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August 3, 2016 Auroras...
...and their onset in Grand Rapids, MN
A combination of a CME from a filament eruption and a coronal hole high speed stream caused enhanced solar wind conditions. The magnetic field magnitude was over 20 nT, which is about three times its non-enhanced value. I was hoping to do a day to night transition time lapse, but magnetic field vector turned sharply north during the evening, and Bz was around +19 nT, about as high as it could go for the conditions. I was going to shoot on Pokegama Lake, but I headed home instead. I was supposed to drive to the Twin Cities the next days, so the northward Bz was, overall, probably a good thing for me.
In my compulsive habit of checking the solar wind conditions, I decided to pull up the SWPC 6-hour plot one more time before going to bed. Well, surely enough, the magnetic field vector had flipped, and Bz was now -20 nT. The auroras had been so dim on the northern horizon that their visibility was afforded only by a moonless, clear night. That was about to change.
Given the time it typically takes for the sudden field change to get to Earth from the measuring satellites' position at the L1 point and then through the magnetotail, I had about 45 minutes before the auroras brightened. Sometimes, this takes a little longer when the conditions have been very quiet for a while or if Bz has been strongly north. This gave me time to prepare myself to simply fall into bed when I returned home from shooting so I could get at least a few hours sleep before my drive the next day. I went to one of the close spots on the north side of town.
When I arrived, there was really nothing going on. The Bz drop was not yet affecting the auroras. My first image was at 11:50 PM, and there was no brightening for another 10 minutes. Between midnight and 12:30 AM, the auroral oval steadily brightened and moved south. The onset of structure occurred only gradually over the ensuing 20 minutes between then and 12:50 AM. This transition from an unstructured, static arc to dancing auroras often happens in less than a minute, requiring the photographer to scramble to change exposure settings to accommodate the sudden brightening. No such scramble was necessary this time, but I did have to change exposure nonetheless-- just calmly as opposed to doing so while uttering four letter words.
The structured show continued for about 40 minutes, which is pretty long. By 1:30, things were settling down, so I made the decision then to pack things up and make that leap into bed.
There was barely a glow on the northern horizon when I started shooting at 11:51 PM.
An hour later, at 12:50, a bright show was in progress. I took another shot at this same time from my other camera, pointing more westward (see below).
For about 15 minutes, I watched as the auroras danced to my north. The bright, structured phase of the substorm lasted a little longer than average.
More structures and colors.
Auroras now fill the 24mm field of view.
Here is the time series of solar wind. During the gap in this plot (ACE satellite), the new DSCOVR satellite showed very northward Bz (the red line in this plot).
VIIRS imagery from the NPP Suomi satellite showing the position of the auroras 15 minutes after I stopped shooting
Zoomed in, eastern view at the same time as above.
Bz went well south again later in the night, and I was worried I might have missed a show later on, but the satellite showed not much going on at 3:25 AM, and daylight approached shortly after that (you can see it on the right side of this image).
The dark spot in the center of this image is the coronal hole that provided the disturbed conditions for the auroras.
This might be my favorite image from the night. It was taken with the Samyang f1.4 lens as the bright show was starting. Compare it with the second image above.
Another image from the Samyang.
I often have a tough time deciding which images to put here when there are so many to choose from.
I decided to switch from 24mm to 16mm. Here's the wider field of view.
Another 16mm image.
One of the last images of the night, as the show began to fade a little. It was still covering a large part of the sky, but I had to use a longer exposure here.
