Working on the northernlights was a pain staking, time comsuming task. Making sure the colors look realistic while at the same time making it flow as life like as possible.
Create a new layer call it "guide line". After you decide on a direction of the northern lights use pencil tool to sketch out a path for the northernlights (it doesn't have to be perfect, its just a guide line).

New Layer-call it "blueglow". Using your Square tool start making a series of vertical rectangles across the guide line that you just made. Filled with a light blue. Have the rectangles overlap more on the upper side of the line.

While the layer is still selected , if not go to select/load selection, use filter/blur/gaussian blur (settings at 6)

Deselect (Ctrl+D) then hit (Ctrl+F).

New Layer. Do the same as the step 2, but this time instant of blue use a green. Play around with the Hue/Saturation untill you find the color your looking for.

Than dulpicate this layer by draging it down to the new layer button. Move this layer around, change the Hue/Saturation, but since it is a copy erase parts or reduce the size (Ctrl+T). Reduce the Opacity to fade it.

Adding effects. Select one of the light layers and Rotate one of the northernlights 90 degrees counter clockwise. (Edit/Transform /Rotate 90 CCW)

Then Filter/Stylize/Wind.

Rotate it back Edit/Transform /Rotate 90 CW.

Use the Filter/Blur/Motion Blur (try different scale) usually at a 90degree angle.

By using the lasso and the square you can be more specific as to what part of the layer is effected. Example: Select the lasso and draw over the top part of the green northernlight then Filter/Blur/Motion Blur.