This tutorial will show you how you can add aurora or northern lights to you night photos.

Step 1: Find your background photo. I'll be using this factory shot at night. Then go to Layer > New > Layer and press OK

Step 2: Choose your Brush Tool and draw something like this in your new layer

Step 3: Go to Filter > Distort > Wave , and play around with the wavelength to get desired result.

Step 4: Go to Filter > Blur > Gaussian Blur, and play around with the radius to get desired result.

Step 5: Go to Filter > Blur > Motion Blur, and play around with the angle and distance to get desired result.

Step 6: Change the layer opacity to around 40%

Step 7: Now add some contrast to your aurora using Image > adjustments > Curves

Step 8: You can Transform your aurora using Edit > Transform > Perspective to make it looks better...and, here's my final image..

You can change the color of your aurora quickly by using image hue/saturation.