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Whistler Wave Trapping in a Narrow Density Trough

The propagation of linear whistler waves is investigated in a plasma where the field-aligned density trough is produced without electron heating, in the artificial way different from previous experiments. Whistler waves are observed to be trapped in the narrow trough of width \(2d{\lesssim}\lambda_{//}\) (parallel wave length). The phase and amplitude profile of ducted whistlers is measured along and across the trough, controlling the density depth Δ n e arbitrarily. The transverse amplitude decay outside the trough is found to become maximum at critical frequency. The measured transverse decay-length as well as the wave dispersion is in quantitative agreement with the trapping theory.