Parametric Ionospheric Amplification

• At VLF (near 15 kHz,) ``stimulated natural emissions'' have been observed.

• A VLF signal which penetrates the ionosphere aligns itself with the magnetic field of the earth at phenomenally high altitudes (one to two earth radii.) At this altitude, the RF ionizes the ambient electrons, which can later be used to amplify signals on the same frequency.

• Possibly an analogous phenomenon exists on HF.