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How Do Scientists Classify Solar Flares?
This image from NASA's Solar Dynamics Observatory shows the sun as it unleashed an X5.4-class solar flare at 7:04 p.m. EST on March 6, 2012 (0002 March 7 GMT). The flare appears as the bright spot in the upper left.
Credit: NASA/SDO

The sun is a violent place, one that seethes with solar flares that blast radiation, heat and charged particles out into space.

A pair of X-class flares erupted Tuesday (Mar. 6) in a one-two punch, unleashing gigantic bursts of material into space, some of which may rain down upon Earth in the coming days. The first storm was also the more powerful, ranking as an X5.4-class flare, according to an alert from the Space Weather Prediction Center operated by the National Weather Service. It is the strongest solar flare yet for 2012. But what do the numbers mean?

Astronomers rank solar flares in a classification system of five categories: A, B, C, M, and X. Class A flares are the weakest, while class X solar flares are the biggest, and can wallop the Earth with radiation that interferes with radio, GPS systems, and power grids.

The classification system, designed by the National Oceanic and Atmospheric Administration (NOAA), is similar to the Richter scale for earthquakes, in that each category is 10 times stronger than the one before it.

Thus, a B-class solar flare releases 10 times more energy than an A-class flare, while a C-class eruption releases 10 times more than a class B flare (and 100 times more than class A). The scales are further divided into subcategories ranked from 1 to 9.

While class A flares are pretty puny, stronger solar flares can pack a punch.

"The biggest X-class flares are by far the largest explosions in the solar system and are awesome to watch," NASA officials wrote in a statement. "Loops tens of times the size of Earth leap up off the sun's surface when the sun's magnetic fields cross over each other and reconnect. In the biggest events, this reconnection process can produce as much energy as a billion hydrogen bombs."

When aimed at Earth, powerful X-class solar flares can pose a threat to astronauts and satellites in space, disrupt satellites in orbit and even damage power grids on the planet's surface.

The strongest solar flare ever recorded occurred in 2003, and was so powerful it maxed out the sensors measuring it, which topped out at class X15. Scientists think this flare was probably closer to class X28, in reality.

Solar activity varies on an 11-year cycle, with the sun going through quiet and rowdy times periodically. The current solar cycle is known as Solar Cycle 24. Feb. 15, 2011 saw the first X-class flare of the current solar cycle, with more following over the summer.

With the sun recently coming out of a lull and gearing up for a solar maximum expected in 2013, this should bring many more strong solar flares, NASA scientists have said.