So, after trying to get to several minor Arkfalls in a row with little luck (zipping around like a lunatic on the four-wheeler), I finally got to a couple of them before they ended/vanished. However, I'm not 100% sure how these are meant to work.

I get that we fight waves of enemies, get ranked afterward, and gain XP from participating. What I don't get is the "no reward" message that pops up. (I've gotten no higher than third, bear in mind.) What kind of rewards are given for these Arkfalls, and what must one do within the Arkfall to get whatever those rewards are? Are there even rewards that would change that message in minor Arkfalls, or is that just for the major ones they describe where the sky coloration changes, etc.?