Conclusions

We have shown that the presence of dust particles which acquire a proportion of the negative charge in a plasma can strongly affect the dispersion relation of obliquely propagating Alfvén waves in interstellar dusty clouds. The consequences are that the low frequency wave has a helicon wave character (elliptically polarized) rather than an Alfvén wave character (linearly polarized), and that the behaviour of the wave near the Alfvén resonance is modified, with possibly enhanced Alfvén resonance absorption in such a plasma. For the cloud parameters considered here, the resonant frequency is , for a wavelength along the magnetic field of AU). At that frequency, the damping length is times the wavelength for parallel propagation (and the damping time is for a real wavenumber), while for oblique propagation at the resonant frequency the wavelength and damping length are both AU.

We note that the dust-charge fluctuations as well as the dynamics of the dust charging process can lead to the appearance of an imaginary part in the dispersion equation (Tsytovich and Havnes 1993), leading to a damping of the wave in addition to the collisional damping that we have considered. However such a damping depends on the electric field component parallel to the magnetic field, which is small for Alfvén and magnetoacoustic waves in a low gas pressure plasma as considered here, so we have neglected it.

The results we have obtained in the specific context of waves in dusty interstellar clouds should also be useful for understanding the effects of propagation and absorption of Alfvén waves in multicomponent dusty plasmas such as those in planetary dust rings and the Earth's mesopause.

© Copyright Astronomical Society of Australia 1997