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Around 04:00 UTC on Monday morning, January 23, 2012, the Sun let loose a pretty big flare and coronal mass ejection. Although there have been bigger events in recent months, this one happened to line up in such a way that the blast of subatomic particles unleashed headed straight for Earth. It's causing what may be the biggest space weather event in the past several years for Earth: people at high latitudes can expect lots of bright and beautiful aurorae.
I'll explain what all that is in a second, but first here's a video of what this looked like from NASA's SOHO satellite.
Wow! Make sure you set it to high def.
So what happened here? The sunspot cluster called Active Region 11402 happened.
Sunspots are regions where the magnetic field lines of the Sun get tangled up. A vast amount of energy is stored in these lines, and if they get squeezed too much, they can release that energy all at once. When this happens, we call it a solar flare, and it can be mind-numbing: yesterday's flare exploded with the energy of hundreds of millions of nuclear bombs!
In the image above, the sunspots are caught in mid-flare, seen in the far ultraviolet by NASA's Solar Dynamics Observatory (it's colored green to make it easier to see what's what). We think of sunspots as being dark (see the image of AR 11402 below), but that's only in visible light, the kind we see. In more energetic ultraviolet light, they are brilliant bright due to their magnetic activity.
A huge blast of subatomic particles was accelerated by the explosion. The first wave arrived within a few of hours of the light itself... meaning they were traveling at a significant fraction of the speed of light!
But shortly after the flare there was a coronal mass ejection: a larger scale but somewhat less intense event. This also launches particles into space, and these are aimed right at us. The bulk of the particles are traveling at slower speeds -- a mere 2200 km/sec, or 5 million miles per hour -- and is expected to hit us at 14:00 UTC Tuesday morning or so. That's basically now as I write this! Those particles interact with Earth's magnetic field in a complicated process that sends them sleeting down into our atmosphere. We're in no real danger from this, but the particles canstrip the electrons off of atoms high in the air, and when the electrons recombine the atoms glow excite the electrons in atoms high in the air, and when the electrons give up that energy the atoms glow. That's what causes the aurorae -- the northern and southern lights.
If you live in high latitudes you might be able to see quite the display when it's dark -- people in eastern Europe and Asia are favored for this, since this happens after sunset there. But the storm is big enough and will probably last long enough that everyone should check after dark: look north if you live in the northern hemisphere and south if you're south of the Equator. There's no way in advance to know just how big this will be; it might fizzle, or it might be possible to see it farther away from the poles than usual. Can't hurt to look! Also, Universe Today has been collecting pictures of aurorae from the solar blast earlier this week. No doubt they'll have more from this one as well.
Although big, this flare was classified by NASA as being about M9 class -- powerful, but not as energetic as an X class flare. One of those popped off last September, and shortly after that a smaller M flare erupted, which also triggered a gorgeous plasma fountain called a filament on the Sun's surface.
As I said, we're in no real danger here on Earth, and Universe Today has a good article describing why the astronauts are probably not in danger on the space station, either. Even if this were larger storm, the astronauts can take shelter in more well-protected parts of the station, too. Bigger storms can hurt us even on Earth by inducing huge currents in power lines which can overload the grid. That does happen -- it happened in Quebec in March of 1989 -- and it may very well happen again as the Sun gets more active over the next few years. [UPDATE: a ground current surge from today's event was reported in Norway.]
But we should be OK from this one. If you can, get outside and look for the aurorae! I've never seen a good one, and I'm still hoping this solar cycle will let me see my first.
Image credit: NASA/SOHO; NASA/SDO