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This is a paid asset, but now you can download AuroraBorealis Pack Free.
Detail this asset from Unity Store: Original Link
AuroraBorealis Pack v1.0
These models allow you to add beautiful auroras to your scene. Because it is a full 3D models you can use them both from the ground and from space.
This pack contains:
1) 5 different aurora shapes
2) 5 models of different quality for each shape
3) 6 different noise types
4) 18 special textures for varying parameters
5) Specialized shader
6) README instructions.