Solar, Space, and Geomagnetic Weather, Part IV
By Stephanie Osborn
“Interstellar Woman of Mystery”
Rocket Scientist and Novelist
“So what the heck are CMEs? I mean, what causes them? In detail this time, please!” you ask. (Or maybe you don’t. But chances are, SOMEONE reading this IS asking it.)
Coronal Mass Ejections are gigantic explosions that occur, usually in the vicinity of particularly active sunspot groups (though not always). We’re still discovering what they are, how they occur, and why they do what they do. It seems to get into some complicated electromagnetic physics and something called “magnetic reconnection.”
Think about it like this. Suppose you have two bar magnets, lying near each other but, say, perpendicular to each other. Each has its own magnetic field, with field lines that go out from one pole and arc around to the other pole (remember our discussion of iron filings a couple weeks ago?), but now we’ve got them close enough that those magnetic fields interact. If you’ve ever played with magnets, you know that, if you hold those two suckers just right, you can make one “float” in mid-air, and even make it spin and gyrate. There’s a lotta expensive, executive-desktop toys based on this. Like this one:
The concept behind the toys is mostly “like repels,” but it gets a little more complex when the toys are MOVING. That means the magnets are moving relative to each other, and the magnetic field interaction gets way more complex to model.
Now suppose — just suppose — a field line broke away from its parent magnet and attached the opposite end to the other magnet? Now suppose a whole SEGMENT of field lines did that. Those bar magnets would start dancing a whirligig, and the magnetic field would go crazy.
Now suppose that the bar magnets are swirling plasma gases, and the field lines are running through more swirling plasma.
THAT is magnetic reconnection. The end result is that a whole bunch of energy gets transferred from the field into kinetic energy. This heats up the plasma AND accelerates it, and, at least on the surface of a star like our Sun, a titanic explosion is the result. A great big blob of plasma goes flying out into space, and that blob is a “coronal mass ejection,” because a big mass of the corona just got ejected from the Sun. (Imaginative name, huh?)
What does magnetic reconnection look like? It looks exactly like this:
So you can think of the reconnection event as being like a titanic nuclear bomb, and the CME is the mushroom cloud moving up…and out, into interplanetary space. (How big a bomb? Well, solar flares vary in size and power. But NASA solar scientists estimate they’re “the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time.” Keep in mind that the biggest nuke ever detonated on Earth was Tsar Bomba, at only 50 megatons.)
The vast majority of them aren’t THAT big, and aren’t even Earth-directed. The chances of one smacking Earth aren’t that great. But because there are a lot of them, especially at Solar Max, it happens fairly often. Sometimes it’s just the edge of the expanding bubble, but sometimes it whacks Earth upside the head. And when they come in, they’re coming fast.
Putting it in motion:
And here’s the beginning of a CME, up close and personal, replayed several times to enhance different features of the event.
Moreover, most of the time as the magnetic field re-forms underneath the developing CME, it can actually accelerate the plasma of the CME out, away from the photosphere and into the solar system.
The WSA-Enlil model is often used to help “track” CMEs as they move through the solar system. It also give you a very good idea of how these things DO move in interplanetary space. Here’s what that would look like.
So what are the general parameters of a CME? Depends on where in the solar cycle you are. If you’re near Solar Minimum, they occur about one every 5 days or so. If you’re around Solar Max, expect one every 6 or 7 hours.
How big are they? If you’re talking volume, that’s gonna depend on how far out from the Sun they are, and how well the interplanetary medium is allowing them to hold together. Note, in the Enlil model, how the thing spreads out as it moves outward.
If you’re talking how massive, well, on average they’re about 3,520,000,000 lb (1,600,000,000,000 kg). That’s over three and a half billion pounds of plasma. On average, their speed is about 304 mi/s or 1.1 million mph (490km/s). IF, however, one follows close on the heels of another, so that the first one has swept most of the interplanetary medium out of the way (decreasing drag), the speed can increase to 2,000 mi/s or 7.2 million mph (3,200 km/s). And with the Sun 93 million miles away, that means a really fast CME can reach Earth in just under 13 hours.
Comet Tales blog/Osborn Cosmic Weather Report: http://stephanie-osborn.blogspot.com/