The amount of light that should be on the ground depends A: on how realistic you want your image to be (what you have now is on the high side of realistic brightness for aurora-lit landscape) and B: how long your exposure equivalent would be in the real world. If you are simulating a longer exposure, you could plausibly have more light on the landscape, however that would significantly blur your aurora as it moves and shifts over time. So for true realism if your landscape is well-lit, your aurora should be quite soft.
- Oshyan