Nighttime sporadic E measurements on an oblique path along the midlatitude trough

Observations of nighttime sporadic E (Es) made within the HF band on a 1400 km path that lies approximately along the midlatitude trough are presented. Although the probability of occurrence of Es (PEs) is generally below that predicted by the ITU‐R model, a significant increase in PEs is found when K p ≥ 6. The signal parameters (azimuth, elevation, and Doppler spreads) also increase for high values of K p . This behavior is consistent with the character of the propagation changing from midlatitude to auroral as the polar cap expands and the trough moves equatorward with increasing K p .