T. Aslaksen

A new damping effect for the dust-acoustic wave

Abstract We have computed the dispersion relation for low-frequency dust-acoustic waves including dust charge variation. A phase difference between the dust charge variation and the wave can lead to a strong damping of the wave. We present analytical expressions for the wave dispersion and the damping rate for dust acoustic waves in a not too dense dusty plasma and discuss possible applications of the theory for dust waves in planetary dust rings.

A kinetic model for dust acoustic waves applied to planetary rings

We have derived a kinetic model for the propagation of low-frequency waves in a dusty plasma containing dust particles and drifting plasma particles. The model includes Landau damping or growth and damping from charge variation on the dust particles, and is applied to dust-acoustic waves in planetary rings. Analytic expressions for the dispersion function are used to examine the stability of this wave mode. The dispersion properties are also found numerically for dense dust clouds or large drift velocities, where the analytical expressions are not applicable. We show how the stability condition depends on the density of dust particles and the wavelength, for plasma and dust parameters which may apply to Saturns F ring, G ring, and E ring, and to Jupiters ring.

Charged dust in the Earth's mesopause; effects on radar backscatter

We discuss to what extent small scale density inhomogeneities in the dust distribution may influence radar backscatter in the summer mesopause. We show for a reasonable range of parameters that falling dust interacting with a neutral gas vortex cannot penetrate to the centre of the vortex. The size of the hole in the dust space density distribution around the vortex centre depends on the vortex size and rotation speed and on the free fall velocity of the dust. If the dust contains a non-negligible fraction of the space charge (this requires that the dust is charged by the photoelectric effect), a substantial gradient in the positive charge density across the dust hole edge results. The positive charge density profile in the thin boundary layer will depend on the size distribution of the dust grains. A corresponding gradient in the electron density will appear and we discuss the conditions under which it will lead to a radar reflection of the magnitude observed in the summer polar mesosphere (PMSE - Polar Mesospheric Summer Echoes). This mechanism appears to have the potential of explaining the observed characteristics of the radar echoes such as the strong wavelength dependence and the narrow and sometimes complex and composite spectral signal profiles.

Probing the properties of planetary ring dust by the observation of Mach cones

Compressive dust acoustic waves can be excited in dusty plasmas. Big boulders in planetary rings move at the Keplerian velocity, while smaller dust particles move at a slightly different velocity due to the action of the Lorentz force. If the difference in velocity Δυ is larger than the dust acoustic wave velocity, αd, a wake will be formed with an opening angle of 2θ where sin θ = |αd/Δυ|. The discovery of wakes and the measurement of their opening angles by the space experiment Cassini to Saturn will yield added information on the dusty plasma conditions in regions through which Cassini will not pass. We find that in some regions the waves that are excited by the boulders may be weak because a large fraction of the interacting dust is absorbed rather than deflected by the boulder. For a given dust size the most favourable conditions for the observations of wakes exist in two fairly narrow regions, one inside and one outside the corotation radius. The favorable regions are closest to the corotation radius for the smallest dust particles and progressively further away for larger dust particles.

Collisionless braking of dust particles in the electrostatic field of planetary dust rings

We show that a dust test particle moving in a periodic orbit in electrostatic fields, as those due to planetary rings, will experience a net deceleration also in the absence of any dust-dust collisions. The varying charge on the moving dust, as it moves in and out of regions of differing electron and ion densities (due to changes in the ring electrostatic potential), will be out of phase with the equilibrium charge in a way which leads to a net braking of the particle. This effect has been shown to damp the coherent oscillations of electrostatically supported dust rings [1] and to damp the oscillatons of levitated dust particles in plasma sheaths at surfaces of solid bodies [2]. We show that this effect will lead to a damping of internal random velocities in a planetary ring, at a rate which in many cases can be much faster than that due to dust-dust collisions. For moderate radial motions of dust test particles relative to a ring we also find that there can be an efficient braking and pick-up of particles already at the ring edges. We conclude that when the collisionless braking of random velocities dominates over dust-dust braking, an increased rate of collapse of the ring to its minimum electrostatic thickness may occur and possibly also an increased radial stability of the ring. The pick-up effect at the ring radial edges is a potentially imortant factor in forming their structure.

