The area where it's the lowest ionospheric level where radio waves can't be transmitted. I belive it's between 70 and 90km. It's also the only region that the free electrons floating around at night combine back in with the oxygen ions to form oxygen molecules.
So it's like a sort of a 'hole in the sky' for planes to hide in to avoid radar at night. Nifty , eh?
Well , now looking at Wiki , it's 50 to 70km.