Absorption of Very Low Frequency Waves in the Whistler Mode Propagation at Low Latitudes

Abstract

Absorption of very low frequency (VLF) waves in the whistler mode of propagation at low geomagnetic latitudes of 24° and 15° have been calculated for various ionospheric conditions during sunspot maxima, average and minima. The very high absorption values of 60dB, 46dB, 22dB at 24° and 104dB, 90dB, 42dB at 15° of VLF waves (10Kc/s, 5Kc/s and 1Kc/s) for two ionospheric paths between 50-200Km height can explain the non occurrence of whistlers and VLF emissions such as chorus and hiss at low geomagnetic latitudes during day time.The absorption contributed by F-region though generally smaller than that contributed by D and E layers is never negligible and at times of high F-layer concentrations during the magnetic storms the F-layer absorption may even be dominant.The absorption of VLF waves is low at night and as such can not explain the low occurrence of whistlers during night time. However, when the angle between the magnetic field and the wave normal is taken into account it is found that the absorption index increases considerably and hence the whistler energy may not be received on the ground especially at geomagnetic latitudes below 15°.Comparing the computed values of absorption of VLF waves by Altman and Cory at 40° geomagnetic latitude and Laschy at 55° latitude with our values, for low latitudes it is found that the absorption increases rapidly as the latitude decreases which is in conformity with the results of Helliwell and Altman and Cory.