The EMP effect depends on there being atmosphere to generate electrical fields from the bomb gamma flux. Being close enough and line of sight from the atmospher

The EMP effect depends on there being atmosphere to generate electrical fields from the bomb gamma flux. Being close enough and line of sight from the atmosphere can be sufficient to generate the effect in the nearby atmosphere. 400km is apparently the optimum height for coverage but not strength. Too far and inverse-square ensures the flux isn't sufficient. So just detonating a bomb "in space" may not do anything depending on the direct path distance to the satellites in question (the bomb gamma flux etc. of course poses its own threat). Look it up, the physics are interesting.