So my players have located an abandoned church that has an infernal aura of 3, lowering their casting rolls by 3. They have time to burn as they are waiting for the Regio access to appear that they suspect exists at night. However while waiting they are casting various spells, on finding their spells were harder to cast they started going outside the boundary of the church to cast any spells and then come back into the Aura once the spell is cast, either with a duration or concentration.
When they walk into the Aura (any none magic Aura) can that cause a dispel effect or any other effect on the spell?
At night when the Aura increases to 4 , the same question?
When they step through into the Regio...same question?
And Slightly related is the Aura effect of the Regio set at the highest level of the Aura (4 in this case) or is there any other increase due to being in the Regio?
I don't think it will cancel the effect.
However, it will lower the casting total if Penetration is relevant. So it might be one case where the Penetration total could end up being negative, making ineffective against anything with just a little bit of Penetration or positive casting total.
You might HR that if the final casting total become lower than -10, the effect is effectively dispelled (a bit similar that when casting a spell if missed by more than 10, the casting fail). But there is nothing in the rules supporting this proposal.