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Blue Staircase & the Blue Towers
Hi Colin,
that blue ray is very subtle but it is definitely a part of the surge pushing from west to east. The colour tells the story. Blue is a sign of bleaching by the sun. It happens more during aurorae australis that occur between mid October through to mid February.
It can happen in two ways. Firstly if the rays are particularly tall, and they can reach up to 1,000 km in height, then there is a chance that the rays will stick out above the earth's shadow. In doing this the normaly red rays are turned blue. This is for aurorae that are a long way from you. This is more prevalent the closer you are to either evening or morning twilight.
In your case the single ray looks to be a rogue one that is well in advance, further north, of the main display. This would bring it out of the earth's shadow as the left hand side is closer to the dawn twilight.
In my photo attached from the Great Storm of Nov 24th 2001, you can see how the tops of the rays are being bleached by the sun. I've also attached "The Blue Towers." The tallest rays I've ever seen as you can see from the annotated version the rays went beyond my zenith! It took three 28mm shots to get them all in. These must have easily been 1,000 km tall. The base of this display was overhead at Queenstown in the South Island, about 600 km SW of me. BTW the eye doesn't see iot as blue as that. It is more a soft blue/white colour. The reds/orange/crimson colours though can be just as the camera shows them on the big storms.
So, while most of your display lay inside the earth's shadow that one ray stuck its head out into the sunlight!
Cheers,
Ian
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