Propagational features of higher harmonic tweeks at low latitudes

Higher harmonic tweeks observed for the first time at the low latitude station Varanasi (geomag. lat. 14 55 N) are reported. The analysis of data shows that higher harmonic tweeks are usually not associated with whistlers and occur when the ionization in the lower ionosphere would not increases with height. The Earth-ionosphere waveguide dispersion features play an important role in the propagation of broad band lightning generated signals and their occasional observation as higher harmonic tweeks. It is shown that the conductivity of ground and sea mixed path, forming the lower surface of the waveguide, provide an estimate of the travelled distances of higher harmonic tweeks in the waveguide. The attenuation factors are computed which shows that as the harmonic number increases their probability of observation decreases. The attenuation increases as the frequencies approach the cut-off frequencies and also as the layer height falls. The tweek activity is found to increase during periods of magnetic disturbances.