A case of artificial triggering of VLF magnetospheric noise during the drift of a whistler duct across magnetic shells

A case study is presented of the concurrent and apparently related behavior of four magnetospheric phenomena: (1) whistler ‘ducts,’ (2) cross-L drifts of tubes of ionization, (3) an abrupt upper intensity cutoff of ducted whistlers at ƒHo/2, where ƒHo is the minimum electron gyrofrequency along the path, and (4) artificial triggering of VLF whistler-mode noise at ƒ ∼ ƒHo/2. The event occurred on June 17, 1965, near local midnight, and involved the inward drift of a whistler duct through ∼0.2 L near L = 3. Triggering of magnetospheric noise by NAA transmissions at 17.8 kHz occurred when the minimum gyrofrequency on the drifting path became twice the NAA frequency.