One of the players in my group has a Bonisagus mage who is interested in regios: their nature and formation.
So, I have reviewed extant material on it, mainly found in RoP: Magic, and find some rules on the subject, and a discussion of practice, which I take as how it is, but not how Hermetic Magic Theory deals with regios.
In practice, a magic aura develops where a sufficiently potent magic event, object, or entity is or was (to put it simply). A regio develops when the supernatural something has impacted reality enough to cause a permanent connection between the mundane and the deep magical realm, resulting in a magical 'pocket.'
I haven't found any living or past mage who investigated magic auras and regios in the existing text. Perhaps I've missed one?
Possibly Hermetic Theory on the subject is deeply intrinsic; the subject was defined by Bonisagus, magi know these things and the theory is well understood, so that they just don't have much reason to discuss it? That seems unlikely.
I am aware of one maga Bonisagus who pursues a theory of magic related to auras; Marcella, whose main interest is in genius loci, which I interpret as having some knowledge of the relationship between auras and entities. Beyond this, magi seem curiously indifferent to magic auras except as places to live.
I plan for our Bonisagus researcher to plumb the apparently quite poorly developed knowledge of magic auras and regios. Some theoretical concepts for him to encounter, including false and contradictory ones:
Auras are an imprint of the presence of magical beings, including magical spirits. Once an aura develops, it tends to persist, attract both fluid vis and other magical beings, and may continue even if the original being departs. (Repeat for magical objects or events; theorists tend to quibble.)
Auras are originated by magical events performed by magical entities (daimons, dragons, or other). A place where a powerful event occurs sometimes becomes charged, and thus attractive to vis, other magical beings, and may grow or shrink as environmental factors come into play.
Auras are simply a place where a regio is prone to manifest and spring forth. The aura forms first, and this attracts other magical things, beings, vis, and so on. (This is not well supported, I think.)
Magic auras are where mortals think they are. If a place is strange to mortals, an aura manifests there. (Quite a lot like faerie.)
Magic is where God says so, sinner.
Side note: So far as I've found original research seems very focused on practical mechanical issues, and not so much on investigative deep causes. Is there any guidance I've missed deep in the ArM5 text on investigating the underlying structure of reality? I suspect I'll just have the player mage go to a place, investigate it, reflect investigation in Magic Theory, and proceed.