A couple of things you could try. With an 8 second exposure for ambient lighting any flash applied behind will be of such brief duration that it won't show up much at all. It would be similar to my first dissapointment with lightning when I used too long of an exposure. If you have another flash available to provide light from the front it should do much to help with reducing exposure times. The other item to experiment with is the vertical angle of your backlight. When I have captured the glow around trees and plants outside, without the help of ice or rain, the sun has always been only slightly angled away from being direct backlight. Putting it in words this way I am starting to wonder if the 'glow' might be diffraction on the edges of objects.