Dusty plasmas can be considered as tenuous, dilute or dense when the dust fugacity parameter f ≡ 4πnd0λD2R ∼ NDR/λD
∼1, or ≫1,
where nd0, λD
denote, respectively, the dust number density, the plasma Debye length and the dust grain size (radius), and ND = nd0λD3
is the dust plasma parameter. Dense dusty plasmas are shown to support a new kind of ultra low-frequency electrostatic dust mode which may be called the “Dust–Coulomb Wave” (DCW). In contrast to the dust–acoustic wave (DAW) and the dust–lattice wave (DLW) which exist even for constant grain charge, DCWs are accompanied by dust charge as well as number density perturbations which are proportional to each other. For frequencies much smaller than the grain charging frequency, DCWs propagate as normal modes with the phase speed CDC ≡ qd0/,
where qd0 (md)
is the charge (mass) of the dust grains. In the long wavelength limit, the DCW phase speed is much smaller than that of DAW (CDA),
and scales as ∼ CDA/.
Thus, for a given wave number, the frequency regime for the existence of DCW is much lower than the DAW regime. A comparison between the three types of dust–modes (DCWs, DAWs, and DLWs) has been carried out. © 1999 American Institute of Physics.