Monday, April 30, 2012

Solar, Space, and Geomagnetic Weather, Part 1

A lot of my friends and fans over on Facebook have become followers of my solar and aurora alerts there, and it has been suggested that I make this a regular part of my blog, so I thought I'd explain what it is and why it's important.

All three - solar weather, space weather, and geomagnetic weather - are interconnected. This is because the Sun has a magnetic field that extends far past the Earth, and so the Earth's magnetic field interacts with it. "Space Weather" is essentially a term for the conditions of space in the general vicinity of Earth, but not necessarily inside the Earth's magnetic field.

We are also sitting inside the atmosphere of the Sun, which is called the corona. It generates a wind, usually coming out from the Sun and spiraling away – yeah, the “solar wind.” Granted, the corona isn't very dense, but it's dense enough to create some effects, and we're working on using it to our benefit, like in solar sails and such, which can use the solar wind as much as light pressure (different blog post) to maneuver around the Solar System like the spaceborne clipper ships of old.

But when the Sun gets...agitated, we'll can get a lot denser. Coronal holes move from the poles down to lower latitudes, and the Sun's face develops an astronomical case of acne. This usually occurs around the time of solar maximum.

Whoa. Waitaminit. What's “solar maximum”?

Our Sun has cycles that it goes through. Some are short and some are long. These cycles are related to its magnetic field and to sunspots. In fact, many variable star astronomers such as myself consider that the Sun is at least a borderline variable star because of this; some consider it outright variable. We'll leave that to a later discussion. For now, let's just look at those cycles and why they exist.

The Sun is a gigantic ball of plasma, a gas of ionized particules like protons and electrons. It spins on an axis. These two facts, when combined, create an electic current. An electric current, in turn, generates a magnetic field. This is why the Sun has a magnetic field, and it looks like a bar magnet – a “dipole.” (Remember elementary school when you put a piece of paper on a bar magnet and sprinkled iron filings on it? It made a cool bunch of lines that arced from one end of the magnet to the other, and then fanned out at the very ends. That's what I'm talking about.) The polar areas normally have “coronal holes,” because of the open-ended lines. The plasma flows out, away from the Sun, at high speeds (200-600km/s, 124-373 mi/s or 447,000-1,340,000 mph).

But since the Sun isn't solid like a bar magnet, the plasma doesn't all have to spin around the axis at the same speed – and it doesn't. The poles don't spin at the same rate as the equator, and the deeper layers don't spin at the same rate as the surface.

So let's think about those lines of iron filings again. Our bar magnet has gone and gotten itself all twisted up because it isn't solid, so the lines of iron filings get all twisted up, too. Now, scientists are still working on this, but the best we can figure out now is that sunspots are places where “snarls” form in the magnetic lines, and break through to the surface. (In the last couple of years we've learned how to look “deeper” into the Sun to see these snarls below the visible surface. Remember that. It'll come into play later on, when we start talking about the Sun as a variable star.) This means that sunspots have magnetic fields, sometimes very complicated. There are almost always at least two – one is a north magnetic pole, the other a south pole. (When there is just one, it is usually funny-shaped and one end will be North and the opposite end South. And sometimes there's a whole cluster, which gets really complicated.) And most all of the spots on the Sun will have the same N/S orientation.

It turns out that every 11 years, there is a peak in the number of sunspots, and a minimum in the number of sunspots. We aren't quite sure why, because we don't have all the theory worked out yet. But we've all heard of Solar Maximum and Solar Minimum, and that's what those terms mean. Solar Max is when we have the most spots, and Solar Min is when we have the least.

(To be continued.)

-Stephanie Osborn