Hi Bridget
I'm also doing the May trip to Alaska so I look forward to meeting you (I booked on the Arctic Circle excursion this morning-sounds amazing!)
As for the Northern Lights, I'm afraid that Monty is right and in May/June you'll be unlikely to see them. As a tip for next year though, if you are thinking about future JY trips, I did Iceland in March 2015 and we saw a fantastic display. The locals told us it was probably the best in 12 years due to solar flares that preceded the eclipse that happened to days after we returned home.
You may not be lucky to get a repeat of that and, to be fair, Northern Lights spotting can really all be down to luck, but I can definitely recommend Iceland in March, and not only for the lights, the whole trip is incredible.
Anyway, Alaska here we come, see you at Heathrow!
All the best
Mandy