Suppression effects associated with VLF transmitter signals injected into the magnetosphere

Magnetospheric growth of coherent VLF signals transmitted from Antarctica is inhibited by whistler mode echoes of earlier transmitter signals. The echoes reduce 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. Another suppression phenomenon, also discovered from VLF transmissions, is the suppression of mid-latitude hiss by as much as 6db in a band up to 200 Hz wide just below the transmitter frequency. This quiet band develops in 5 to 25 s and lasts up to a minute after the end of transmissions. There is evidence that power-line radiation in the magnetosphere at harmonics of 60 Hz also produces quiet bands.