Composite of one of my Iceland Aurora shots (available on my homepage as wallpaper) and an old photo of Bannalpsee (Lake Bannalp) in Switzerland.
Once I finished combining the Aurora sky with the once daylight scenery of Bannalpsee, I used Flaming Pear's great Photoshop plugin -Flood- to create the Aurora reflections. I little tweak merges both layers so that the original ripples are combined with the new "fake" ripples of the Aurora sky. Then I mostly corrected minor artifacts and color fringing and last but not least, used Nik Color Efex to play around with the colors.
Let me know in the comments whether you liked it or what you'd improve. Enjoy!
Adobe Photoshop CC, Nik Color Efex 4.0, Flaming Pear Flood.