Wednesday, July 18, 2012

Weather Whys Wednesday: Dissecting Solar Flares

Late last week, we learned of the X-Class solar flare released from the sun. Since we are likely to hear more about these sorts of events down the line, namely because of the impact they can have on satellites, the power grid, etc., here's a quick primer on the definitions and causes of some of the terms you'll hear bandied about.

Solar Flare erupting from the sun, from October 2003.
Credit: NASA
First, what's a solar flare? In simple terms: A heck of a lot of energy that's released by the sun. It's essentially the equivalent of over 1 BILLION megatons of TNT. At least that's what we presume. Basically a sudden "brightening" on the sun's surface is classified as a flare. Another term you'll often hear associated with a solar flare is a "CME," or coronal mass ejection. Usually occurring concurrently with a flare, a CME is a giant burst of solar wind and magnetic fields above the sun's corona. Additional details can be found here for CMEs and here for flares.

How are solar flares classified? They're measured by satellites above the earth, which look to quantify the flux...the unit being Watts per square meter. So the class rankings (from weakest to strongest) are A, B, C, M, X. They're similar to earthquakes, in that each class up represents a 10-fold increase in strength of the flare. For example, an M class solar flare is 10 times stronger than a C class flare. Classes A through C generally have minimal to no impact here on Earth. M class can have minor impacts, mainly at the poles. X class flares are the big boys though, and they can pack the punch and cause the problems. BUT...just because a solar flare is X-class does not mean it's going to destroy the electrical grid...as you might think from watching the news every time there is an X class flare.

Visible auroras: A fairly common result of solar flares
and CMEs. This, from 2003. Credit: NASA
Now, within each class, there's a linear increase from 1 to 9. So B2 is twice as strong as a B1 flare. Within the X class however, there's not a specific end to the scale. The strongest recorded flare was back in 2003 and was at least an X28 class flare...as the satellite sensors cut out presumably before it peaked. Last week's flare measured at X1.4. But it was followed by a strong CME and it was directed right at Earth. As you might expect, that's important from an aurora standpoint (as well as other effects). Direction, strength, speed, and orientation will always allow for variable impacts...and like most events in the atmosphere or space, no two are exactly alike in terms of composition or impact.

There's no way to know how strong it was, but the 1859 Carrington Event solar flare would dwarf any solar event in modern history. The Carrington Event produced a visible aurora as far south as the Caribbean, destroyed telegraph lines, and produced auroras bright enough in the Northeast to allow people to read newspapers at night. The Carrington Event is the main reason we've seen such dire news stories in recent years describing how serious solar storms can be. A dose of reality though: While certainly possible again in the future, an event of the magnitude of a Carrington Event is extraordinarily rare...and in hundreds of years of observations, it has only occurred once. While solar flares are important to study and prepare for, you needn't panic about it every time one occurs.