How does being in a foreign Aegis affect one's certamen totals.
Not at all, I would say.
One could argue that Certamen isn't spell casting, but one could also argue that it is. Personally, IMS, in order to forestall any question on the subject, I decided that the Aegis was designed with Certamen in mind, and thus has no negative effect. Also, I think it would be considered extremely bad form to take advantage of an opponent in a foreign Aegis during certamen, and might call into question the legality of a certamen duel under such circumstances.
A) why within the Aegis?
B) Why don't you hand out Aegis tokens?
Certamen uses totals with supernatural abilities (arts). IMO/IMS, affected by it.
"Certamen... is the ceremony by which two magi conduct a magical duel..." (p 89, 1st line under "Certamen")
Whether "magic" or supernatural, it's not mundane - this is not a debate or mere theoretical contest - and so would, RAW, be subject to the Aegis imo. (Doesn't have to be - this is more a social/RP decision than something that changes game mechanics, as it doesn't have to echo into any other aspect of "Hermetic Theory", so ysmv without prejudice one way or the other. It is a proprietal Hermetic activity, so maybe it was designed around Aegis - either way can work independently of anything else.)
But if an Aegis does affect Certamen in your saga, it would be good to consider how that would affect the social aspect of the AM world...
Because the mage who lives there wants to win?
The Code says nothing about who gets to decide "place and time", as was historically customary in some RL eras (one picks the weapons, the other chooses placed and time, or whatever). With Certamen, "Here and Now!" is often assumed, but not specified by anything in the Code afaik, and any dis/advantage of Aegis would certainly be a serious argument against that (as any who enter an Aegis without token* would be immediately vulnerable to such a "here and now" custom).
(* and considering that a token can be revoked at any time by any mage involved in that Aegis ceremony, even that safeguard would be thin at best.)
However, a similar complaint could be raised if a mage had a Deleterious Circumstance tied to the current location, or a (relevant) Susceptibility to X Power - and those penalties are on each mage to consider when issuing/accepting the challenge.* Likewise, if a mage had a Positive Cycle or Special Circumstances, there is nothing stopping them from taking advantage of that if they can.
(* It's been asked before what happens if Certamen is challenged against a mage who is currently suffering from a Restriction, or cannot meet a Necessary Condition - either these do not affect Certamen or magi w/ such Flaws are very polite and very nervous in such situations - ysmv.)
For me, I'd lean toward "neutral ground" as the expected default w/ regard to Aegis (anything else would be "bullying"?), but other advantages are considered "being clever", the equivalent to working a fencing opponent onto stairs to gain an advantage. But no one should be expected to enter a duel with one hand tied behind their back.
if you do a certanem in a foreign aegis, you will get a token, so you don't have any trouble
I ruled that while Aegis didnt impact Certamen, aura might do so.
It also depends on how you come down on where the rules for an Aegis needing to penetrate parma is too.
The text says the magi merge parma, it doesn't say they drop it...
However unless protected by a Parma, any Tribunal protected by an Aegis would not allow familiars into the hall easily.
I would say that the Certamen ritual is designed in such a way so that the participants have to forgo all forms of MR whatever the source. Otherwise I'm making my next certamen champion with true faith and a relic.
The Aegis spell description(p.161)says "Magi... must subtract... from all of their casting totals...Penetration totals for all magical creatures must be reduced by the same amount."
Certamen does not involve any casting totals or penetration totals.
(Did you notice the presumably sloppy wording of "magical creatures" rather than "creatures of might" allowing demons and faeries free reign once they make it inside of the aegis is presumably not the intent.)
Am I correct in thinking that Certamen is a Supernatural Ability, so would come under aura modifiers?
p 183 says that auras affect all supernatural activities so certamen is affected. Certamen isn't an ability (in the game mechanics sense of something that you spend xp on) at all.
Even if Certamen is affected by the aura (which it might be, I'm not sure), it shouldn't (usually) make any difference as both combatants are affected the same by the prevailing aura.
Unless one of the magi is weird, of course.