rank, follow
亞 centers on the idea of being second or subordinate: ranking below something else, or following after in order.
亞 is an ancient pictograph often interpreted as a representation of a building with a cross-shaped floor plan, later used abstractly to mean 'second' or 'subordinate'. The exact historical development is uncertain.
The symmetrical cross-like shape of 亞 looks like a secondary, inner chamber within a larger structure, always following the main hall.
For ア, picture a subordinate following behind with an 'ah' of realization: ah -> ア, as they understand their rank.