Monday, November 21, 2011

Predicting Aurora: Kp index

I was recently talking with my brother, who spent his summer closer to the auroral oval than I did. He had a lot of opportunities to see really good aurora and asked what some of my methods are for knowing when the lights will be bright. So I'm going to do this blogging series, Predicting Aurora.

It will be fun.

So one thing he was checking was the Kp index. Kp index is averaged over three hours from ground magnetometers across the US and Canada. Basically, when a lot of geomagnetic activity will occur in conjunction with auroral activity. The geomagnetic fluctuations can be observed on the ground. Then they are averaged and put into this arbitrary 0-9 scale. Usually if Kp is below 3 aurora will be limited or be at very high altitudes. Kp between 4-6 will make really awesome aurora (as long as it is close to midnight over the US & Canada, and not noon). If Kp goes above 6, anyone living below latitudes of 45 degrees should go outside because you could be getting a rare auroral treat.

This is where I go to view the Kp index.

There is another great advantage of the Kp index. You can keep track of it on longer timescales than the 3 day plot linked above, and this gives you a method for predicting activity ~27 days in advance. From the Earth, we observe the sun to have a rotation period of ~27 days. So, if we see a sunspot and observe aurora to occur in conjunction with that sunspot, it is likely that sunspot will reappear again once the sun makes a full rotation. Obviously sunspots don't last forever, so we might only see each one two or three times, but it is still a good predictor.

And lucky for us, someone has already taken the time to keep an updated plot of the Kp index on this conveniently useful cycle of 27 days. So if you go here, you will find that often, spikes of high Kp are on top of each other.

Next up will be how to predict aurora ~3 days in advance.

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