Ground Riser Whistlers at Low Latitudes

Abstract

An attempt has been made to explain theoretically the characteristics of ground risers reported by Dikshit et al. [<i>Nature, Lond., </i> 230 (1971), 114]. From computations employing Haselgrove two-dimensional ray tracing equations and the diffusive equilibrium model for electrons and ions in the magnetosphere, a riser whistler having all the characteristics of the reported ground risers was computed. The riser waves, radiated from lightning discharge, with frequencies lying between 2.8 and 7 kHz and penetrating the ionosphere at different entry points, are found to reflect at different heights during their quasi-transverse propagation but form, alter reflection, a bunch of waves to be received as a ground riser. The upper cutoff frequency (7 kHz) of the ground riser is explained in terms of the non-reflection of vlf waves in the same hemisphere as that of the originating atmospherics and crossing over the equatorial plane to be propagated in the opposite hemisphere; the lower cutoff frequency (2.8 kHz) of the whistler is explained in terms of deviation, after reflection, from the arrival latitude of the riser waves.