Set the camera exposure before you leave to go out photographing. A good starting point is: f/2.8, ISO 1600 and the maximum shutter speed as determined by your lens, such as 20 seconds. Reduce the shutter speed to five to ten seconds or less to stop the movement of the auroras, as needed after reviewing the image on the LCD screen. Sometimes they move faster and sometimes slower, so the shutter speed will vary. Look for increased detail and lines in the auroras.
With a narrower aperture, such as f/2.8 instead of f/1.4, the image will typically exhibit less chromatic aberration and be sharper across the frame. I recommend using f/2.8 but use a wider f/1.4 aperture if the auroras are dim or fast moving.
Finally, adjust the ISO to create a good exposure at a desired shutter speed and aperture. Starting with 1600 ISO however will vary depending on how bright the auroras are that night. I recommend not going over 6400 ISO.
Use Live View to review the RGB histogram before taking an exposure. Set your histogram to RGB instead of Brightness View. In Brightness View you only know if all three color-channels are clipped and with RGB you can see if the individual red, green or blue channels are clipped. When making an exposure, reduce exposure as needed to prevent a spike on the right side of the histogram. Any clipping on the right will mean that you have overexposed and lost detail highlights.
Be sure to have some detail in the mid-tone and highlight areas of the histogram with auroras and information into the far right bar of the histogram. This will provide a good exposure. You can also turn on Highlight Alert to show “blinkies” for overexposure as an extra precaution.
Do not use the LCD screen to determine exposure because your eyes will adjust to the dim light, making an underexposed image look good.
For more on understanding histograms, refer to this article.