Subionospheric VLF “imaging” of lightning‐induced electron precipitation from the magnetosphere

Abstract

High‐resolution measurements of subionospheric VLF signals at multiple sites are used as a new tool to assess the spatial distribution and occurrence of ionospheric disturbances associated with lightning‐induced electron precipitation. Simultaneous observations in California, Saskatchewan, and Quebec of VLF signals from multiple sources allow the monitoring of event activity over a coarse grid covering the continental United States. Association of the observed VLF signal perturbations with lightning is often made on the basis of time correlation with prominent radio atmospherics. Simultaneous observations of individual events on subionospheric paths that “cross” one another are used to locate the disturbed ionospheric region(s). Absence of perturbations on nearby paths permits assessment of the spatial extent of the region with a varying degree of accuracy, depending on the distribution of signal paths. In one case distinctly different onset delays (with respect to causative discharges) consistent with predictions of whistler‐particle scattering theory were found, and were interpreted as being due to two separate regions separated in L value by ∼0.4 L being excited in individual events. Occurrence statistics over the course of October 1987 exhibit generally higher levels of activity at the lower‐latitude end of the 2 < L < 4 range, although it is difficult to separately assess the role of the source lightning distribution. Occurrence statistics of simultaneous events on crossing paths are consistent with the spatial extent of the disturbed ionospheric regions being less than a few hundred kilometers. The occurrence statistics also suggest that scattering from disturbances located at distances of >100 km off the great circle paths is not significant.