What settings should a Nikon d3100 camera be on to be used at night?

I want to take photos of the northern lights tonight And need to know how to set my camera to capture some great photos!

Get a tripod. Put the camera on said tripod. Frame your subject. Turn on the 2 second self-timer. ISO 100 or 200.f5.6 or 8. Start out with a 2 second exposure. Hit the shutter release button. 2 seconds might be too long or too short, so adjust your exposure accordingly.

I would suggest putting the camera on a tripod, attaching a shutter release cable, and experimenting with the ISO/aperture/shutter speed.

ISO settings from 400 to 1600 can deliver excellent results (depending on the camera). The length of your exposures will depend on how large of an opening (f-stop) your lens has, and the brightness from the moon.

I agree, time exposure is the only way to achieve such shots. A tripod is essential. However I'd go one step further. Rather than the 2-second delay (I've gotten camera shake at 2-seconds) use the
10-second timer. That way you know that the movement caused by you pressing the shutter release will have settled down by the time your shutter opens.

Good luck, have fun… Wish I were there to shoot with you.

Auroral activity is expected to be low to moderate tonight or tomorrow.
http://www.gi.alaska.edu/AuroraForecast
Where are you? I would not expect activity to be visible from the UK. It should be nice up in Tromso, Norway.
Anyway, you might try ISO 800 and 15 to 30 seconds at 18mm f3.5 if you are using the kit lens. A problem with using very long exposures with aurorae is that they are dynamic, and longer exposures become increasingly blob-like.
If there's no bright light to lock your autofocus on, use a strong flashlight to illuminate a stop sign, license plate, or other reflectorized sign or object from a pretty good distance.
Set white balance to daylight.
A way to get decent dexterity without freezing your fingers is to use two layers of glove liners with a handwarmer pack sandwiched between them.
It is good to have a tripod with a quick release plate.

This was taken from Dawson City, Yukon Territory with a Nikon D60, 15 seconds, ISO 800, 28mm AF-D lens set to f3.2. I think the Geophysical Institute had rated activity about low to moderate that night.

It helps to get away from city light pollution. At least try to get to the edge of town, as I did with the shot with the little town of Dawson City behind me and wilderness ahead of me.
In this shot from Frame Lake in Yellowknife there's more scattered city light around me:

In this shot taken about 15 miles outside of Fairbanks, Alaska, I had wonderful conditions, but there was weak activity:

For this one about 8 miles north of Fairbanks, there was pretty high activity, but clouds and a full moon: