleys, than like the jagged peaks of a heart monitor, says Art Poland, an astrophysicist with George Mason University.

"We're just on the other side of the top of the sine wave right now," says Dr. Poland. "You get your biggest flares and biggest magnetic disruptions on the way down."

It's hard to pinpoint the precise timing of a solar maximum, especially in advance. In recent years, NASA was estimating the maximum would come in 2011, or 2012, or maybe 2013. "You only know afterwards," says Poland, who is the project scientist on SOHO, an observatory orbiting the sun. "You're pinning real data to a sine wave, and it's not a perfect sine wave, so it's hard to tell."

The first X-class flare of the current solar cycle occurred in February 2011. The largest X-class flare so far in this cycle was an X6.9 on Aug. 9, 2011.

What's an "X-class" solar flare?

Solar flares are classified by strength. When the system was created in the mid-1960s, the smallest solar flares were classified as C-class, moderate flares were M-class, and the largest were X-class. Since then, more refined instruments have been able to measure even smaller flares, called A-class and B-class.