July 16, 2017 Auroras
Pokegama Lake, Grand Rapids, MN
Solar Cycle 24 is gradually winding down, and there are fewer days with visible sunspots on the sun. This week was an exception in a long absence of solar flare-induced auroras. A nice sunspot group (2665) rotated into an Earth-facing position and fired off a long duration M2.4 flare. The flare was associated with a full halo coronal mass ejection visible on LASCO C3 imagery. WSA-Enlil models had indicated an arrival at Earth sometime mid- to late-day on the 16th, but I had the feeling it would be a little earlier than that.
Late on the 15th, the low energy proton flux started to increase, and I thought a CME impact was imminent, but there are often false alarms. The evening went by without an impact, and I was rather sleepy after a full day outside, so I decided to head for bed. I told a friend to call me if the CME impact took place.
I got a call probably within 30 minutes of falling asleep, but I continued to sleep for another hour or so beyond that, finally waking up after 2:00 AM when I checked my phone. I saw the missed call, and I checked to see that the CME impact had indeed occurred, and Bz was hovering around -10 nT. The auroras would definitely be out, and I had another hour or more to see them.
I hopped in the car and headed to my spot on Pokegama Lake. When I arrived, the auroral oval was barely discernable because the last quarter moon had risen, and forest fire smoke, which is very common this time of year, covered part of the northern and eastern horizon. I snapped one photo to verify that the band I was seeing was indeed green, and then I waited to see if things improved, not wanting to waste pictures on what I was seeing presently.
Within 20 minutes, the auroras brightened, and some structure started to appear, so I set up my camera to take a picture about every 10 seconds. More structure appeared, and the vertical columns migrated from west to east through the band slowly enough that I did not have to decrease the exposure time of my shots. I continued shooting as the columns passed over the house boats in front of me.
After about 30 minutes, the northern horizon began to brighten noticeably from the upcoming sunrise, but it was at this same time that Bz dropped to -20 nT. I doubted the auroras would respond to this drop before the sunlight overwhelmed them, but I decided to stay up and watch anyway. The auroras settled back a bit to a diffuse green band, but as the sky brightened, taller, brighter columns appeared, and I had a few more minutes of some pretty nice activity. Then, just when I thought things would start to move overhead, the auroras "flamed out" and retreated to a diffuse band along the northwestern horizon (the northeast being too bright and covered with a band of dense smoke). Apparently, the substorm timing did not coincide with the drop in Bz as it reached Earth.
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| Auroras above a houseboat on Pokegama Lake. |
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| The twilight is getting a lot brighter, but the auroras are keeping up. |
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| Wider angle (16mm) shot as the last visible auroras migrate to the east. |