Does anyone happen to know if a divine aura is diminished at night?
Not as a rule. But auras are fuzzy, and some divine auras recede at night, some on certain days of the year, etc, in canon, so if you need one that does, go for it.
According to Realms of Power: The Divine, page 10, sidebar:
Similarly, Infernal auras increase by one at night (Realms of Power: The Infernal, page 10) and decrease on holy days. A Divine aura that overlaps with a variable Infernal or Faerie or Magic aura could also effectively fluctuate, even if its numerical value remains unchanged, as the other aura variably dominates or is dominated by the Divine aura.
That being said - your story's aura does whatever you want it to. 
Thanks! That's very helpful. 