Sunday, February 16, 2014

Auroral Forecasting: A Study Guide

There's two main things I look at when I'm trying to figure out what the aurora's about to do: Solar wind and Earth's magnetosphere. I guess that much is obvious since the whole show is caused by the interaction of those two things.

I'm sort of assuming you know the basic idea of how auroras work here: The Sun emits plasma which makes up the solar wind, when the solar wind collides with Earth's magnetosphere (if the solar wind is 'south' or 'negative') it strips magnetic field lines off the dayside of Earth and drapes them back behind the planet, where they 'reconnect' and accelerate plasma down the field lines to make aurorae. It still happens when the solar wind is north, BTW, just the geometry is different so where the plasma comes down to Earth is different.

So what you mostly care about is how much stuff is coming in on the solar wind, if the magnetic field geometry is right (southwards), and how much Earth's magnetosphere is being squished and stretched in response. I look at the readouts from two satellites to get this: ACE which sits about an hour upstream of Earth measuring the solar wind, and GOES-15 which is a geosynchronous satellite sitting at 135W longitude measuring (among other things) the shape of Earth's magnetosphere.

Here's a readout of the last two hours at ACE. I like this two hour display but everyone else seems to like the longer ones. Whatever. Either way, you look for the speed or density to be high (the amount of stuff coming in) and for Bz to be negative (which means the field is southward, and the more southward the better). Since ACE is about an hour upstream, whatever you see happening at ACE will reach Earth an hour later.

And here's the readout from GOES. This is measuring the component of Earth's magnetic field parallel to the dipole axis (at the location of the satellite... you'll see a daily up/down cycle which is the satellite going around the planet once a day). In simple terms, when the satellite is near midnight (marked 'M' on the plot), a high value means the field is not very compressed, and a low value means the field is compressed and storing magnetic energy. When that energy releases you see the value suddenly jump upwards, and that probably means some plasma has been accelerated down the field lines and you should expect to see the aurora brighten in roughly 10 or 15 minutes. The higher the jump up, the more energy released, and the more auroral activity you should expect. When the value drops below about 50 you should expect a release soon.

And oh, but they give it as a three day plot? How awkward is that when you're looking for features on the scale of minutes? So this 3 hour plot is better.

With that in mind, I put together something of a 'study guide' display. It's one thing to hear this stuff as theory, but it's much better to see it in practice. So I took the timelapse movie of the night's activity from AASAP and overlaid the last hour of ACE Bz data and the last 30 minutes of GOES Hp data. This is useful I think for anyone from a beginner to a seasoned vet because it lets you watch in a minute or two how these factors work together over a whole night. Here's the video from Feb 08, when we had a fairly short but quite spectacular display:

So if you watch, Bz was around zero or weakly south from the beginning, leading to some slow but steady buildup of magnetic energy in Earth's magnetosphere, reflected in the Hp number being down around 40nT. Around 06:00UT Bz turned more solidly south, and one hour later that led to a pretty strong buildup of magnetic energy as Hp went down to around 20nT. Bz jumped back sharply to the north around 07:00UT, and when that jump reached Earth an hour later it triggered the release of all that stored magnetic energy - GOES Hp jumped nearly 50nT in 10 minutes! That's a really large jump, and as expected 15 minutes later we see that westward traveling surge come in from the east (right) side of the image and the aurora gets very bright and active. Once the energy is used up the bright arcs fade away, leaving a large mass of drifting, pulsating diffuse aurora, which is pretty typical - this is the recovery phase of the substorm. Bz remains north for a few hours, and so isn't 'driving' the aurora very much and we just get some very weak diffuse stuff for a while, but around 12:00UT it turns south again. This late in the evening the GOES satellite is no longer well positioned, but an hour later we do see the aurora brighten up and get more active. This late in the evening Poker Flat is coming out of the 'auroral electrojet' where the brightest, most active displays happen, so instead of super bright arcs like earlier the display is dominated by large patches that drift around and pulsate. This continues for the remainder of the night as Bz remains southward.

So yeah. I'll be making more of these overlays when I get good example nights, and I hope people can learn by watching them. I chose this night as the debut because we had that really impressive 50nT jump in Hp. Here's what the GOES 3-day plot of that looked like:

And here's a 5 hour plot:

And, in case you were wondering what that looked like in person, here's a video from an intensified CCD video camera. It's black and white, and kind of noisy, but it can record the aurora in real time video, so that's something. I sped it up by about 4x to better see the dynamics of the westward traveling surge come in, and the arc movement. This should give a good idea as to how much it was actually moving around.

Sorry about the big black thing blocking part of the sky. The camera is sensitive enough that the Moon will ruin it, so that's the lawnmower battery we set next to the camera to block the moonlight. It's very high-tech.

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