So what the heck are CMEs?
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.
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?)
The vast majority of them aren't THAT big, and aren't even Earth-directed. The chances of one smacking Earth aren't that big. 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.
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. 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 trillion 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 fast CME can reach Earth in just under 13 hours.