Aurora Wave
What are we cooking?
Hint: Click on the image!
Step 1
Open up Photoshop an create a new file as below size
Step 2
Now you have a big white canvas. Select the Paint Bucket Tool or just press G on your keyboard. Change the colour to black. Now paint your white canvas with black. Just click in the middle of the canvas.
Please ignore any unnecessary item on the preview image. I’m just too lazy to crop it.
Step 3
Right click on the Paint Bucket Tool and choose Gradient Tool. Change your foreground colour to white. On the top left corner of your photoshop window, there is a drop down box to change the gradient style. Click the arrow and choose the second style (foreground to transparent).
Step 4
Create a new layer (I really hope you know how to do this). Click and drag your cursor down from the top of the canvas to 3/4 of its size.
Now you should have a white gradient just like below.
Step 5
Here comes the interesting part. While still selecting the gradient layer, on the main menu, go to Filter -> Distort -> Wave. Set the numbers just like below.
Don’t hit the OK button yet. You may not see the preview result in the small box. Next time you can try black gradient. Click Randomize. You may hit it few times to get random wave. Now you can click OK. Below is what I got. Isn’t it cool?
Step 6
Time to play with the colour. Double click the gradient layer (at the Layers panel) to open layer style window. Choose gradient overlay. Change the Blend mode to Multiply. Choose a gradient you like or create your own gradient (double click the current gradient). Set the Angle to 0.
Done!
Now you can play with other things like Pattern Overlay, put your picture, and add some text. Have fun exploring! 
PSD File: Aurora Wave.psd (18)
p/s: I would like to see what you have done. Upload it somewhere and post a link in the comment.