Parametric decay of whistlers – A possible source of precursors

Laboratory plasma experiments show that, above a certain threshold field strength, a whistler‐mode wave (WMW) may decay parametrically into a backward propagating WMW and a low frequency ion‐acoustic wave. The required threshold field for a 2 kHz wave in the equatorial plane of the magnetosphere, just inside the plasmapause, is found to be well below 1 mVm−1. It is suggested that the resulting backward travelling WMW may be responsible for triggering precursors ‐ rising frequency emissions which are occasionally observed to precede 2 (or more) hop whistlers.