One of the very best plots I check when wanting to know if I will see any aurora on any given night is this one:
which can be found here.
If you are looking at this plot from the past two nights you will be kicking yourself for not looking outside (because there was probably some good aurora if you are at a high enough latitude).
So what does this plot mean? Along the x-axis you have three days of time, and along the vertical you have magnetic field strength. There are two traces plotted. The red trace (as seen from the label on the right) is at a longitude of W75 (oh look Mark Twain's 176th birthday, thanks google). I live at a longitude of W . The blue trace is from a satellite that is over the West coast. I guess the main purpose for these satellites is for weather measurements. We use them because they have magnetometers to measure the Earth's magnetic field.
What does measuring the Earth's magnetic field have to do with aurora? Stop getting so distracted by Mark Twain and magnetic fields.
Don't worry, the magnetic field is important. Mark Twain is important for things not related to the aurora.
So imagine the Earth's magnetic field as a perfect dipole. Like so:
If you were a satellite, orbiting around the Earth at the same rate as the Earth's rotation, measuring the just the vertical component of the magnetic field, what would you measure? If you said a straight line, you were correct. As you orbit around the Earth, the magnetic field you would measure is constant, say 90 nanotesla.
But the Earth's magnetic field is more interesting than a perfect dipole, because it interacts with the sun's magnetic field, aka the IMF (interplanetary magnetic field), and the solar wind. Remember this post? That is what actually happens. So the Earth's magnetic field ends up looking more like this:
The field lines on the Sun-side of the Earth get shoved in, and the field lines on the opposite side get stretched out into a tail. Ignore the labels on that photo that say "acceleration region" and "auroral electrons" because they are wrong/don't make sense.
So now, as your little-satellite-self orbits around, when you are at noon, you measure a stronger magnetic field, say 100 nT. And when you are at midnight you measure a less strong (vertical component) of magnetic field, say 80 nT. If you plotted this, you would end up with a sinusoidal wave. A sinusoidal wave is a realistic trace to see on the GOES plot, and unfortunately that means there will probably not be any aurora.
The aurora, in the simplest way of thinking of it, is the result of a giant energy release. Which means you need to somehow get energy into the magnetosphere system. This energy comes from the solar wind, which is why we look at ACE as part of predicting aurora. When a lot of energy is being put into the system that magnetic field will no longer be nice and stable, but highly variable. If you look at the plot at midnight (conveniently labeled with an 'M' (and there is an 'N' for noon)), you can sometimes see that tail getting stretched out like crazy, so you are only measuring a magnetic field of 25 nT or even less. This measurement is indicative of a huge amount of energy being put into the system, and that energy needs to get released, and the beautiful result is an amazing light show.
And since we are talking about predicting the aurora, I'll mention that if you see a sudden spike in the magnetic field measured at GOES, this is indicative of aurora to come in 30-60ish minutes.
The main idea: smooth sinusoidal curve is no good, jagged trace with big spikes and dips is very good.
I guess the last main thing I look at for predicting the aurora is the optics from the ground, so that will be up next!