I have been lucky enough to see them twice on the flight deck. Once on a 'polar route' flight from the sand pit to the states, and another time over ABZ. Depending on winds, routing and solar activity, you may stand a reasonable chance. Stay sat on the left and glued to the window
You may well see them over northern Canada. Depending on where you are sat, you may struggle to see them due to the Nav/Strobe Lights. When they have been spotted on the light deck we lower all the lighting to the lowest possible setting and still only tend to see a faint glow.
Lastly, best of luck! You need some to stand a chance of a sighting ^