VLF strip holographic imaging of lightning-associated ionospheric disturbances

A linear array of very low frequency (VLF) receivers, deployed along a line roughly perpendicular to the direction of signal propagation, enables the determination of the size and location of D region disturbances produced by lightning-induced electron precipitation (LEP) bursts or by lightning-induced heating in the vicinity of the great circle VLF propagation paths. The configuration essentially constitutes a strip hologram so that the width of both the amplitude and phase perturbation of the signal pattern recorded along the strip are simply related to the size and location of the disturbance for the range of altitude profiles of disturbed ionization expected for LEP events. The validity of the approach is demonstrated both analytically for single waveguide-mode propagation and also for a realistic propagation path (Annapolis-Stanford) by using a three-dimensional numerical model of VLF propagation and scattering in the Earth-ionosphere waveguide. We also discuss the criteria by which the spacing of discrete elements along the strip can be optimally choosen.