Incorrect here. First, the text says that "Places where raw vis manifests may also form a tether for a Magic aura". This is further diluted by the statement that "a source of vis not already in the presence of a tether may be given a tether score". In other words, the connection of vis sources and tether is not automatic, can be ignored, and further, there is nothing in the text that suggests other tether sources create a vis source.
The text isn't explicit as to whether a permanent increase create a new tether, but I think if I have to come up with an answer, I would go with the preternatural tether "Magic residues" which is what fits better with what normally triggers a check on the aura strengthening table. But there are cases where I can see a natural tether arise from a magi's action, for example, by casting Tear the Earth Asunder.
The real question to me is what does it take to reach a natural aura of 5, since preternatural auras are relatively easy to stack.
These are two different things tracked by two different scores. The Base Strength is found by combining the Tethers and would be its "natural level", the level that it will eventually settle around (in play it will actually settle 1 point lower than this without magical activity). The Current Strength is is what has been called the "permanent level" in this thread. It is the number used when the Aura affects spell casting and protects things from acclimation.
The Tethers together (and only the Tethers) form the Base Strength of the Aura, which can and often is different from the Current Strength of the Aura. Changes in the Current Strength do not effect the Base Strength.
Because changes in the Current Strength have no effect on the Base Strength, along with a modifier to the Strengthening (and Weakening) roll based on the difference between Current and Base, I do not see how an increase in the Current Strength would result in a new Tether forming. After all one of the easiest way to get a consistent and long term positive modifier on the Strengthening roll is to create new Preternatural Tethers (import Things of Might, cast high level rituals [level 50+], and long term use of Magical Monuments such as the standing stone at Durenmar).
Natural Tethers all have a Strength of only 1 or 2, which means to get a 5 requires at least three of them. Looking over the descriptions of them will show that creating new ones is also much more difficult. All of them would require large scale changes to the landscape, with the addition that the Plants one would also often require an exceptional amount of time. Because you need three of them to hit a total of 5 and they require so much change, there is a high likelihood of damaging or destroying one when you increase or create another.
Part of the issue here is that this particular book is very poorly written and very unclear on a great many topics- note the frequent use of the word implies, your reading is certainly a legitimate one but it is far from the only one.
Quite specially it is left unclear what is meant by a permanent increase in the aura when you have a base strength and a current strength but no indication as to whether a permanent change applies to the base strength or not.
RoP: M, p. 8 at the bottom of the Tethers section
Base aura score = natural aura score + preternatural aura score
Natural aura score (maximum 5) = sum of all natural tethers
PreterNatural aura score (maximum 5) = strongest preternatural tether + 1 per additional tether
What makes up the Base is clearly defined and has no relation to the Current. No where in the text is it implied that it could be otherwise.
Additionally RoP: M, p. 11 on the Strengthening Table clearly states
Total: 15 – 20 Current aura score increases 1 (permanent).
So that permanent change is to the Current, rather than Base. The text on the Weakening Table is nearly identical, only swapping out "increases" with "decreases".
Considering how little RPG authors get paid, I'm really uncomfortable with seeing one attacked.
Yes, the base and current are clearly spelled out and different, except that on p. 11 on the aura variation table the paranthetical notation (permenant) can be taken to mean that it modifies the base aura instead, especially if this replaces the straight accumulation of 1 point of aura per century and would also explain where preternatural tethers such as monuments get their tether strength from initially, since nobody can make a tether intentionally.
As to the book I do not blame the author- it is the job of an editor to make sure these things are clear.
It actually clearly states Current.
Are you trying to make this personal?