Determination of source thunderstorms for VHF emissions observed by the FORTE satellite

Abstract

This paper explains a method of locating groups of satellite-observed VHF signals from lightning that emanate from isolated storm regions. LF/VLF signals recorded and located by the National Lightning Detection Network (NLDN) are often accompanied by broadband VHF emissions of sufficient intensity to trigger satellite-based RF receivers. It has previously been shown that events recorded by the FORTE satellite and the NLDN, which have occurrence-time differences of 0.3 ms or less, can be assumed to originate at the same approximate geolocation with 97.5% reliability. For such event pairs, the VHF events recorded by FORTE were assigned the NLDN locations. We use graphical analysis to identify groups of satellite data that can be assumed to originate from the same storms as the located events. To accomplish this, a value of total electron content (TEC) is derived from the frequency dispersion of each VHF signal recorded by FORTE. Scatterplots of TEC versus time often reveal curved clusters of satellite data. The variation of TEC with satellite elevation angle, and time, is consistent with a model in which the length of a signal's required propagation path through the ionosphere changes with satellite and storm position. We use the values of TEC for the located satellite data, a simple model of the ionosphere, and NLDN data to help us identify TEC clusters that can be assumed to originate from relatively compact, and unambiguous, geolocations. For the time period of April through September of 1998 we geolocate 65 groups of satellite data records. A total of 6131 satellite data records, which were not previously located, have been assigned the median NLDN coordinates of their corresponding storm.