We have a magus in my group that has invented a moon-duration version of "Disguise of the Transformed Image," and intends to cast it on a regular basis for story reasons.
If this magus successfully casts the spell in a non-aura, but then a week later enters a high-powered Dominion aura, are there any rules governing how the spell might be affected? Logically, it seems to make sense that the illusion might flicker or turn off, but I'm not sure.
I know I remember reading a long essay on Species in one of the books, but can't seem to find it.
If this question has already been discussed, please direct me to the thread. If this is in the RAW, please direct me there.
When a magus casts a spell they take bonuses or penalties for their present location. The effects are described on page 183 of the core book.
Spells do not get extinguished or changed when they move to different auras. There isn't to my knowledge a place where this is clearly stated but if it was the author's intent to have spells wink out or change when entering a different aura I"m confident that they would have said so.
The magic resistance of creatures changes by moving between auras but that should not have any bearing on your particular scenario. The magus casts his image alteration spell, to do this he might need to penetrate his own magic resistance (but probably won't need to, he could develop it to work at range personal, or he could suppress his resistance, or he could cast it upon himself when his parma is down). Once the spell is working his image is really changed. The image will interact with mundane light to send off mundane species, in much the same way as if the magus had changed himself with a muto corpus spell. Observers will see the transformed magus not his untransformed image regardless of their magic resistance.
The Hermetic illusion spell, its workings and generated species are not affected by being carried into the Dominion. But Divine Supernatural Abilities like Second Sight, Divine Powers like Intervention (RoP:TD p. 52), and all out miracles (RoP:TD p.60f, TC p.12f Devotion) have good chances to penetrate and show up the illusion in the Dominion.