The contribution of anisotropic conductivity in the ionosphere to lightning flash bearing deviations in the ELF/ULF range

Lightning flash bearing deviations in the ELF/ULF range are monitored at tw o independently operating measurement instruments in Hollister, California, and Silberborn, Germany. The lightning flash bearing deviation at Holliste r exhibits a rotational dependence similar to 12 degrees, associated with t he conductivity contrast between the Earth's crust and the nearby Pacific O cean. The bearing deviations at both stations exhibit a pronounced diurnal local time variation similar to 11 degrees. This diurnal variation is attri buted to the anisotropic conductivity in the ionosphere during day- and nig httime conditions. No bearing deviation dependency from the source-receiver distance can be distinguished. Correction for the mean rotational dependen ce at Hollister and the mean diurnal variation at both locations results in residual bearing deviations similar to +/-10 degrees which are related to the variability of anisotropic conductivity in the ionosphere on a subdiurn al time scale.