Andrew Z. answered 03/16/19
Master's in Creative Writing, 12 Years Experience in Fiction & Poetry
Hello Christiana,
you are correct in your first presumption, the speaker is expected to see an aurorea borealis some where along his journey, but not an absolute flood of light. They will not be completely illuminated, as a "cascade of light" would do, but rather will see flashes of brilliance and beauty along the way.
"nubbed treasure" is a little more cryptic, but this is part of the art of poetry: meaning doesn't need to and often isn't totally direct. A nub is a small lump, usually extending from something greater. I believe what the Heaney meant by "trust the feel of what nubbed treasure your hands have known" is that one should trust in the things that have truly rewarded them, even if those rewards were meager or small.