The effects of ionospheric horizontal electron density gradients on whistler mode signals

Whistler mode group delays observed at Faraday, Antarctica (65° S, 64° W) and Dunedin, New Zealand (46° S, 171° E) show sudden increases of the order of hundreds of milliseconds within 15 minutes. These events (‘discontinuities’) are observed during sunrise or sunset at the duct entry regions, close to the receiver's conjugate point. The sudden increase in group delay can be explained as a tilting of the up-going wave towards the sun by horizontal electron density gradients associated with the passage of the dawn/dusk terminator. The waves become trapped into higher L-shell ducts. The majority of the events are seen during June-August and can be understood in terms of the orientation of the terminator with respect to the field aligned ducts. The position of the source VLF transmitter relative to the duct entry region is found to be important in determining the contribution of ionospheric electron density gradients to the L-shell distribution of the whistler mode signals.