Abstract
Sonagrams of a large number of whistlers recorded during the magnetic storm of 13th to 15th January, 1967, have been presented to illustrate the formation of ducts corresponding to path latitudes of 24°, 27°, and 31°and the VLF ducts formation during the main phase of the magnetic storm has been attributed to the enhanced electron density produced in the F2-region heights and above upto about 2 earth's radii. A correlation between the Kp-index and the whistler occurrence during the storm has been drawn and the enhanced whistler activity has been explained in terms of additional VLF ducts produced for whistler mode of propagation.
From a large number of whistlers recorded during the storm, it is inferred, contrary to the idea of Smith, that though the formation of ducts can take place in much less than 30 minutes, once the duct has formed it can persist for at least 2 to 3 days.
It is found that usually whistlers have pure gliding tones during quiet days while most of the whistlers recorded during the disturbed days show a large amount of diffuseness, which has been attributed to the propagation of VLF waves within a wide area of the duct. This is in contrast to the idea of Crouchley and Finn who show that diffuseness increases with dispersion.
From the diffuseness of sonagrams of the whistlers, the effective width of the VLF ducts for the waves of 5Kc/s have been calculated and are found to vary from 15km to 25km for normal whistlers and 40 to 180km for those recorded during the magnetically disturbed days.
