For much of the radio spectrum the frequencies are simply too high to be refracted back to the earth given the range electron densities which exist and propagation angles required. This is not true in the HF band of frequencies between a few hundred hertz and about 30 megahertz. There a delicate balance exists where, depending on the natural variations in density of the ionsphere, radio waves may be either refracted back to the earth or passed out into space. For the case of vertical incidence and a given ionospheric density there is a critical frequency above which no reflection will be received. Inversly, you can say that for a given frequency there is a "critical" density needed to return reflections. The relationship between the three parameters (Nec = electron density (cm^-3), v = frequency, and a = incident angle measured from vertical) is given by the following expression, (Tascione, 1988):
It should be noted that this expression is for the critical situation only. Where the angle is approximately known based on the circuit geometry and the frequency is known precisely, the density calculated will be the lowest density that, below which, no return (reception) will be possible. A radio wave of a given frequency therefor will continue to travel upwards into an ionsphere layer until it reaches the elevation where the density rises to the "critical" level (Nec) and the wave will be turned back to earth.
For most HF radio communication the main reflecting layer is the F layer situated between 200 to 300 KM altitude. The D and E layers are generally too weakly ionized to be of much importance except during the day over shorter distances. Ionization, although falling, persists through much of the night in the F layer as well. Intense ionization of the F layer causes it to warm and split into a second, higher, level during the day, returning to a single layer at night. There are also seasonal variations on the theme. The E layer, about 100-150 KM up, can persist into the night to some extant and in addition there are occasional "clouds" or masses of E level ionization which can drift causing sporadic radio openings when the right geometries exist. The D layer at about 80 to 90 KM is only useful for propogation during the day recombining quickly after sunset. In the negative sense, it absorbs signals passing through it to or from the F layer.