Masked the sky and put
My Aurora designed in Illustrator in place of it.
step 2 of 2
Used this shot of the moon I took a couple of months ago clipped the moon and placed it as layer under the aurora, masked the moon then deleted where the moon would be in the aurora layer, duplicated the aurora layer first though.
Then set the new aurora layer to 60% transparency to give the sweep of the colors across the moon.