RAPID ONSET, RAPID DECAY (RORD), PHASE AND AMPLITUDE PERTURBATIONS OF VLF SUBIONOSPHERIC TRANSMISSIONS

RaPid onset (few ms), rapid decay (approximately 1 s) perturbations or RORDs occur frequently on the west-to-east signal from NWC to Dunedin, more often than not with classic Trimpis. They do not appear on an NWC mimic signal directly injected into the antenna and so cannot be broadband bursts. There is no delay between the initiating sferic and RORD start, implying that they are produced not by whistler-induced electron precipitation but directly by lightning. Observations on a multi element array show that classic Trimpis and RORDs initiated by the same steric usually come from measurably different directions, so the lightning-induced ionisation enhancements (LIEs) which cause them must be laterally displaced. They may also be vertically displaced to explain the differing decay rates (30s versus 1s). We conclude that RORDs are VLF echoes from vertical columns of ionisation at around 40km altitude and having vertical dimensions of some tens of km and horizontal dimensions of 1-2km, since such a column would scatter sufficient signal to fit observed amplitudes. Cloud-to-ionosphere (CID) lightning discharges (also called ''cloud-to-space'' and ''cloud-to-stratosphere'' discharges) of these visible dimensions have been observed on mountain observatories and on board the Space Shuttle.