Tuesday, February 21, 2012

Pulsating Aurora

There were auroras again Sunday night, but after launching the rocket Saturday and spending Saturday night and Sunday packing equipment and flying back to Fairbanks, and then a big celebratory dinner for everyone involved with the launch, I was more interested in sleeping than watching the aurora. I woke up at 3am and looked outside, and the aurora was still going. Okay, if you insist. I set my camera just outside the front door and went back to bed. Here is the video:

For the first few seconds (about 20 minutes of real time) you see the arc that was just south of me, but then things change. The arc seems to expand and fade, and what's left are a bunch of patches that flicker in brightness while seeming to drift past like clouds. This is a pulsating aurora.

Pulsating auroras are pretty common, yet few people see them. One reason is they typically occur in the early morning hours when most people are asleep. Another is they're dim (I shot this at f/4, 8 seconds and ISO3200!) so your eyes need to be pretty dark adjusted. The patches brighten and dim on the order of seconds to up to a minute or so, but typically around 10-20 seconds. Of course, every second of this video shows 5 minutes of real time, so you see them flicker several times per second because of how fast the video is.*

The Earth's magnetic field lines convect over the pole towards the night side, then back towards the day side through the auroral oval. So here in Fairbanks, right under the oval, we have field lines drifting towards the Sun, which means east-to-west before midnight and west-to-east after midnight. I can't find an animation or diagram of this online, so here's a 60-second MSPaint drawing I just did:

This is looking down at the Earth from above the north pole, and the Sun is towards the top of the screen. The green circle represents the auroral oval, and the black arrowed lines show the path of the magnetic field lines as they convect down across the polar cap (the area inside the oval) and then back towards the Sun through the oval. The Earth rotates couterclockwise when viewed from here, so the red dot represents the location of Fairbanks before midnight (on the left) and after midnight (on the right).

So one thing to notice in the video is that the pulsating patches are drifting west-to-east, because this was taken after midnight (video is facing south). This produces what I think is a pleasing effect as the stars drift through the shot in the opposite direction as the auroral patches. But here's something even more interesting: The patches are tied to processes occurring in the magnetic field, so as you watch the patches drift across the sky, you're actually watching the magnetic field lines convect past. The field lines are invisible (or nonexistent, or whatever), but when you watch those patches drift through, the field lines are sticking down through those patches, and so moving past at the same speed as the patches.

Or so we think. This is still an area of active research :)


*not to be confused with flickering aurora, which brightens and dims many times per second in real time. This cannot be filmed without very expensive equipment. Think on the order of $50,000.

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