Diagnostic of dusty plasma conditions by the observation of Mach cones caused by dust acoustic waves

The theory of dust acoustic waves in dusty plasmas is well developed for conditions with monosized dust particles. It was proposed [O. Havnes et al., J. Geophys. Res. 100, 1731 (1995)] that such waves may be generated in planetary dust rings by boulders moving through the dust at a velocity that is somewhat higher than the dust acoustic velocity. In this manner Mach cones can form with an opening angle that is dependent on the boulder velocity and the local dust acoustic velocity. Measurements of this opening angle will therefore provide additional information on dusty plasma conditions since the dust acoustic velocity can be determined when the boulder velocity is known. We suggest that this method may also be applied under laboratory conditions if a suitable controlled disturbance can be made and we discuss how the dust acoustic velocity and resulting Mach cones are affected if the dusty plasma have a distribution of dust sizes.

The effects of electrostatic forces and random velocities on dust ring thickness profiles in rings with a dust size distribution

We have considered vertical density profiles of dust rings with a dust size distribution. The density profiles are determined by random vertical velocities of the dust, the vertical component of gravity, and the electrostatic lifting force on the charged dust due to the self-screening electric field of the dust ring. We reconsider the case of electrostatically supported dust rings with monosized dust and no random vertical velocity (Havnes and Morfill, 1984). We thereafter look at a pure electrostatically supported dust ring with dust size distribution, which we find will be perfectly separated according to dust size with the largest dust particles in the ring plane and the smallest and lightest furthest above the plane. When the dust particles are given a random velocity, this leads to an increasing mixing of the different dust sizes as the random velocity is increased. The electrostatic effect will still lead to separation of dust particles according to size, and we find that a general trend in dust rings will be the existence of a halo of small dust particles with a degree of mixing of dust sizes near the ring plane which is dependent on the dust random kinetic energy and on the dust and plasma parameters.

Mach cones in dusty plasmas in planetary rings and in laboratory experiments

Abstract We have discussed in more detail the possibilities of extracting information on the dusty plasma conditions in planetary rings and in laboratories by observing the V-shaped Mach cone pattern around a charged body moving through or close to a layer of dusty plasma. Based on the existing theories for dust acoustic waves and accelerations of dust orbits at the front of the body we find that, if the normal plasma parameters are known, we should be able to extract information on the dust average sizes and size distribution, the dust number density and material density. With more refined theories for the dust acoustic wave and dust bow shocks, including a dust size distribution it should be possible to find additional and more accurate information on the total plasma conditions.

The Outbursts of Compact Radio Sources: Limitations of Compton Scattering Models and the Possibility of Pitch Angle Scattering

The self-consistent determination of emergent flux and electron distribution in synchrotron self-Compton (SSC) models is considered in the limit when Compton cooling dominates synchrotron cooling and the Thomson optical depth is small. An approximate analytic description is found that agrees well with numerical solutions. The results are used to evaluate the importance of Compton cooling during the initial/high frequency phase of radio outbursts in flat spectrum compact radio sources. It is concluded that the main observational features are not likely due to a Compton phase in an adiabatically expanding source. It is shown explicitly why this conclusion differs from that of earlier calculations. An alternative model is considered, in which the injected electron distribution is anisotropic. The subsequent pitch angle scattering phase is identified with the high frequency radio outburst. The salient features expected from such a model agree qualitatively with those observed. The observed burst of 3C 345 is discussed in some detail; in particular, the importance of the different time evolutions between 1990 and 1994 of the burst in the initial and late phases is stressed. It is emphasized that in order to constrain models for the outbursts of compact radio sources, detailed observations of the initial phase are essential.

The dust direct current self‐bias potential in a time varying plasma sheath

We introduce an important physical effect, namely, the response of the dust particle charge to an oscillating electric field in a plasma sheath. The ac field may lower the average dust potential well below the corresponding floating potential Uf in a dc sheath. The resulting dust potential will be called the dust dc self‐bias potential Udc and is generally a sensitive function of the ac potential amplitude and of the ratio ωch/ω between the so‐called dust charging frequency ωch and the ac frequency ω. The difference between Udc and Uf maximizes for ωch≪ω and approaches 0 for ωch≫ω. We propose that the dust dc self‐bias effect is important in all plasmas having a rapid potential variation and always has to be taken into account when calculating the charge and the force on dust particles in such plasmas. Since levitation of dust particles above surfaces normally requires negative dust particles, the dust dc self‐bias effect may explain why some experiments indicate that levitation of dust particles is easie...