By analogy with “afire”, we have here another instance of “a-” preposition 1, being in the situation.  But not so fast!  That comparison was fabricated after the development of the word – the adjective is from the past participle of the verb “alight”, which uses the “a-” prefix 1, motion onward, adding intensity.  That would be the second verb “alight” the meaning of which is “illuminate”.  Yet we must not grab too fast onto that meaning, as there’s plenty of evidence supporting “a-” prefix 3 with the senses “on” or “onto”.

The Chapter 15 word – the first verb “alight” – also, thank goodness, uses the “a-” prefix 1, motion onward.  Its obsolete meaning “to lighten” has moved over time to mean “descend and settle” when used of a creature in flight.

• 06.064 Then he set one alight with bright blue fire,
• 06.065 and setting others alight,
• 06.070 had set it alight
• 13.017 Oin with a small pine-torch alight
• 15.013 He alighted stiffly on the ground before them,

“a-, prefix1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“a-, prefix3.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“alight, adj. and adv.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“alight, v.1.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.

“alight, v.2.” OED Online. Oxford University Press, March 2015. Web. 18 May 2015.