July 25, 2016 Auroras
Grand Rapids, MN
There must have been a weak CME impact earlier in the day from the activity from sunspot group 2567. A transient passed by the ACE satellite in the morning, and the magnetic field had a disturbed look most of the day. By evening, things had calmed down a bit with Bz steady around -3 nT. I thought there could be some weak auroras, but I was not in a hurry to get out with the solar wind data not looking terribly strong.
I was wrong about that. For the conditions we had, the show was actually pretty good, and I arrived at McKinney Lake just in time (midnight CDT) to see a substorm erupt. I arrived at the lake thinking there was a band of auroras on the horizon partially blocked by a band of clouds lower on the horizon. I was wrong about that. The "clouds" I saw were actually the sharply defined lower edge of the band. Such a sharp edge is often present shortly before a substorm. Only when I looked at my shots and saw the stars close to the horizon did I realize what it was, and by the time I looked up again, the brightening was already starting.
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| 12:07 AM. Immediately before the substorm, there was a pretty well-defined auroral arc. |
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| On my next shot, structure was starting to appear. This was the beginning of an auroral substorm. |
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| The peak of brightness often occurs within one minute of the onset of the substorm. In this case, it was five minutes. |
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| A secondary peak in intensity occurred 25 minutes after the start of the substorm. The white lights in the center on the horizon are from a car or truck on Highway 38. |
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| Auroras overwhelm the brightness of the Highway 38 traffic, but the truck has also moved on. |
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| A further spreading of the auroral bands. |
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| Peak extent at 12:41 AM. The auroras faded after this, and I took my last shot at 12:45 AM. |
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