When making a magic item, can the Environmental Trigger feature be set for when entering an aura of specific Realm/strength?
Yes: that's an InVi trigger.
Isn't an InVi trigger a linked trigger, as opposed to an environmental trigger?
I'm not sure what Timothy Ferguson meant, but to answer your original question yes, entering an aura of a given Realm and/or strength appears to qualify as an environmental trigger by the Rules As Written.
ArM5 (p99) says that "changes in the aura modifier" are a possible environmental trigger. So, "entering an aura" or "leaving an aura" (from/to a different level/sort of aura) would certainly switch such a trigger.
Whether such an environmental trigger can be made to respond in a more subtle way (such as "only when entering an infernal aura", or "only when the aura modifier increases") is less clear. A generous reading of RAW would say "yes". A harsher rule would say "no; you need an InVi effect and a linked trigger". Unless there are explicit examples somewhere it looks (to me) like a YSMV question.
I would go with "No" for the following reason: you need a spell to identify in which aura you are. So if it was possible to trigger an enchantment when you enter a specific aura, if means that magical item have a realm sensitivity embedded in them.
The best way to trigger something specific to a Realm would probably be:
- an InVi spell detecting if the aura you are in is aligned to the Infernal realm, with a +3 environmental trigger link to entering an aura
- a second effect with a spell trigger linked to the first effect if positive.
So you will have this chain of effect:
- the item enters an aura, it triggers the first effect;
- the first effect detect if it is an Infernal aura;
- if yes, it triggers the second effect.
I believe (serf's parma), that item can only trigger one effect per round, so the second effect will only take effect at the end of the second round you entered the aura (if it matters).
For obvious reason, an environmental trigger is better than a constant detection effect (minimise warping). Second benefit, the first effect will trigger even if there is a protection spell preventing aura detection since it is an environmental trigger and not magical effect needing to penetrate (since it is not specifically written in the rule, you can have a different interpretation). However, the spell detecting the aura could be stopped by the spell protection. On the other hand, if the environmental trigger is linked to a simple CrIm doing a little "Ding", the protection cannot stop it from happening, the CrIm is not triggering the aura 
.
I was considering a corrupted magus using an Aegis to prevent the detection of an Infernal Aura... and his counter-part, a witch hunter equipped with a simple magical bell ringing each time he enters an aura - it does not tell him anything else that he entered or left an aura, just enough to make him aware where he is stepping (CrIm 1 (base), Mom, Self, Ind, +3 for env. trigger, +10 for unlimited use = 14, easy lesser enchantment).
As to how fine a "resolution" one can have, note that Environmental Trigger can respond to specific changes in Aura modifier, since it can obviously respond to specific events that end spell durations, and the wording for the two responses is the same. So you can at the very least distinguish Magical and Faerie Auras (that give a positive modifier) from Divine and Infernal ones (that give a negative modifier). And you can do more: say you set your trigger to respond to Auras that have a -1 or -2 modifier - then it automatically detects weak Infernal Auras.
The only "points of confusion" really are modifiers +1 to +5 (which could be magical auras of the corresponding strength, or faerie auras of twice that strength), and modifiers of -3,-6 and -9 (which could be strong infernal auras or much weaker divine ones).
I thought about that, and I did not want to mention it as I find, it is a bit streching the ability to have automated trigger more sensitive that certain spells. It is a base effect 1 to detect the presence of an aura, a base effect 2 to measure the power of an aura.
However, you need one spell per Realm to detect it and there are no guideline to detect an Aura's realm of origin (I do remember an older edition spell called "Detecting the Font of Power" if I am correct), so it is left to appreciation if by RAW there is a spell which can detect the type of aura you are in.
PLAYERS can know the aura their character are in, because of the modifier they are getting on their casting spell. Can magi do the same ? The will feel that they can cast spells with more ease or difficulty, but can they pinpoint exactly by how much ? If you are a senior wizard with a casting total of +50, can you feel a difference of +/-3 vs +/-5 ? or even between +1 and -1 ? Unless you know a spell which level is exactly your casting total, it is not possible to feel that you exerted yourself more.
So can an item be more sensitive than a magus or does it need specific spells with linked triggers ?
It is not explicitly cover by RAW so left for troupe approval.
Yes there is? That's what the base 1 InVi guideline does; c.f Sense of Magical Power (ArM5, page 157). You do need a separate version for each flavour.
Personally, I do like a reasonably generous interpretation of environmental trigger. It has to be specific (and clearly defined at design time, so that it is clear in play whether it triggers or not), but it can be complicated. So, I would actually be pretty happy (in my saga) with a trigger like "entering an infernal aura of magnitude 3 or more".
