Quasi-periodic particle precipitation and Trimpi activity at Halley,Antarctica

The relationship between quasi-periodic VLF emissions and micropulsations is briefly reviewed, and then discussed with reference to an event recorded at Halley, Antarctica, on day 257 in 1986. VLF emissions at 2 kHz with a quasi-period of 9 s were observed simultaneously with Pi1 and Pe1 micropulsations. Also observed was a quasi-periodic Trimpi event on the amplitudes and phases of the VLF transmitters NAA and NSS. It is deduced that the VLF emissions are modulated in the generation region by a hydromagnetic wave, giving rise to particle precipitation. The emissions are also modulated by the bounce period of the generating particles. The Trimpi effect is due to 120 keV electrons being precipitated into the lower ionosphere by the interaction with the VLF emissions. This event shows that the Trimpi effect can be used to detect particle precipitation occurring during the ULF/VLF interaction, and can give information which helps to define the mechanism reponsible for the interaction.