HF propagation characteristics of artificial ionospheric layers

Artificial ionospheric layers descending from the background F-region ionosphere can be produced by high-power HF transmissions with effective radiated powers of 10⁵->;10⁶ W, especially near harmonics of the electron gyrofrequency. Multiple high-speed DPS-4D ionosondes were utilized to obtain direct and oblique ionospheric soundings from a large number of artificial layers during an experimental campaign at the High Frequency Active Auroral Research Program (HAARP) facility in November 2009. Initial analysis of this data shows remote oblique echoes to be qualitatively similar to direct echoes received at the transmitter site. This pattern holds when the artificial layers are created in different locations including vertical, magnetic zenith, and midway between the two ionosonde sites. Artificial layers can have higher critical frequencies and maximum useable frequencies (MUF) than the background ionosphere when the background is decaying after sunset, and echo amplitudes can be significantly stronger, even when the shorter path lengths are taken into account.