They are back over the Oregon and they showed up in full force tonight after 2 weeks of quiet skies. They appeared like a storm, taking over the horizon, the Brightest and most amazing display mother nature has shown the Northwest US yet!
A mini-meta analysis of what work existed back in the 70s/80's showed a relationship that depended on whether the observer was located in Europe, or in North America. That would seem to hint at the geomagnetic pole's position relative to the observer making a difference to the effect.
Isn't this simply the result of where higher geomagnetic latitudes are located at favourable NLC latitudes, giving NLC observers in northern USA and Canada an enviable advantage when it comes to seeing simultaneous displays of NLC and aurora? Here in Scotland I've only been lucky enough to observe this on one occasion - and that's over 24 seasons. The phenomenon is reported from Canada most years.
Yes, I've read the relevant passages on this but find it almost impossible to draw any firm conclusions from it - indeed, the authors readily admit as much in their summary.
My own direct observations have been entirely inconclusive: I've observed bright, vigorous aurora fade with NLC later forming, or, not forming; bright NLC fading with aurora then appearing, or, not appearing; aurora and NLC together with no unusual change to the NLC, and, bright NLC present with no visible aurora during sustained (2-3 days) periods of high Kp values.
This is an interesting area to study and one that visual observers can contribute greatly to. We should always be alert to the possibility of the aurora appearing when we observe NLC and, if we hit lucky, be ready to record as much detail as possible.