We had snow last night and it is still snowing today, so I sat down and analyzed some of the high speed photometer data from Monday evening. The Interplanetary Magnetic Field (IMF) was nicely southward from about 04-08 UT, see the attached plot from the ACE website.
The ACE spacecraft is between the Earth and the sun, and measures the IMF orientation, as well as the solar wind speed and density. When the IMF is southward, it can "cancel" out the magnetic field of the earth which points northward. Magnetic fields can store energy (inductors are an example of this), and when the Earth's magnetic field "reconnects" with the IMF, some of the magnetic field energy is released into kinetic energy of the particles. This increase in kinetic energy results in increased aurora.
It turns out that the energy is not distributed evenly around the auroral oval when the IMF turns south. Rather, the most likely place to see an increase in auroral activity is on the opposite side of the earth from the sun. At Toolik Lake magnetic midnight is about 11:30 UT, so the aurora we were looking at was pre magnetic midnight.
At around 08:00 UT on Monday 4 March 2008, we were looking at a quiet arc that was in the magnetic zenith. The magnetic zenith is the location where the magnetic field comes straight into the Earth. At Toolik this happens at about 203 degrees Azimuth (with 0 degrees being magnetic north), and 80 degrees elevation. This means we were looking straight up the magnetic field line.
We took regular fram rate (30 frames a second) video with a Watec Supreme camera that is not intensified, as well as high speed video with a Phantom 7 camera with a VideoScope intensifier. Additionally we had a 16 channel multi anode photometer from Hamamatsu that we digitize at 20,000 samples per second. This photmeter is an easy way to look for high speed flickering in the aurora. We take the digitized time series output of the photometer, and turn it into a spectrogram, which shows power at different frequencies as an image.
The spectrogram shown to the right comes from pixel number 8 
(the center pixel in the linear array of 16 pixels). The data were taken at 0801UT. The top panel in the image shows the digitized output, while the bottom panel shows the spectrogram. In the spectrogram, increasing frequency in Hertz is long the y axis, while time runs along the x axis. the color bar is in dB, with reds and yellows being higher power than blues. As you can see, there is quite a bit of power down around 5-10 Hz. In the first ~15 seconds of the spectrogram, there is also power at higher frequencies, 50-80 Hz. The eye can detect frequencies up to about 10-15 Hz, so the lower frequencies are what are seen by the eye in the Watec images.
(the center pixel in the linear array of 16 pixels). The data were taken at 0801UT. The top panel in the image shows the digitized output, while the bottom panel shows the spectrogram. In the spectrogram, increasing frequency in Hertz is long the y axis, while time runs along the x axis. the color bar is in dB, with reds and yellows being higher power than blues. As you can see, there is quite a bit of power down around 5-10 Hz. In the first ~15 seconds of the spectrogram, there is also power at higher frequencies, 50-80 Hz. The eye can detect frequencies up to about 10-15 Hz, so the lower frequencies are what are seen by the eye in the Watec images. Each file is 30 seconds long, and takes the computer a bit over 30 seconds to write to disc. 
So the next file was about a minute later at 08:03 UT. The spectrogram for channel 8 is shown to the left, and you can see the higher frequencies have disappeared, leaving the lower "narrow band tone" at around 8-9 Hz. A little later in the spectrogram the higher frequencies are seen again, but weaker than they were in the first spectrogram.
So the next file was about a minute later at 08:03 UT. The spectrogram for channel 8 is shown to the left, and you can see the higher frequencies have disappeared, leaving the lower "narrow band tone" at around 8-9 Hz. A little later in the spectrogram the higher frequencies are seen again, but weaker than they were in the first spectrogram. This is very similar to what Hans and I published in a Geophysical Review Letters article back in 1998.
We hope to obtain optical observations of flickering aurora at the same time that Jim LaBelle's Dartmouth group measures radio frequency auroral roar.
1 comment:
Geoff,
I thought there were some images of the aurora from the night you took the spectra...Can you add those in so the students can see what your eyes were looking at?
Delores
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