What do streamers in the dust tail tell about the cometary nucleus?

The interrelations between the physical parameters of a cometary nucleus, and the morphology of the dust tail and its streamers are systematically investigated by means of a model developed by Beißer (1990a), involving a rotating nucleus. The analysis of streamers in the tail, using direct modeling and synchrone grids, provides a suitable tool to deduce substantial informations on the nucleus' state of rotation. Opportunities and limitations of this analysis are discussed. Dust emission parameters like the distribution of active regions on the nucleus, or the emission characteristics can only be determined if certain other physical properties of the nucleus have been independently measured before.     

Dust tail of the active distant Comet C/2003 WT42 (LINEAR) studied with photometric and spectroscopic observations

We present the study of dust environment of dynamically new Comet C/2003 WT42 (LINEAR) based on spectroscopic and photometric observations. The comet was observed before and after the perihelion passage at heliocentric distances from 5.2 to 9.5 AU. Although the comet moved beyond the zone where water ice sublimation could be significant, its bright coma and extended dust tail evidenced the high level of physical activity. Afρ values exceeded 3000 cm likely reaching its maximum before the perihelion passage. At the same time, the spectrum of the comet did not reveal molecular emission features above the reflected continuum. Reddening of the continuum derived from the cometary spectrum is nonlinear along the dispersion with the steeper slop in the blue region. The pair of the blue and red continuum images was analyzed to estimate a color of the comet. The mean normalized reflectivity gradient derived from the innermost part of the cometary coma equals to 8% per 1000 Å that is typical for Oort cloud objects. However, the color map shows that the reddening of the cometary dust varies over the coma increasing to 15% per 1000 Å along the tail axis. The photometric images were fitted with a Monte Carlo model to construct the theoretical brightness distribution of the cometary coma and tail and to investigate the development of the cometary activity along the orbit. As the dust particles of distant comets are expected to be icy, we propose here the model, which describes the tail formation taking into account sublimation of grains along their orbits. The chemical composition and structure of these particles are assumed to correspond with Greenberg’s interstellar dust model of comet dust. All images were fitted with the close values of the model parameters. According to the results of the modeling, the physical activity of the comet is mainly determined by two active areas with outflows into the wide cones. The obliquity of the rotation axis of the nucleus equals to 20° relative to the comet’s orbital plane. The grains occupying the coma and tail are rather large amounting to 1 mm in size, with the exponential size distribution of a^−4.5. The outflow velocities of the dust particles vary from a few centimeters to tens of meters per second depending on their sizes. Our observations and the model findings evidence that the activity of the nucleus decreased sharply to a low-level phase at the end of April–beginning of May 2007. About 190 days later, in the first half of November 2007 the nucleus stopped any activity, however, the remnant tail did not disappear for more than 1.5 years at least.     

Viscous boundary layer between the solar wind and cometary plasmas

The interaction between the solar wind and cometary ionospheres downstream from the subsolar region is modeled in terms of viscous MHD flow theory. Calculations of the flow stremalines within the mixing region indicate that, as a result of viscous action, both the solar wind particles and the cometary material should be gradually directed towards the interior of the plasma wake to reinforce the formation of a type 1 tail. This behavior supports the notion that a transverse force acting on cometary plasma particles is actually responsible for the collapse of tail ray structures as suggested by Öpik (1964), Wurm (1968, 1975) and Wurm and Mammano (1972).     

