Some unusual discrete VLF emissions observed at a low-latitude ground station at Agra

A detailed analysis of the VLF emissions data obtained during occasional whistler campaigns at the low-latitude ground station Agra (geomagnetic latitude 17°1¢ N, L = 1.15) has yielded some unusual discrete VLF emissions of the rising type. These include (1) emissions occurring at time intervals increasing in geommetrical progression, (2) emissions occuring simultaneously in di€erent frequency ranges and (3) emissions observed during daytime. In the present study, the observed characteristics of these emissions are described and interpreted. It is shown that the increasing time delay between di€erent components of the emissions match closely with the propagation time delays between di€erent hops of a whistler of dispersion 19 s, the unusual occurrence of the emissions in two di€erent frequency ranges approximately at the same time may possibly be linked with their generation at two di€erent locations, and the occurrence of emissions during daytime may be due to propagation under the in ̄uence of equatorial anomaly.