Yes, I did that first actually (following a tutorial from the Gnomon Workshop DVDs) but the problem with that is that the glow appears all around the sphere, even when there is a dark area to the sphere. And I want the glow to appear only at the horizon.

I tried adjusting the Threshold attribute of the ShaderGlow node, but that was not very satisfactory.

If it helps I can post a low res figure of what i am trying to achieve, as i am afraid that i am not being very clear?

thanks