Global lightning distribution and whistlers observed at Dunedin, New Zealand

Whistlers observed at Dunedin, New Zealand, are an enigma since they do not conform to the classical model of whistler production developed by Storey (1953). It is generally accepted that the causative lightning stroke for a whistler observed on the ground at a particular location was located in the neighbourhood of the conjugate point, and generated an electromagnetic signal which propagated in a plasmaspheric duct stretched along a magnetic field line linking the two hemispheres. The causative stroke is thought to have occurred within reasonable proximity of one footpoint of this field line, while the observer was located in the vicinity of the other footpoint. Support for this model has come from a number of previous studies of whistler-lightning observations and whistler-induced particle precipitation. However, as demonstrated here, this model does not always apply. Whistlers detected at Dunedin are nearly as common as those at Tihany, Hungary, despite there being at least 3 orders of magnitude more lightning in Tihany's conjugate region compared to that of Dunedin. Furthermore, whereas Tihany whistlers are generally observed at night, consistent with historical observations, Dunedin whistlers occur predominantly during the day. This paper aims to resolve two paradoxes regarding whistler occurrence at Dunedin: (i) an observation rate which is at variance with conjugate lightning activity, and (ii) a diurnal occurrence peak during daylight. The technique developed by Collier et al. (2009) is used to diagnose the location of the source lightning for Dunedin whistlers. It is found that the majority of the causative strokes occur within a region extending down the west coast of Central America.