First observations of Trimpi events at Durban (L = 1.69) using an OMSKI receiver

We report observations of transient ionospheric disturbances at VLF reflection heights, manifested as perturbations on sub-ionospherically propagating VLF transmitter signals: Trimpi events. Four VLF transmitter signals were monitored at Durban using an OM- SKI receiver. Both traditional Trimpi events and a new dome shaped event were observed at Durban. Occurrence of Trimpis at low L values can be interpreted as the precipitation of very high energy electrons which are capable of producing events during daylight, or as evidence of the fast Trimpi mechanism which could dominate at low L. The average winter event rate observed at Durban was quite low (7 per day on four transmitter signals) but during one very active period on the NAA-Durban signal 88 Trimpi events were observed in 2 hours. This activity was linked to an isolated thunderstorm occurring over the signal path over South Africa. 26 refs., 14 figs., 2 tabs