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February 7, 2021 Auroras
Lake Wabana
This was the first corotating interaction region in quite some time that was well-timed for the moon phase, clouds, and time of day to give us an aurora show. Bz had dropped south during the day, and the initial speedup in solar wind had happened then, too. That gotten the magnetosphere into a somewhat unsettled state. Then, a second round of southward Bz (briefly as low as -10 nT) occurred in mid-evening, allowing a nice round of auroras in the late evening.
I left home just after 7:30 in the evening and initially stopped at Prairie Lake to see what might be there. My intention was to shoot a few frames and then move on to another location since I get a bunch of shots here anyway, and there are other locations that can be nice to shoot from in the winter but are inaccessible in summer because the lakes are not frozen then.
The scene at Prairie Lake was a rather interesting one. There were a couple bands of auroras, and neither appeared to be the typical, pre-substorm band. They were essentially flat "blobs" of aurora. The band would start at the horizon then migrate southward and higher in the sky, and then a second band would occasionally form above that band (even farther southward). Once I was convinced a substorm would not happen in the next fifteen minutes, I decided to move to another lake. The temperature was approaching -20 deg F, and the wind was blowing, so I needed a shooting location with a bit less fetch for a northwest wind.
Initial, multi-banded aurora viewed from Prairie Lake.
They got as high as 20 degrees above the horizon and looked like the "Blobbert" type of aurora.
Pughole Lake has a nice boat landing on the west part of the lake that would be protected from a northwest wind, so I drove there first. I got out of the car and saw a pretty similar scene to what I saw at Prairie Lake. There was a green band on the horizon, and there was another band just fading out much higher in the sky-- probably about 25 degrees above the horizon. With the view of the northern sky limited to mostly the eastern half, the band on the horizon was hard to see from the landing, so I decided to move to a location where I could see the horizon better. Also, Pughole Lake was another location that is accessible year-round. It would be good to use one of the winter locations.
I drove to Wabana Lake. I also considered Trout Lake, which also has a good view of the northern horizon, but that location has a bit of a hike to get there, and it has quite a bit of exposure to the northwest wind. The Wabana location has a point that comes out from the landing that I could tuck behind to get out of the strongest wind. This worked. I considered driving out on the lake, but I know I'd be tempted to hop back in the car, and the headlights, etc. would illuminate the shot whenever I did that, so the car stayed on land.
At first (around 9:15 PM), all I saw was what looked like the remnants of a substorm long past. The auroras were dim and had receded to the northern horizon. There were a few dim, red columns appearing, but the show was much weaker than what I had expected given what I had seen at Prairie and Pughole. Also, Bz had dropped again to -10 nT. There should be more of a show, but auroras often ebb even when solar wind conditions were good, so it was best to wait, even though Bz was now becoming more north.
First look at the auroras on Wabana Lake. They were pretty dim at 9:15 PM.
I stood around for about an hour before a classic pre-substorm band appeared on the northern horizon. I also had to run around a bit to stay warm. I began regular interval shots again at about 10:20 when I figured this band might be the one to eventually provide a show. I continued shooting as the band got brighter and moved southward toward me, reaching about 10 degrees above the horizon. This meant that they were slightly under 300 miles (under 500km) to my north (you can calculate this based on geometry and assuming the base of the auroras is about 100km above ground level).
Auroral band as it appeared at 10:53 PM.
I continued to watch and wait for the substorm, being ready to quickly swap a battery from my warm front pocket should the cold became too much for the ones that were in the camera, but the camera kept going, so I kept running around to stay warm. I had been watching this band for better than 30 minutes. These bands can stay steady for hours before they break into a substorm, but this one seemed to be brightening faster than that. One thing to do to guage the imminence of a substorm is to look at the intensity of the band (I compare it to the glare from Grand Rapids to my south-- if it's getting intense, it will be at least as bright or brighter than that) and also its position. This one moved pretty quickly from about three degrees above the horizon to ten degrees above the horizon, but then it stayed steady around ten degrees.
There were a couple false alarms, but then at 11:05 PM, the substorm began. I had a chance to gradually drop the exposure time on my camera as the light intensity increased, and I brought it down to about four seconds. This still overexposed a shot or two, but this phase of the substorm usually doesn't last too long, and I can keep a setting the exposes the rest of the sky better while not causing the bright columns to wash out too much (their movement causes them to blur unless you use an exposure time less than one second). I shot as the dancing auroras filled the sky above the northern horizon. The contrast soon fades, but as this happens, there are often more colors. Reds start to enter the picture, and the exposure works out a little better for the camera. After a few minutes of this, the whole display gradually dims and retreats a bit to the northern horizon. At this point, you can either choose to wait another couple hours for the next substorm or to pack it up and go home. This evening, I went home.
The substorm starts. This shot was taken at 11:06 PM.
An image where the brightest features are overexposed. There were narrower, faster-moving columns than this image shows.
Best match between exposure and structure. 11:09 PM.
11:12 PM. I can increase my exposure time now-- back up to six seconds at ISO 1600, f1.4.
11:30 PM. My final image. I packed up and started for home.
I sometimes look back at VIIRS satellite images to see what the auroras looked like from space while I was viewing them from ground. Unfortunately, the earliest satellite passes are typically around 0730 UTC, so unless I stay up really late, this comparison is not possible. I guess the satellite gives me an indication of what I would see if I hadn't gone to bed. Here are some images from this night.
The first image shows that the aurora band redeveloped within about an hour after I had stopped shooting. At sometime between that and the second image (2:04 AM), there would likely have been another substorm, so the two hour wait I mentioned above would have been reasonably accurate. After that, there would probably have been a longer period of pulsating (on time periods between one and ten seconds) auroras, which seem to be the dominant type, later in the night, after a couple substorms have occurred.
0626 UTC. The band reintensified. I probably would have seen it about four degrees above the horizon.
0804 UTC (2:04 AM CST). I would have seen blobs of aurora to the north along the horizon.
0947 UTC overpass. 3:47 AM CST. I would probably see pulsating blobs in the northern sky up to 20 degrees above the horizon.
Here is what the solar wind looked like. There were periods of Bz in the -5 to -10 nT range (this image does not really capture the absolute minimum) as the density of solar wind increased and dropped back down and as the speed of the solar wind increased from just under 400 km/s to almost 550 km/s. This isn't a particularly large speed increase, so given what it was, this was a pretty good show.
Solar wind timeseries. My shooting time interval (7:55 PM to 11:30 PM CST) is roughly indicated by the rectangle.
Finally, here is a picture of what the coronal hole looked like (from Solar Dynamics Observatory UV imagery) three days before the solar wind stream hit Earth.
A look at the coronal hole. The particular region of interest is inside the rectangle (other dark areas also indicate likely high speed solar wind).
