The northern lights are caused by cosmic charged particles interacting with the earth's upper atmoshphere. The particles can be trapped by the Lorentz force, so that we can see them where the magnetic field is nearly perpendicular to the earth's surface.
Now, if one looks more closely one can see that the colors seem to dissappear continously into space but have a more or less sharp border at the bottom. At least it seems like there is a discontinuity of northern lights at their bottom.
Is it because the particles cannot enter the lower athmosphere? Or is the transition region not so narrow anyway and only appears so? What is the real reason it looks like this?