An explanation of non-observation of transmitted VLF signals on ground

Generally upper cut-offs in VLF signals observed on the ground occur at f=fH/2f=fH/2 (f   is wave frequency, fHfH is electron gyrofrequency). But in an experiment conducted on January 23–24, 1988 a cut-off occurred at 2400 Hz (or 2500–2550 Hz also sometimes) though transmitted signals were of 1900, 2150, 2400, 2650 and 2900 Hz frequencies. The theoretical cut-off in this case has a value of ∼3.3 kHz corresponding to the line of propagation of 63.7° geomagnetic latitude (L=5.1L=5.1). Sonwalkar et al. [1997. Journal of Geophysical Research 102, 14363–14380] suggested that unavailability of sufficient number of energetic electrons may be a reason of non-reception of these signals. To examine this possibility, pitch angle dependence of temporal wave growth (Γ  ) and power gain (in dB) is studied. Our analysis shows that energetic electron number density was sufficient to amplify the transmitted signals, and it was scarcity of suitable thermal anisotropy, A(ER)A(ER), because of which some signals could not be received at conjugate point.