Cutoff frequencies of intermediate frequency waves in a bounded magnetoplasma

Abstract

The paper presents a study of the cutoff frequencies of a class of electromagnetic waves, designated here as intermediate frequency waves, which can propagate in a plasma column enclosed in a conducting cylinder and embedded in a static axial magnetic field. These waves reduce to lower hybrid waves or magnetohydrodynamic TE waves under suitable conditions. The cutoff frequencies of the intermediate frequency modes have been investigated, covering all possible values of ωpe,ωce, and a, where ωpe is the plasma frequency, ωce is the electron cyclotron frequency, and a is the plasma radius. The study includes all the possible types of modes, namely, circularly symmetric I0m modes and asymmetric Inm modes with n ? 1, each Inm mode being split into two, called I+nm and I−nm modes, corresponding to two different polarizations of the field components. The cutoff frequency ωc 0 of any Inm mode with n ? 0 is found to be confined to the frequency domain 0 to ωnm, where ωnm is the cutoff frequency of the TEnm mode in an empty waveguide. The zero frequency is approached if ωpe or a tends to infinity or ωce tends to zero, the other parameters remaining constant in each case. The frequency ωc 0 tends to ωnm if ωce is sufficiently large for any given set of ωpe and a. The study of intermediate frequency modes reveals that these modes can broadly be divided into the following two categories: (1) I−n1 modes with n ? 1, and (2) modes other than I−n1 modes. The nature of variation of ωco ith ωpe for fixed values of ωce and a for I−n1 modes is, in general, quite different from that for the other modes. Furthermore, ωc 0 tends to ωci as a tends to zero with ωpe and ωce held fixed in the case of I−n1 modes, while for other modes ωc 0 tends to ωlh under the same condition, where ωci is the ion cyclotron frequency and ωlh is the lower hybrid frequency. Any Inm mode with n ? 0 is found to reduce to the magnetohydrodynamic TEnm mode at or near the cutoff under the condition: ωpi≫ωnm, where ωpi is the ion plasma frequency. Finally, it is shown how the charge density in a bounded magnetoplasma can easily be determined in most experimental situations by using the cutoff frequency characteristic of the dominant I−11 mode.