D. A. Mendis

A note on the possible electrostatic disruption of bacteria

Recently published scanning electron and transmission electron photomicrographs of cells of the bacterium Escherichia coli exposed to plasma glow discharges at atmospheric pressure indicate physical disruption of their outer cell membranes. However, the mechanism of cell disruption was unclear. Here, we propose and model an electrophysical mechanism for this phenomenon, namely, the electrostatic disruption of the cell membrane, which takes place when it has acquired a sufficient electrostatic charge that the outward electrostatic stress exceeds its tensile strength. It also appears that surface roughness or irregularity would render it more sensitive to electrostatic disruption.

UV-induced Coulomb crystallization in a dusty gas

Conditions are investigated for forming a Coulomb lattice of dust grains which are charged positively by the ultraviolet induced photoemission of electrons. >

Positively charged dust crystals induced by radiative heating

In this paper, we consider theoretically conditions for forming crystals of positively charged dust, when the dust is heated by a laser or by a strong infrared source and emits electrons thermionically. We discuss parameters for grains dispersed in an inert gas at pressure of a few mbar. We briefly discuss levitation by forces associated with gas flow or photon flux. A possible experimental scheme for achieving such a crystal is briefly outlined.

Secondary emission from small dust grains at high electron energies

A previous model for secondary electron emission from small grains is modified to calculate yields for micron sized grains, both spherical and cylindrical, when the primary electrons constitute a high energy parallel beam. It is found that, in general, the secondary electron yield is significantly higher than for the case of normal incidence. Moreover, the equilibrium potentials of the grain are always positive due to this enhanced secondary emission. These results are compared with experimental data recently available for micron sized glass particles, and equilibrium potentials, calculated based on the model presented here, and are found to be in reasonably good agreement with their measured potentials. >

UV-induced Coulomb crystallization of dust grains in high-pressure gas

A scheme for forming a Coulomb lattice of positively charged dust grains in a high-pressure (P>torr) gas is discussed theoretically. The grains are charged positively by photoemission in the presence of a flux of ultraviolet (UV) photons with energy larger than the work function of the grains, but lower than the ionization potential of the background gas. Methods for levitating and containing the dust using photophoretic forces (both UV and non-UV), gas drag, and electrostatic forces are considered.

Neutral hydrogen in cometary comas

The strong Lα radiation observed recently in comets Tago-Sato-Kosaka and Bennett can be explained in terms of the resonant scattering of solar Lα radiation on neutral hydrogen formed by the photo-dissociation of H2O which is vaporized from a nucleus having an ice core. A complete hydrodynamic description of an atmosphere composed of H2O and its daughter products OH, H and O coupled through frictional interaction as well as production and loss processes is given. Numerical results are computed in a typical case, and it is found that a temperature of about 3000 K for the cometary atmosphere provides the best fit with observation.

The role of field emission in the electrostatic disruption of cosmic dust

The role of field emission in mitigating the process of electrostatic disruption of charged cosmic dust grains is discussed. It is shown that this process prevents the disruption of even the smallest grains independent of the plasma environment if the tensile strengthFt of such grains is greather than about 3.6×107 dynes cm−2. On the other hand, if such grains are very fragile (i.e.,Ft≲106 dynes cm−2, as in ‘cometary’ grains) even field emission cannot always prevent their electrostatic disruption. In this case it is required that the magnitude of the potential of the grains is ≲0.15 V, which places stringent upper limits on the ambient plasma temperatureT. For pure hydrogen and pure oxygen plasmas,T≲700 K andT≲460 K, respectively.

The neutral atmospheres of comets

In this paper we have endeavored to critically evaluate our present understanding of cometary atmospheres. Following a brief introduction of the significance of the study of cometary atmospheres (Section 1), the relevant photometric and spectroscopic observations are summarized in Section 2.The interaction with the solar radiation, with regard to both the excitation of the observed species as well as the dissociation of stable molecules evaporating from the nucleus, is considered in Sections 3 and 4. The gas phase chemistry likely to take place in the dense inner coma is next considered in Section 5.The exospheric and hydrodynamic models of the expanding cometary atmosphere are considered in detail in Section 6, and both their limitations as well as possible improvements are discussed.The observed chemical composition of the neutral atmosphere and the inferred chemical composition of the volatile component of the nucleus, together with possible variations between different classes of comets is next considered in Section 7, and their possible cosmogonic significance is discussed.In conclusion, some of the important directions in which future research should progress, in order to provide more complete and secure knowledge of cometary atmospheres, are stressed (Section 8).

NEUTRAL ATMOSPHERES OF COMETS: A DISTRIBUTED SOURCE MODEL

It is argued that the typical nuclear region of a comet, under a variety of circumstances, consists of a central icy nucleus surrounded by an extended icy halo of grains forming a supplementary source for the observed atmospheric constituents.A complete hydrodynamic description of a neutral atmosphere corresponding to such a distributed source model when the predominant ‘parent-molecule’ is H2O is given, and numerical results corresponding to two different velocity distributions of the icy halo consistent with smaller and larger grains are computed. The applicability and relevance of these models are discussed.

A note on the very small grains (VSGs) observed at Halley's comet

The most striking feature in the spatial distribution of the smallest dust grains observed at Halleys comet by the VEGA-1 spacecraft is the sharp glitch at a cometocentric distance of about 180 000 km, which approximately corresponds to the so-called cometopause inside which the contaminated solar wind plasma was rapidly cooled. We propose that this glitch was caused by the electrostatic disruption of larger composite grains which rapidly charged up as they traversed the cometopause. The clear asymmetry in the distribution between the inbound and outbound portion of the spacecraft trajectory is also consistent with the dynamical effects of grain charging although other causes are not excluded.