Simultaneous VHF and transequatorial HF observations in the presence of bottomside equatorial spread F

Simultaneous transequatorial HF and VHF incoherent scatter measurements were conducted in Peru in January 1994. On the evening of January 13, conditions as measured at VHF by the Jicamarca Radio Observatory were comparatively mild. The spread F observed on this evening was confined to the bottomside of the F layer and relatively weak. Yet dramatic propagation effects were seen in the HF. Strong off great circle signals with more than 10 Hz in Doppler spread were measured. Large spreads in angle of arrival, as much as 20 deg, were also seen. The strength of the off great circle returns are comparable to the strength of signals arriving from the direction of the great circle and are believed to result from total reflections from horizontal gradients in the ionosphere. Jicamarca VHF measurements substantiate this hypothesis. We present data from a single evening during which two separate events occurred. From the combined VHF and HF measurements, we conclude that the source of the off great circle reflections consists of a longitudinal density gradient which is immediately followed by a region of unstable plasma. The unstable plasma affecting the HF signals is shown to consist of a sequence of finger‐like structures having a nominal separation of 15 km which remain below the height of maximum F region electron density.