Northern lights in Svolvaer mid September?

Not very likely.

While northern lights depend on a number of variables, one thing to consider is hours of darkness. You might be thinking, the nights will be long in September in far northern Norway, but in fact, not so. Your visit would be not long before the September equinox, when day and night hours are identical, 12 and 12, anywhere on Earth. Generally speaking, anytime between June 20 and Sept 20, the farther north you are (1) the more hours of daylight you will have, but (2) the faster the daylight hours will be decreasing day to day. Out of curiosity I compared sunrise and sunset times on Sept 12 in Tromso with my town of Washington DC. Tromso has 13 1/2 hours of daylight, DC 12 1/2.

Full moon is September 18th. Moon phases will affect ability to see NL. Better chances when no moon.

I am not aware of any reliable data which prefers one place before
another above polar circle.

There are many factors to consider and all places north of the arctic circle are not equal.

Thanks, both of you... I read that there was a chance to see the NL in September with Svolvaer's location in the Arctic Circle, but I forgot to check the moon cycle. I think that makes this unlikely, and probably not worth the effort to sail further north than Trondheim. Thanks for the input!

I read that there was a chance to see the NL in September with
Svolvaer's location in the Arctic Circle

That's correct, there is a chance. Just like buying a ticket means there is a chance you'll win the lottery.

The question is, how big is the chance. The moon is a factor as mentioned, and during your dates there won't be any night in Tromsø, and only 30 minutes of night your last day in Svolvær. So you'll have to rely on the twilight to see the lights.

Agree -- decided to carry on to Tromso after all because I do want to see the Lofoten Islands and Tromso regardless of the northern lights. PS: I read that the moon phases have little impact on seeing the northern lights, which depend on solar winds colliding with earth's atmosphere

Good for you to treat the lights as a bonus.

I was talking to someone at the weekend who had been on a NL cruise in February- but had like me only ever had a very weak show which wasn't captured on their camera. But didn't mind as the scenery (and the cruise itself, as she was sailing on Fred Olsen- the Norway cruise expert) more than made up for it- and she accepts it is just luck. All the really good NL shows I have seen have been in the UK and Canada.