Hi SethO,
The Physical Sky Object uses a parallel light source, which has no Volumetric options. The Atmosphere in the Physical Sky is not enabling such effect, to my knowledge.
To get this effect, I would set up a light source that is similar to the Sun Light Object in the Physical Sky, and use here the Inverse Volumetric options to create “light” behind the object. Yes, the opposite of what your are looking for. (Note that I have set the Sun in the Physical sky to black)
Render the scene file below in the editor view, and then in the Picture Viewer. In the Picture viewer I have set up an Inverse filter, to illustrate the idea. This is an extra render pass then, which will allow for a step in the compositing to adjust the scene as you need it. You can either use a complete Black material in the render settings for an Material Overwrite, or the Compositing Tag, set to Matte object:black. I guess the Material overwrite will do.
The Sun Tag was switched off after the position was given, as it overwrites the light settings.
Perhaps a simple Blend-mode Subtract will do, or a multiply/divide based on your filter settings, for the compositing.
The Sun Tag of the new Light is there to help to get the Sun position applied to the light source. The Distance need to be set. The falloff of the new Sun is set to none, as with its distance and the small change here it would be wrong.
Scene file
https://www.amazon.com/clouddrive/share/74Wtxh4BWZv8bWR8ImLXllvOVGr5PgaykVZalVQgbrX
Screen shot (rendered in integer so it is more obvious, the float didn’t read well for a screen shot)
https://www.amazon.com/clouddrive/share/USuZHRrIU3Czu2EnSejudHkofwJaCjjC9tU6o1FF6me
All the best
