VLF Remote Sensing - A new computational model

Very Low Frequency (VLF) radio waves have long been used as a tool for longrange remote-sensing of the ionosphere. Changes in the electrical properties of the ionosphere cause changes in the received phase and amplitude of the radio wave. Comparing experimental observations with simulations based on the predicted changes to the ionosphere allows us to understand what is happening. This technique has been successful in understanding many different types of ionospheric changes, from the influence of the Sun, including day– night differences and solar flares to nuclear weapons tests, and electron precipitation from the Earth’s Van Allen Radiation belts to the effects of lightning related phenomena such as EMP and quasi-electrostatic heating. A new VLF propagation simulation is created here with the aim that it has a minimum of assumptions and is as general as possible. The simulation approach is based on previous work by Nagano et al. [1975]. However, we expand on their basic approach to allow propagation in a realistic Earthionosphere waveguide allowing for the spherical nature of the Earth and changing conditions along the path. The simulation is able to reproduce some simple propagation situations, particularly close to the transmitter and when the transmitter and receiver are on the ground, which is a common modelling situation. Despite some shortcomings considerable progress has been made towards the stated goals. Useful modelling can be achieved using the simulation, particularly with short range (< 3000 to 4000 km) transmitter to receiver paths under mostly homogenous conditions.