Echo-induced suppression of coherent VLF transmitter signals in the magnetosphere

Abstract

Magnetospheric growth of coherent VLF signals transmitted from Siple Station, Antarctica, is inhibited by whistler mode echoes of earlier transmitter signals. This new phenomenon, called echo-induced suppression, is observed at least a third of the time that transmissions from Siple Station are detected at the receiving station in Roberval, Quebec, Canada. Suppression levels as high as 20 dB are observed. Though the echo is usually much weaker than the direct signal, the level of suppression is directly related to the amplitude of the echo. The echoes reduce the triggering of emissions as well as the growth of the signal. Echo-induced suppression is not explained by linear wave-wave interference. The echoes are thought to restrict growth by reducing the coherence of the total input signal. These new results suggest that coherent waves tend to limit their own growth even when particle distribution modification is unimportant. Other phenomena, such as whistler-induced suppression of signal growth, are thought to be related. Wave-induced growth suppression provides an indirect verification of the existence of discrete ducts for the propagation of VLF signals in the magnetosphere. It also provides a new tool for investigating wave-particle interactions in the magnetosphere.