Propagation of artificially excited Langmuir waves in the ionosphere

The MPAE heater near Tromsö, Norway was used to excite parametric decay instabilities in the ionosphere, which then were observed by means of the EISCAT radar. During unfavorable geophysical conditions the heater power may not be sufficient to exceed the threshold of excitation for wavelengths matched to the observing radar. Evidence is presented that during such conditions instabilities of longer wavelengths are produced that cannot be seen by the radar immediately, but that become observable after they have propagated for some distance and changed their wavelength according to the dispersion relation.