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 Kp ≥ 6. The signal parameters (azimuth, elevation, and Doppler spreads) also increase for high values of Kp. 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 Kp.