The Abdus Salam International Centre for Theoretical Physics Dust-cyclotron and Dust-lower-hybrid Modes in Self-gravitating Magnetized Dusty Plasmas

A theoretical investigation has been made of two new ultra-low-frequency electrostatic modes, namely, dust-cyclotron mode and dust-lower-hybrid mode, propagating perpendicular to the external magnetic field, in a self-gravitating magnetized two fluid dusty plasma system. It has been shown that the effect of the self-gravitational force, acting on both dust grains and ions, significantly modifies the dispersion properties of both of these two electrostatic modes. It is also found that under certain conditions, this self-gravitational effect can destabilize these ultra-low-frequency electrostatic modes. The implications of these results to some space and astrophysical dusty plasma systems, especially to planetary ring-systems and cometary tails, are briefly mentioned. MIRAMARE TRIESTE July 1999 Regular Associate of the Abdus Salam ICTP. Permanent address. Recently, there has been a great deal of interest in understanding different types of collective processes in dusty plasmas (plasmas with extremely massive and negatively charged dust grains), because of its vital role in the study of astrophysical and space environments, such as, asteroid zones, planetary atmospheres, interstellar media, circumstellar disks, dark molecular clouds, cometary tails, nebulae, earth's environment, etc. [1-7]. These dust grains are invariably immersed in the ambient plasma and radiative background. The interaction of these dust grains with the other plasma particles (viz. electrons and ions) is due to the charge carried by them. The dust grains are charged by a number of competing processes, depending upon the local conditions, such as, photo-electric emission stimulated by the ultra-violet radiation, collisional charging by electrons and ions, disruption and secondary emission due to the Maxwellian stress, etc. [8-12]. It has been found that the presence of static charged dust grains modifies the existing plasma wave spectra [13-20]. Bliokh and Yaroshenko [13] studied electrostatic waves in dusty plasmas and applied their results in interpreting spoke-like structures in Saturn's rings (revealed by Voyager space mission [21]). Angelis et al. [14] investigated the propagation of ion-acoustic waves in a dusty plasma, in which a spatial inhomogeneity is created by a distribution of immobile dust particles [22]. They [14] applied their results in interpreting the low frequency noise enhancement observed by the Vega and Giotto space probes in the dusty regions of Halley's comet [23]. On the other hand, it has been shown both theoretically [24-33] and experimentally [34,35] that the dust charge dynamics introduces different new eigen modes, such as, dust-acoustic mode, dust-ion-acoustic mode, dust-drift mode, etc. [24-35]. These collective processes or wave phenomena in a dusty plasma (containing extremely massive dust grains) can be studied in either of the three possible regimes, namely, (i) the electromagnetic force is much greater than the gravitational force, (ii) the electromagnetic force is of the same order of the magnitude as the gravitational force, and (iii) the gravitational force is much greater than the electromagnetic force. Case (i) corresponds to usual laboratory plasma situations where Coulombic interaction is primary responsible for the plasma dielectric behavior. Case (ii) corresponds to planetary atmospheres and interstellar media [3, 36-38] where the thickness of the Jovian ring, spoke formation in Saturn's rings, etc. are thought to be due to the balance of these two forces. Case (iii) generally corresponds to astrophysical plasmas where the formation of large-scale structure is attributed to gravitational condensation [39]. Most of these studies [24-35] on these new modes (associated with extremely massive dust grains) are concerned with situation (i) but not with cases (ii) and (iii). Recently, a number of investigations [40-43] have been made on dust-acoustic waves in a self-gravitating dusty plasma