Michael Quinion, Ologies and Isms: Word Beginnings and Endings (2002) offers a useful summary of the origin and application of this type of a- prefix:
a- Towards, of, in, into, or at; marking some ongoing process or state; movement onwards or away. {Old English prepositions of or on (sometimes as unstressed an), or the Old English prefix a-.}
The Old English prepositions were originally separate words, but became reduced to a- and attached to the words they once modified. The process can be seen in alive, which in Old English was two words, on life, literally 'in life'; others of similar type are aside, akin, and anew. Some examples are verbs derived from Old English a-, which had an idea about it of an action or an intensification of an action: arise, abide, and awake.
Some adjectives imply a continuing or active state, and have much the same force as a present participle ending in -ing: ablaze, abuzz, afire, afoot, aglow, astride. Others combine the prefix with a present participle, usually hyphenated; such words imply an ongoing process or activity: a-brewing, a-roving, a-hunting, a-wasting; though they are mostly now archaic, literary, or dialectal, the form has had a small revival in recent decades, as in Bob Dylan's song lyric The times they are a-changing.
So we have a prefix form that may appear in nouns (e.g., aside), verbs (e.g., arise), adjectives (e.g., aplenty), adverbs (e.g., apart), or prepositions (e.g., above) and may indicate, variously, movement in a particular direction, intensification, or ongoingness. And as if this multipurpose utility weren't complicated enough, an entirely separate prefix a- (originating in the Greek negation prefix α-) attaches to many English words with the meaning "not."
