NICARE I HF propagation experiment results and interpretation

The NICARE I chemical release created a large electron density depletion in the nighttime F region of the ionosphere at an altitude of approximately 300 km which persisted for at least 30 min. A three‐dimensional ray‐tracing code (TRACKER) predicted that the lenslike refractive perturbation would create new off‐axis modes in long‐distance HF radio paths which reflected in the ionosphere near the depletion. Channel measurements during this experiment bear this out. The multipath created by these new modes induced an increased fading rate on the received signals. Analysis of the time history of the direction of arrival of the new modes indicates that after the initial expansion of the release the western edge continued to move westward while the eastward edge remained stable in location. This behavior is consistent with model predictions of the behavior of the depletion.