RAPID ONSET, RAPID DECAY (RORD), PHASE AND AMPLITUDE PERTURBATIONS OFVLF 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 N WC mimic signal directly injected into the antenna and so cannot be br oadband bursts. There is no delay between the initiating sferic and RO RD start, implying that they are produced not by whistler-induced elec tron precipitation but directly by lightning. Observations on a multi element array show that classic Trimpis and RORDs initiated by the sam e steric usually come from measurably different directions, so the lig htning-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 altitud e and having vertical dimensions of some tens of km and horizontal dim ensions of 1-2km, since such a column would scatter sufficient signal to fit observed amplitudes. Cloud-to-ionosphere (CID) lightning discha rges (also called ''cloud-to-space'' and ''cloud-to-stratosphere'' dis charges) of these visible dimensions have been observed on mountain ob servatories and on board the Space Shuttle.