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.