Variable Phase Propagation Velocity for Long Range Lightning Location System

Lightning Location System (LLS) is of key importance to numerous meteorological, industrial and aviation systems worldwide. A crucial input parameter of a LLS which utilizes time-of-arrival (TOA) method is the wave propagation velocity at low frequencies. For example, the WWLLN network use group velocity approach, which is assumed to be constant near the speed of light [e.g. Dowden et al., 2002]. The detected lightning signals are normally a mixture of ground waves and sky waves (i.e. ionospheric hops), which are associated with different elevation angle of the incident wave [e.g., Fullekrug et al., 2015]. In this study, we introduce the new concept of "phase propagation velocity" as observed by the receiver considering the elevation angle. It is found that the radio waves from two submarine communication transmitters at 20.9 kHz and 23.4 kHz exhibit phase propagation velocities that are 0.51% slower and 0.64% faster than the speed of light as a result of sky wave contributions and ground effects. Here, we apply our new technique, using a variable phase propagation velocity, to the TOA method for the first time. This method was applied to electric field recordings from a long range LLS ( 500km) that consists of four radio receivers in Western Europe. The lightning locations inferred from variable velocities improve the accuracy of locations inferred from a fixed velocity by 0.89-1.06 km when compared to the lightning locations reported by the UK Met Office. The observed phase propagation velocities depend on the ground and ionosphere conditions along the propagation paths. The distribution of the observed phase propagation velocities in small geographic areas fit a normal distribution that is not centered at the speed of light. Consequently, representative velocities can be calculated for many small geographic areas to produce a velocity map over central France where numerous lightning discharges occurred. This map reflects the impact of sky waves and ground effects on the calculation of lightning locations as a result of the network configuration. The phase propagation velocities also vary with different ionospheric conditions, e.g. day-night differences.