[PK]

The answers are contained mostly in Powers but also in Characters.

Aura turns your transient attack ability into a switchable one. Aura is a requirement for Always On because you cannot take Always On on a transient ability, only a switchable one. Aura by itself does not reduce your range, but it is incompatible with any sort of normal ranged attack, and thus requires the Melee (or, one would assume, Emanation) limitation.

So, if you want a character whose firey aura fills up the two yards around him, I would buy it as Aura, Area Effect 2 yards, and Emanation, with the option of also taking Always On. You could not just take Always On, Area Effect and Emanation, as Always On is not a valid limitation on a transient ability -- Aura is needed as well.