Long-range tracking of thunderstorms using sferic measurements

[1] Each lightning stroke launches an electromagnetic pulse, known as a radio atmospheric or sferic that propagates in the waveguide between the Earth and the ionosphere. Sferics, with most of their energy contained in the frequency band from 3 Hz to 30 kHz, can propagate with low loss to distances of tens of megameters. Using very low frequency (VLF) magnetic field measurements at Palmer Station, Antarctica, the azimuths of sferics originating in North America (similar to10,000 km range) are found to be measurable to less than 1 degree accuracy. Analysis of the arrival azimuths at Palmer Station allows the assessment of lightning flash rates of storms occurring in diverse regions of the globe including North America and Africa. Arrival azimuths are compared with flash level data from the National Lightning Detection Network (NLDN) and 83.6% of the sferics that could be matched to NLDN flashes were within 2 degrees of the predicted azimuths for those flashes. Results are also compared with data from the satellite-based Optical Transient Detector (OTD), and the arrival azimuths of sferics detected at Palmer Station are consistent with the location of lightning occurring elsewhere around the world. The relative distribution of and the relationship between ground flashes and cloud flashes are discussed for a specific localized thunderstorm. Evidence that sferics generated by intracloud discharges were detected at Palmer Station is also presented.