Here's the settings I used in the atmosphere's Transparency function editor for the above example. I blended the image map with a perlin noise node to soften the line. I also bumped up the UV map Y-scale to 3.0 (that function is located in the half-round icon above the image map node) to thicken the line and eliminate the visible repeats of the image map across the sky.

With a little work on the image map, the blending of the noise node and the UV mapping, I hope you can use this idea to create the effect you're looking for.