“…This signature could be present at the fundamental and harmonic frequencies of the field line (assuming these frequencies are present in the fast mode waves), keeping in mind that the position of the turning point of a fast mode wave depends on its frequency, so the cavity-mode frequencies will not obey the simple relation f,= nf•, where f is the frequency, fx is the fundamental and n indicates a harmonic [Samson et al, 1991a]. However, the problem is somewhat complicated because, in reality, a transient frequency is also excited [Allan et al, 1985[Allan et al, , 1986 Whether discretely driven resonances are excited by a cavity mode, a waveguide or some other source, their observation may be distinguished from those driven by stationary broadband sources. In the former case, wave power is observed over a narrow range of L shells, as thin as 0.3 to 0.6 RE in the equatorial plane [Samson and Rankin, 1994].…”