VLF SFERICS OF VERY LARGE VIRTUAL SOURCE STRENGTH.

Two stations in New England, spaced 68.6 miles apart, recorded sferic waveforms in the frequency band 2.4 to 310 kHz. Individual sferic sources whose virtual locations, as determined by triangulation with crossed-loop direction finders, fell within two areas of 290 square miles each were identified, and those producing peak fields equivalent to 20 Vm at 50 miles at both stations were selected. In over two years of recording there were only 27 such sources, the largest giving 58 Vm at both stations. Waveforms from the same source had nearly the same shape at the two stations, but occasional amplitude disparities as great as 21 were noted. It is shown that such cases are consistent with an idealized model in which the source is an elevated, time-varying dipole moment with its axis inclined away from the vertical. On the ground plane, the electric field pattern is then directive and the magnetic field lines are circles about a virtual source displaced horizontally from the actual source. Crossed-loop direction finders would thus indicate the direction of the virtual source, rather than the true source. All 27 sferics had rise times in the 4 to 20 microsec range, but differed in initial polarity, duration, and number of oscillations. Those recorded on the same day from virtual sources in the same area tended to have the same polarity, but over the entire recording period the polarities were nearly equally divided between positive and negative. These sferics appeared as single events, rather than multiple events, and occurred predominantly in the daytime from April through October, usually in conjunction with cold fronts and squall lines in or near the monitored areas. Author