Downshifted peak features of stimulated electromagnetic emissions during a two-pump wave heating experiment

Abstract

Experimental results of the downshifted peak (DP) in stimulated electromagnetic emissions under two-pump wave ionospheric heating near the third electron gyroharmonic frequency are presented. The European Incoherent Scatter Scientific Association heating antenna array was divided into two parts, one of which worked at constant pump wave frequency f1 and the other part worked at varied pump wave frequency f2 which was not larger than f1. It was found that when the second pump wave was turned on at different frequency with f1, the f1 DP power declined by more than 10 dB with respect to the background noise level, while the downshifted maximum belonging to f1 was further enhanced. The time needed to reach a steady state for DP was shortened from approximately 10 s under cold background conditions belonging to f1, which was nearly consistent with growth time of small-scale artificially field-aligned irregularity (AFAI), to less than 1 s under the preconditioned heating belonging to f2 with pre-existing AFAI. According to the difference in DP temporal evolution under two experimental conditions, it could be deduced that AFAI plays an important role in the DP generation process. Similar to single-pump wave heating, the frequency offset of DP decreases as f2 increases toward the third electron gyroharmonic frequency. These experimental findings provide new insights into the theoretical study of ionospheric plasma nonlinearity.