Tail Rays Research Articles

The fountain model for plasma envelopes of comets

By regarding a comet as a source of CO molecules, we investigate the dynamics of the CO + envelopes of major plasma comets. Electron-collisional ionization within the envelopes is the main mechanism for plasma production. By analyzing the flow within it, a thin envelope is shown to be dynamically unstable at enhanced solar wind velocities, as long as the mean expansion velocity of the CO molecules is greater than 3 km/sec. An unstable envelope always contracts in towards the head; it also acts as the source of a sunward fountain of plasma which initiates a new outermost envelope. The material of each envelope flows into a number of tail rays, and the phenomenon of ‘closing-in’ rays is related to the decreasing pressure within the aging envelopes. The existence of an innermost stationary envelope, from which the main tail rays arise, is predicted. The varied and rapidly changing tail structure is largely due to the inherent instabilities of the ionization and plasma interaction mechanisms. The model is also applicable to smaller comets which neither possess distinct plasma envelopes nor exhibit fountain behaviour.

Angiostrongylus sandarsae sp. n. (Nematoda: Metastrongyloidea), a Lungworm of Rodents in Mozambique, East Africa

Angiostrongylus sandarsae sp. n., from the pulmonary arteries of Mastomys natalensis in Mozambique, possesses a gubernaculum and can be differentiated from the closely related A. gubernaculatus by the reduced length of the esophagus, spicules, female tail, and ventroventral bursal ray. The desirability of combing the genera Angiocaulus and Rodentocaulus with Angiostrongylus until more information is available, is discussed. The pathology associated with the presence of the parasite in the lungs is also discussed. In a study of lungworms among field rats in the vicinity of Beira, Mozambique, September 1965, 15 out of 27 Mastomys natalensis and one Gerbil tatera trapped, harbored an undescribed species of metastrongylid lungworm. Although this nematode possesses a guberaculum, it has many characters in common with members of the genus Angiostrongylus. For reasons which are indicated below, it appears justifiable at this time to assign this nematode to the genus Angiostrongylus until more is known of the congeneric characters of this genus and those of Angiocaulus and Rodentocaulus. The description of the new species is based on specimens collected from Mastomys only. Unless otherwise indicated, the measurements are in microns. The drawings were made with the aid of a camera lucida. The name is in honor of Dr. D. F. Sandars, who together with Dr. M. J. Mackerras, in 1955 described the obligatory migration of the larvae of Angiostrongylus cantonensis in the central nervous system of the rodent host. Angiostrongylus sandarsae sp. n. (Figs. 1-6)

Structure and kinematics of cometary type I tails

A summary of our knowledge concerning the structure and the changes in structure of Type I tails of comets is given. The study of the structure and the structural changes within the “heads” of the tails (coma region) rests nearly exclusively on photographic material of Comet Morehouse 1908. Eddington's paper of 1910 about this comet, based on about 150 Greenwich 30-inch reflector plates is critically reviewed by having this whole material available for a study. Most of Eddington's conclusions, which all support the old “fountain model”, are accepted. Öpik's new transverse force, which accelerates the ions slowly to the tail axis, is also accepted. The characteristic kinematical properties of the tail material are qualitatively and partly quantitatively clear. These properties are consistent only with a tail model in which the visible tail rays and streamers represent narrow jets within an otherwise highly diluted ion atmosphere.

The physics of cometary plasma

Abstract Assuming that plasma instabilities and possibly an enhanced interplanetary magnetic field ensure the fluid behaviour of cometary plasma, the production and flow of plasma in a carbon monoxide comet under the influence of the solar wind is investigated. The solar wind heats and compresses the cometary plasma. CO+ production occurs mainly through electron-collisional ionization of the CO, in a region of slowly moving plasma sunward of the comet nucleus. This region is determined as the position of balance of the solar and cometary plasma momentum fluxes and for various sizes of comets its distance lies in the range 2–50,000 km. Some 20–50 times further out, however, charge exchange processes (plus some photoionizations) become more important than electron collisions in causing ionization. Only the giant comets are efficient at ionizing the CO, and their plasma mostly occurs in envelopes at a distance of few times 104 km, whence it flows at high velocity into tail rays. In the lesser comets much of the plasma is cooled to energies of a few eV through the electron collisional ionizations and excitations of CO molecules. This plasma flows on through the head and, before it reaches the tail, promotes the dissociation of complex molecules. Here may be identified the so-called ‘active region’ or ‘false nucleus’.

Comet tail streamers in the solar wind

Transverse plasma instabilities are suggested as the basis of the mechanism which couples the motions of the solar wind and cometary plasma. In a plasma with a non-isotropic distribution of particle velocities, collective interaction of the charged particles can lead to electromagnetic instabilities. In a shearing flow these plasma instabilities draw energy from that of the shear and ensure a short time of free flight for diffusing ions. We derive a coefficient of viscosity for a mixed CO +, H + and electron plasma based on this mechanism. Using in addition the related transport coefficients of thermal diffusion and conductivity we describe the flow of a cometary streamer in the supersonic solar wind. Viscous heating of the cometary plasma is shown to be important and is taken into account in the derivation of a similarity solution for the steady flow of a straight streamer. The length of a typical streamer is predicted to be about one hundred times its width, showing that the stability of comet rays against diffusion can be explained without invoking a magnetic field. The hydrodynamic stability and extreme straightness of the tail rays are due to the hypersonic nature of the flow. Finally, the variation with distance of the density and velocity of ions in the tail rays is deduced, and is compared with observations.

The axes of the type I tails of comets

The axes of the type I tails of comets are defined by the directions (position angles at the nucleus) to which the tail rays are “closing in”. The axes are normally lagging behind the prolonged radius vector. It is shown that the directions of the tail axes are determined by the forces which regulate the outflow of the tail material from its source in the head. A direct influence of the “solar wind” on the position of the tail axes is not apparent.

Die ionisierung in den kometen

New arguments are added to those published earlier which support the view that the ionization of the molecules in comets is intrinsic to the cometary atmospheres themselves. The ionization occurs within a very limited region in front of the nucleus and the ions are expelled only within a small cone directed towards the Sun, and in the form of narrow rays. The outbursts have next a high velocity (estimated at 30 to 50 km/sec), which decreases after about 15 to 20 hours to a few km/sec. Each outburst leads to the formation of a shrinking parabolic envelope around the nucleus. The shrinking of the envelopes is accompanied by a closing-in of the tail rays to the tail axis.—The true mechanism of ionization (hich must be a kind of discharge) remains as yet obscure.