Intensities and polarization rates of whistler mode VLF signals observed from a ground network near L = 4

Abstract

VLF signals transmitted from Siple station in Antarctica that propagated through the ionosphere and magnetosphere to the northern hemisphere and triggered emissions generated by the Siple signals in the magnetosphere are radiated from wave exit areas on the ionospheric lower boundary. The locations and extent of the wave exit areas are deduced from the wave intensity distributions measured on the ground. In this method we use measurements of wave magnetic intensity made simultaneously at six stations. The validity of this method is assessed by comparing the results with values simply estimated from a mathematical evaluation and with results of full wave calculations carried out by Nagano et al. (1986, 1987). We find close agreement among results obtained from the three methods and conclude that the wave intensity distribution analysis is valid. The observed distribution of polarization rate also agrees well with the results of the full wave calculations carried out by Nagano et al. (1986, 1987) and with those obtained by the mathematical evaluation. As a result of analyzing a triggered emission in detail, the extent of the wave exit area of the downgoing triggered emission is estimated to be 25-50 km or so at an altitude of 80 km. We believe that this value relates to the horizontal extent of the duct through which the triggered emission propagated in the magnetosphere and ionosphere and that essentially only direct waves transmitted from the wave source arrive within a range of 200 km from the wave intensity peak on the ground.