In the game I plan on running I want there to be an area that has a magic aura during the day and a infernal aura at night. (low aura rank 1 or 2 may be)
-Is there a RAW way for this to occur?
-How would you stop it once somthing like that started?
-How would it affect a magical regio in the same area? (RAW or otherwise)
Yup. See RoP: The Infernal page 10-11. All infernal auras increase by one at night. There can be a magical and an infernal aura, equally strong. During the day, the magical aura holds sway (say, magical aura 2), and at night the infernal aura becomes stronger, dominant and affects the environment (infernal aura 3).
Increasing the magical aura will help, though the infernal may still hold sway on especially unholy nights.
I wonder how such a fluctuating aura would affect labwork. Experimenting in the lab would be scary.
Paired auras. Magic predominates because "it was there first". +1 to infernal aura at night. TADDA! Magic aura superceded 
The +1 to infernal at night has been around since 2nd edition at least. Dunno about the current edition, but it is certainly feasible using this simple mechanic.
Cheers,
Xavi
It's been long established that many infernal Auras are stronger (+1, occasionally +2) at night and that in an area with overlapping auras only the strongest is 'active'.
That's so canon I would be more surprised if asked 'how that worked' than if my players just ran with it.
Ok, i didn't know that that +1 at night was an official thing, i think it was mentioned in the core book tho. So yay my idea makes perfect sense!
Also for clarification the regio wouldn't have a fluctuating aura, because it's stronger magically and i don't think regio's can have multiple aura types since they're basically pocket dimensions of their aura type. correct?
You could have a nice infernal regio only accessible at night... accounts for all those covenfolk disappearing between the barn and the outhouse.
Why care about raw in this case? If you want a magic aura during the day and an infernal one at night - have it!
Well i'm gonna do it any way. I was just curious, and if theres a RAW way to explain it(and there is) all the better!