So, wait... Sam casts a spell and gets the exact result necessary for that spell to work. It has no penetration. He walks forward into a divine aura of 1. The spell continues? What if he walks forward into an aegis of 1? It stops? What if he walks forward into a magic aura of (jesus how does this math work) 5 then into an aegis of 1? Aegis cuts your casting total by 5 for each point and Magic aura boosts it by one for one. We've been playing it as changing auras don't affect currently running spells but moving into an aegis does. Right?
Penetration is determined at casting as far as I know. Auras don't normally dispel; that is something special done by the Aegis. It's not the reduction of Casting Score that does it, though; it's the bit about dispelling foreign effects entering the Aegis.
And the casting is over, not done again. How about this? You cast 5 Moon-duration spells, succeeding on the dot in a Magic 5 aura. You now walk out of that aura a few days later with those spells running. Should you now fall unconcious from losing 5 Fatigue?
I don't think the spells drop, or the caster suffers any ill-effects. The spells were invoked using the benefit of the aura's power, and are now stand alone effects which are tracked with a FoTeNN/Pen score.
It is best to distinguish caster and target (ArM5 p.111ff Ranges, Durations and Targets) here. Once a magus has successfully cast his spell with all the adjustments for auras, Aegides and so on, that spell takes effect on the target. It generally moves with the target, can be analyzed, dispelled and so on on the target, no matter where the magus is and what he does. The caster can control some spell effects while he remains within range of the spell. Some Virtues (like HoH:TL p. 101 Mutantum Magic, p.107 Tethered Magic) allow to retain an even stronger connection to a spell cast with them. An Aegis can block a cast spell moving into it on its target (ArM5 p.161).
Typically an Aura does not interact with already cast spells: but Magic Resistance is modified by the aura. So if a maga targets an archbishop in his cathedral from outside with a ReMe spell, her casting is influenced by the aura she is in, and the archbishop's resistance by the cathedral's aura (ArM5 p.183f).
Many already cast spells - like magical flames on a sword - are still resisted, and that resistance is modified by the aura the being resisting them is in (ArM5 p.183f). This can lead to a cast spell losing effectiveness as it moves into an aura after casting: HoH:TL p.73 Aura of Inconsequence is an interesting example.
This all kind of makes me wonder what effect the Aegis of the Hearth has on a spell the caster casts on himself prior to entering its area, e.g. Illusion of Ennobled Presence, given that the target will not be anything within the Aegis at that moment.