What maximum frequency can be reflected from ionosphere?

The ionosphere extends from a height of 80 km to 300 km . The refractive index of ionosphere is less than its free space value. That is, it behaves as a rarer medium. As we go deep into the ionosphere, the refractive index keeps on decreasing. The bending of beam (away from the normal) will continue till it reaches critical angle after which it will be reflected back. The different points on earth receive signals reflected from different depths of the ionosphere. There is a critical frequency $f_c$ (5 to 100 MHz ) beyond which the waves cross the ionosphere and do not return back to earth.