Remote sensing by VLF using “Absolute OmniPAL”. 1. Investigation of short path propagation for possible earthquake precursor detection

The latest version of the OmniPAL receiver uses GPS to allow drift free logging of the phase and amplitude of phase stable transmissions. Any drift in phase or amplitude can then be attributed to temporal variation of the lower ionosphere. In this paper, the first six weeks of data logged of JG2AS phase and amplitude at three sites in Japan is examined for perturbations. Two sources are identified. Solar flares during daytime and hiss-induced electron precipitation during nighttime. Both of these sources have durations of the order of an hour and, in principle, could be detected by other means and the LF effects allowed for. However, if earthquake precursors have much longer periods (days), the effects of solar flares and electron precipitation can be ignored