High spatial attenuation of the Siple transmitter signal and natural VLF chorus observed at ground‐based chain stations near Roberval, Quebec

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Abstract
The field intensity of the Siple transmitter signal was measured at four stations along the geomagnetic meridian passing about 100 km west of Roberval, Quebec, during about 1 month from July 10 to August 10, 1979. The distance between adjacent stations was 70–90 km, and the L values covered by the four stations were from 4.15 to 4.7. It is found that the field intensity of the Siple transmitter signal falls off very rapidly with distance. A typical rate of attenuation with distance is found to be as high as −7 db per 100 km, which is nearly 1 order of magnitude higher than the value expected from the earth‐ionosphere cavity propagation theory. The intensity of VLF chorus was measured at six stations along the same meridian on August 8, 1979. The two stations were added to the north of the four stations, and their L values were 4.85 and 5.1. The chorus intensity was found to fall off with distance almost with the same rate as the Siple signal.