I would use two layers in GIMP. The aurora in the top layer. Set that layer mode to "Screen" and adjust the contrast and brightness.
I see the star trails in front of the aurora, which is actually just weird to my eye. So he took a long exposure which I presume brought out the ground level exposure and minimized ISO noise. It could be 1 very long exposure or a layer-stack with many exposures. The aurora appear to be more noisy, so a shorter exposure at a higher ISO setting. He then layered them and combined. But again, the trails in front of the aurora look weird to me and I would have either layered different or thrown out the whole project.
I see the star trails in front of the aurora, which is actually just weird to my eye. .......
Thanks all.
Here is an example I tried in Gimp using "Lighten only" blending mode with two layers. Remark: BF is correct about the flaw of star trails in front due to gimp's (maybe PS too) blending mechanism, but sparky's point (aurora isn't cloud) could also be true. In my example, there are enough objects behind star trails, they should be in front...
When blending 2 (or more) images in Photoshop, if there is any problem with stars showing up in front of trees or other closer subjects, you'd have the option to use a layer mask. With that, you'd brush over the stars to hide them making all the stars appear behind the closer objects.
I never used Gimp to know whether or not that is an option. Glad you were able to give the process a try!
When blending 2 (or more) images in Photoshop, if there is any problem with stars showing up in front of trees or other closer subjects, you'd have the option to use a layer mask. With that, you'd brush over the stars to hide them making all the stars appear behind the closer objects.
I never used Gimp to know whether or not that is an option. Glad you were able to give the process a try!