A COMPARATIVE-STUDY OF THE SUNRISE AND SUNSET EFFECTS ON THE INTEGRATED FIELD INTENSITY OF VLF ATMOSPHERICS

A comparative study is made of the stepped structure of atmospherics d uring sunrise and sunset hours at three harmonically related frequenci es in the VLF band, observed at the coast of the Bay of Bengal. The oc currences of the steps, their duration, time sequence and amplitudes a re critically examined. Our analysis reveals that the start times of t he third and second steps of the sunrise effect are always before grou nd sunrise, while the start of the main step is distributed on both si des of it. During sunset hours, the onset time of the main step is bef ore the local ground sunset, but for the second and third steps, the o nset times are after that. The results are interpreted by considering the multi-hop propagation as well as the distributed noise sources sur rounding the observing site.