| MadSci Network: Earth Sciences |
There is a good Website reference for this material, with
diagrams
and images, if you would like a slightly more technical and complete
answer. It also contains many useful definitions. If you're just asking
about predicting the Northern Lights, and not about what causes them,
skip to the bottom of this answer.
First I will describe what causes the Northern Lights, and then I'll
address the question of magnetic storms.
The Earth has a magnetic field, which can be pictured as lines
coming out of the North pole, wrapping around the planet, and then sticking
back into the South pole. Ions (particles that are not electrically
neutral, but rather have a positive or negative charge) travel along
these field lines -- that is, charged particles in the upper atmosphere
won't just fall to ground like a stone would. Instead, they are
pulled around the planet along the magnetic field, and they enter the
atmosphere either at the North or the South pole. When these
ions enter the atmosphere, they give off light, and so we see
the Northern Lights (or the Southern Lights) ocurring in rings, around
the poles.
Where did the ions come from in the first place? Well, there is a
resevoir of charged particles in our upper atmosphere (the region
called the Ionosphere, logically enough). Some of the ions come from
there, and they move along the field lines back and forth between the
North and South poles. But the ionosphere is also continually replenished
by charged particles that come from the Sun -- the Solar Wind.
The Solar Wind is a constant stream of charged particles that sweeps
through the solar system. Our magnetic field shields us from most of
this stream (remember, the charged particles can't cross the field
lines, so they get deflected and either just go around us, or end up
in our ionosphere).
So the Northern Lights are a natural, continual phenomenon, that
don't require a storm to produce them. But that's not to say that
the sun doesn't affect the Northern Lights. Sometimes, the sun is more
"active" than other times. The sun has an 11 year cycle: we're near
the minimum of this cycle now, so that in about 11 year the sun will
be at it's most active, and 11 years after that it will again be
"quiet." We see much more Auroral activity (Northern Lights) near
solar maximum. But even during solar minimum, sometimes the sun gives
off flares or other strong bursts of energy, so that the Earth gets a
particularly strong hit of particles and energy. When this happens,
the Northern Lights can spread towards the equator, so that people can
see them not only in Alaska, but in the Northern United States, as well.
The phrase "magnetic storm" usually refers to something that happens
when the Earth's magnetic field is suddenly stronger, and there are more
charged particles and electricity in the upper atmosphere. They can be
triggered by solar activity. That is, something happening on the Sun
(a solar flare, for example) will cause the solar wind to be stronger
briefly, and this in turn will cause a magnetic storm at the earth.
A magnetic storm can last from a few hours to several days. During
this time, the Lights are stronger as well. So magnetic storms -- which
humans can't feel or see -- might be made obvious by particularly
brilliant displays of the Aurora.
If you are also asking whether we can predict magnetic storms and
big displays of the Northern Lights, the answer is no, not really.
It would be nice if we could, because magnetic storms damage scientific
and communications satellites, and if we knew when storms were coming we
could find ways to protect these expensive and necessary things.
But if you just want to know when your chances are good, try looking
at the Space Weather
page. This site give a regularly-update image of the sun, and a
"weather forecast" for how active the sun is expected to be for the
next couple days, and how much magnetic activity is going on around
the Earth as well.