Some studies on the stepped fall of atmospherics during sunrise

An analysis of the VLF atmospherics at 10, 20 and 30 kHz recorded over the tropical station Calcutta exhibits stepped-falls during sunrise of which the first two steps observed before ground sunrise are relatively weak while the last one called the main stepped-fall observed around ground sunrise is always clearly visible. Results show that the fall of the main-step as well as its duration are greater than those of the other two steps. The average duration for the third, second and main steps are respectively 21, 31 and 54 minutes. The magnitudes of the fall are frequency dependent, being maximum at 30 kHz. They have also been found to be less during the summer and monsoon months than in winter. The results have been interpreted by considering the source activity and the nature of propagation involved.