Here's why:

* The ionosphere's behavior depends on the frequency of the radio wave: The ionosphere is a layer of the atmosphere filled with charged particles. These particles interact with radio waves in a way that depends on the frequency of the wave.

* High frequency (HF) waves can penetrate the ionosphere: HF waves have enough energy to pass through the ionosphere without being significantly reflected. This is why they are used for long-distance communication.

* Low frequency (LF) and very low frequency (VLF) waves are reflected by the ionosphere: These waves have lower energy and are reflected by the ionosphere. This is why they are used for long-distance communication, particularly for underwater communication.

Therefore, the waves that are not reflected by the ionosphere are high frequency (HF) waves.