The main reason for this is that it encourages the PCs to make "utility" lesser enchanted items. In play, I find that as soon as you start needing an invested item (with several linked effects) it a) becomes a lot more unclear what the item does, b) you sometimes run into warping problems, and c) the PCs decide that they can't be bothered spending several seasons (and lots more vis) faffing about making the item and so solve the problem another way.
This is a question that could be asked about any environmental trigger. When speaking about environmental triggers that can trigger an effect, the rules specifically say that "it can respond to the events that end spell duration (sunrise, sunset, phases of the moon, etc.) and to changes in the modifier applied to magical activities by the local aura." (ArM5, p 99.) How does your device know when it's sunset? How does it know the phase of the moon? Apparently these are things that a spell (or enchanted item) can inherently detect. There's no need for a linked InIg spell to detect when sunrise occurs. It just knows. (How? Um, magic.)
The same could be said for the local aura. A spell 'knows' the local aura. It must if it's to be able to tell whether your efforts at casting it were successful. You might not know the aura strength. But the spell does. Otherwise, how would it know whether the 22 you just rolled successfully casts it or not? Even though the spell (and by extension magic item) doesn't impart this information to you (except indirectly), it nevertheless knows that information. And since every spell in the book doesn't have a linked InVi spell associated with it, I would conclude that a magic item doesn't need one either.
This is further supported by the fact that a change in the local aura is listed as an environmental trigger, not a linked trigger. The rules account for linked triggers. It's the paragraph right after environmental triggers. However, since a change in the local aura is not listed as a linked trigger, it seems reasonable to conclude that there's no linked effect required.
That's my interpretation anyway.
Not clear why the spell needs to "know" anything about the aura. Perhaps, depends on what you think a spell is.
For example, a car doesn't need to know the distance between Paris and Berlin to run out of fuel on the autobahn.
You can definitely sense which aura (if any) you're in, if you're astute. Magi feel invigorated and powerful in Magical Auras, things are bright and closer to the Platonic Ideal. Awed and buffeted, with body pains and headaches in Divine. Foul moods, and old scars, with plant life stunted and warped in Infernal. I can't find the reference to Faerie, but I know they feel different inside one too.
Now, that in mind, it does sound like an InVi trigger.
I agree that detecting a change in aura is definitely doable as an environmental trigger, but anything more specific than that would have to be an InVi linked trigger. Admittedly, if you SG allows you asking how his character feels when the spell "dings", you could extrapolate the aura, strength (is everything crisper than before, or duller?) from that, and activate an effect manually there. Of course that takes time and effort, so maybe just a InVi linked trigger is the way to go.
I mostly agree with you. The keyword for me is "astute". It takes a certain sensibility, it is a bit like "my joints hurt, it is going to rain". Obviously, powerful aura should be either to detect.
But regarding Infernal aura, since demons are perfect liars I would not assume that all infernal auras have noticeable signs.
It is was a place where a massacre happened, with dozens of innocents brutaly murdered, the nature will reflect it. But if it is a place of subtle corruption and temptation, it might looks like falsely peaceful and quite. Maybe slighty bigger flies here and there or a unique black toad.
Also, I am not sure a Divine aura will be hostile to a mage and he will hurts. His magic will be weaker sure, but he is also a human with a soul and should also feel the soothing effect of a Divine aura. As long as he does not cast a spell, can he really differentiate between being in Magical aura and a Divine aura - at least of moderate level ? Possibly as a magus accumulates Twilight level, he will get more attune to the Magical realm, thus reacting more and more like a denizen from the magical realm. At this stage, any Divine aura can make him feel unconfortable.
You're partially correct, however I imagine the feeling would still be very distinct to a Magical Aura.
On the subject of an Infernal aura, if it is attuned (corrupted?) towards temptation, I agree. I still think an astute observer would figure it out sooner rather than later though.
Back to the subject of the thread, assuming an astute person could detect an aura change, is it unreasonable to say an magical item could too? I imagine it would only be an Environmental Trigger if the aura changing is considered a significant change (mostly SG fiat) in the locale. My opinion? Any change in aura type would be Environmental Trigger worthy. A change in strength lower than two within the same aura type would probably need to be InVi, higher than that, I think we fall back to the assumption of a "reasonably astute" man, which while dodgy scientifically does seem reasonable to me.
The ArM5 core rulebook specifically states on page 99 that "changes in the modifier applied to magical activities by the local aura" is an Environmental Trigger.