O. Havnes

First detection of charged dust particles in the Earth's mesosphere

Some theories for the observed anomalous radar backscatter during the summer (polar mesospheric summer echoes, or PMSE) and electron bite outs measured by rockets require the presence of charged dust. To investigate this, two dust probes have been launched in 1994 from Andoya Rocket Range and we here report the results from the dust and an electron probe on the two payloads. The dust probes were designed to block out the electron and ion components at the mesopause but to detect primary currents due to impacts of charged dust and also to detect secondary plasma production during dust impacts. The results indicate that both during PMSE and noctilucent cloud (NLC) conditions, large amounts of dust, with average sizes apparently of about 0.1 μm and less, were present. The number densities Nd can be up to many thousand per cubic centimeter, and the charge density NdZd likewise. Large local gradients in density and charge density of dust are detected. Dust carrying both positive and negative charges can apparently be present on different occasions. In some parts of the NLC/PMSE layers we find that the negative charge density locked in grains is so large that the number of free electrons is significantly reduced there because the dust acts like sinks for electrons, and an electron bite out results. We also find that in one case the presence of positive dust leads to an increase in the local electron density by photoionization. The main uncertainties in the data analysis are the structure of the dust and the secondary plasma production at the comparatively low dust impact velocities (1 km s−1) in the experiment.

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.

On the role of dust in the summer mesopause

Abstract We propose that dust formed at the cool summer mesopause may have optical properties very different from that measured for bulk material of ice. The smallness of the dust and possible surface impurities may lead to high photoelectric yields and low workfunctions. For such reasons the dust in the summer mesopause may, at least occasionally, be charged to substantial positive surface potentials while pure ice, with its high photoelectric workfunction, would be charged to low and negative potentials by collisions with plasma particles. The presence of ‘dressed’ dust particles, with surface potentials of some volts, can lead to enhanced radar backscatter. We also suggest that the apparent reductions in electron density (‘bite-out’), which have been observed in the radar backscatter region, can be caused by the inability of an electrostatic probe to deflect the massive dust particles. The dust density which is required by our model to explain radar backscatter and electron bite-outs is of the order of 10 cm −3 for dust of radius above 5 × 10 −6 cm.

Levitation and dynamics of charged dust in the photoelectron sheath above surfaces in space

Bodies in space subject to solar ultraviolet flux will emit photoelectrons. At steady state the current of escaping photoelectrons is balanced by an influx of particles from the surrounding plasma. When the photoelectrons dominate the space charge close to the surface, it has previously been shown that two steady state potential distributions can exist, one in which the potential decreases from the surface value to zero monotonically and one in which it decreases to a negative minimum and then increases to zero. It has been suggested that the nonmonotonic distribution is the stable one. By assuming planar geometry and a Maxwellian distribution of the emitted photoelectrons the charging of isolated dust particles in the plasma sheath is calculated for both the monotonic and nonmonotonic potential distribution. By increasing photoemission from the surface from zero a transition from an ordinary Debye sheath above a nonilluminated surface to a photoelectron sheath is simulated. Dynamical properties of the dust particles such as oscillations, damping, stability, and trapping are investigated. After being injected into the sheath or electrostatically levitated, dust may be stably suspended above illuminated surfaces in space, even in the case of zero gravitation. However, the smallest particles may escape completely from the body. For all sheath types an unstable layer exists close to the surface where dust cannot collect. The theory is applied to bodies in the solar wind and to the spokes of Saturn.

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.

Electromagnetic wave scattering in dusty plasmas

The cross section for transition scattering of electromagnetic waves on charged dust particles in a plasma is calculated, extending the results of a previous paper [J. Plasma Phys. 42, 429 (1989)] where the case of longitudinal waves has been considered. For the case of nonlinear screening of the charged dust by the plasma particles (i.e., ‖eφ0/Te‖ ≫ 1, where φ0 is the dust grain surface potential and Te is the electron plasma temperature), numerical and analytical results are presented, showing a significant enhancement, proportional to the square of the grain surface charge, in the cross section with respect to scattering by free electrons. The effect is independent of the sign of the charge for wavelengths larger than the Debye length.

First common volume observations of layered plasma structures and polar mesospheric summer echoes by rocket and radar

We report the results from simultaneous radar and rocket measurements of a PMSE event where for the first time the rocket measured dust and plasma within the radar beam. We find very clear correspondence between the measured dust charge density profile and the radar backscatter profile as a function of height. We find that even very small amounts of charged dust is associated with an appreciable PMSE radar backscatter. Although we find it likely that the dust layer corresponds fully with the PMSE layer there is a possibility that the upper part of the PMSE layer may be influenced by ion clusters which are too small to be detected by the rocket dust probe.

METER-SCALE VARIATIONS OF THE CHARGE CARRIED BY MESOSPHERIC DUST

Abstract Results from the first direct rocket probe measurements of the currents due to impacting mesospheric dust are reported. The observations which are corrected for production of secondary electrons during impact show that dust is a major charge carrier during polar mesosphere summer echo (PMSE) conditions and that the charge density of the dust can vary strongly over a vertical distance of several meters only. The dust is negatively charged in the reported case.

Dynamics of dust in a plasma sheath and injection of dust into the plasma sheath above Moon and asteroidal surfaces

We have examined single dust particle dynamics in a plasma sheath near the surface of solid bodies in space, considering conditions which resemble those of planetary system bodies, when photoelectric effect can be neglected. The forces on the dust particles are assumed to be from the electric field in the sheath and from gravitation only. As the dust particles will charge negatively in the sheath, these forces will act in opposite directions and may balance.The charge delay of a moving dust particle is responsible for many of the interesting dynamical properties, and we show that for a stationary plasma, dust motion is unstable to about one Debye length out from the surface of the solid body. This part of the sheath will therefore be devoid of dust particles as they will either fall down, escape completely from the solid body or collect and make damped oscillations at stable positions in the outer part of the sheath. With increasing plasma bulk speed towards the surface, the inner unstable part of the sheath will decrease in thickness.The sources for the dust in the sheath are assumed to be mainly ejecta from meteorites and micrometeorites, but may also, for the smallest solid bodies, be from electrostatic levitation of very small dust particles. We have for different sizes of solid bodies calculated the sizes of ejecta that can be ‘floated’ in the sheath. For the solar wind plasma, the suspended dust particles range from less than 1 Μm for the Moon to about 80 Μm for an asteroid with radius 1 km. These particles create a ‘dust atmosphere’.The results in this paper hold when the dust particle density is so low that the charges on the dust particles do not contribute significantly to the total space charge; a higher density will lead to a modification of the sheath.Our calculations show that ejecta below a certain size will be accelerated in the sheath and totally escape from the body even if they have near zero initial vertical velocity, while ejecta above this size will need a much larger velocity to escape. This is especially significant for the small solid bodies (radius of order km and less) which will therefore act as important sources of micronsized dust. This could be of significance for the dust production and the size distribution of dust in planetary ring systems.