Abstract
The characteristics of the wave exit area on the lower boundary of the ionosphere is inferred from the distribution of the magnetic intensities of the VLF waves simultaneously received at six observation stations near the Siple conjugate point at Roberval in Canada on July 23, 1979. In order to estimate the characteristics of the wave exit area, we have to make some assumptions about the conditions of the input wave injected to it and the propagation mechanism in free space. Applying the principle of Huygence for electromagnetic wave in free space to the wave radiated from the exit area, we compute the distributions of the wave magnetic intensity on the ground. By a least square technique, we infer the location and shape of the exit area making the wave intensity distribution observed on the ground. The obtained results are as follows. It was found that three wave exit areas at least existed over the observation station network. It was confirmed that the signals radiated from the Siple transmitter (Siple waves) were propagated along the ducts in the magnetosphere. The wave exit areas were generally elongated in the east-west direction, suggesting that the shapes of the ducts are also elongated in the east-west direction.