Disconnection Events in Comets

Cometary and solar wind data are compared with the purposeof identifying the solar wind conditions which are associated with comet plasma tail disconnection events (DEs),i.e., when the plasma tail appears disconnected from the cometaryhead. The cometary data are fromThe International Halley Watch Atlas ofLarge-Scale Phenomena (Brandt et al.,1992a). A systematic visual analysis of the atlas images(Voelzke and Matsuura, 1998)revealed, among other morphological structures, 47 DEs alongthe plasma tail of comet P/Halley. This work compares the current competitive theories, based on the triggeringmechanisms, in order to explain the cyclic phenomena of DEs, i.e., the ion production effects, the pressure effects and themagnetic reconnection effects are analysed. The distribution of the DEs in time or heliocentricdistance presents abimodal character possibly associated with the cometary passage through the magnetic sector boundaries in the interplanetary medium.The 47 DEs documented in 47 different images allowed the estimation of 19 onsets of DEs, i.e., the time when the cometsupposedly crossed a frontier between magnetic sectors of the solar wind. The solar wind data are taken from in situ measurements of IMP-8 (King, 1982), which is used to construct the actual variation of solarwind speed, density and dynamic pressureduring the analysed interval. These in situ measurements arereferenced to the comet by standard co-rotationmethods. The preliminary results ofthis research reveal that the DEs onsetsof comet P/Halley are correlated with pressureeffects only in 23% of the analysed cases,therefore these effects should not be the principaltriggering mechanism of DEs.     

Physical characteristics of the plasma tail of comet C/2006 M4 (SWAN)

Wide-angle images with narrow-band filters isolating the radiation of CO⁺ ions and continuum of comet C/2006 M4 (SWAN) were acquired. The brightness distribution in the plasma tail shortly after the outburst of the comet on October 26, 2006, has been derived. The model brightness distribution was calculated, and the parameters of the diffusion model of the plasma tail have been obtained. For the date of observations, the magnetic-field induction in the plasma tail, as well as the lifetime of luminous particles and the lengthwise and transverse diffusion coefficients of ions have been estimated. Possible association between the change in the tail structure and that in the physical parameters along the plasma tail of the comet is discussed.     

On the hydromagnetic model of comets

In this paper we have developed several consequences of Alfvén's (1957) hydromagnetic model of comets. It is shown that such a model not only accounts for the observed morphology and time variations of the fine structure in the plasma tail, but also leads, in a natural way, towards explanations of two of the central problems in cometary physics; namely, the short ionization time-scales of the cometary molecules, and the large velocities and accelerations observed far down the tail.Paper dedicated to Professor Hannes Alfvén on the occasion of his 70th birthday, 30 May 1978.     

Acceleration mechanism of particles in the Type-I cometary plasma

In the paper, the accelerated effect of ions has been discussed. The transversal magnetic disturbance is able to bring about the magnetic annihilation and merge in some cometary area. The non-steady-state reconnection process can transform the magnetic energy of some cometary area into the kinetic energy of plasma. In addition, the two stream instability caused by both solar wind and cometary plasmas exists in Type-I tail, it can also lead the paticles to be accelerated and heated in the plasma tail.     

Dust tail streamers of a comet with rotating nucleus

Synthetic images of the dust tail are presented for a comet which has a rotating nucleus with one predominant dust source fixed to it. The images have been generated using a new computer model which, unlike similar models, allows for the study of dust tails caused by a rotating nucleus with an anisotropic distribution of sources.The dust tail is studied in the post-perihelion phase of a parabolic comet with a perihelion distance of 0.5 AU. One finds that in the case of a rotating nucleus with anisotropic emission characteristics streamers caused solely by the dynamics of the dust particles are forming in the dust tail even if there is no dependence between the solar irradiation angle of the source and the amount of dust emitted. If the dust emission depends on the solar irradiation angle of the dust source, then the brightest tail regions do not necessarily coincide with the synchrones for the times of maximum dust emission.As a consequence, a thorough analysis of streamer patterns in a cometary dust tail requires assumptions on the rotational state and the dust source distribution of the nucleus. Otherwise, it seems not possible to discern between streamers which are caused dynamically by nucleus rotation and others which reflect variations in the emission activity.     

Physical characteristics of the plasma tail of comet C/2009 R1 (McNaught)

We determined brightness distribution in the plasma tail of comet C/2009 R1 (McNaught) using observations with a small Newtonian reflector (200/1000) on June 9?C12, 2010. Images of the comet were detected using short exposures with a Canon CMOS APS-C camera. The brightness distribution is simulated and the parameters of the cometary plasma tail are obtained within the diffusion model. The magnetic field induction in the cometary tail, lifetime of light particles, and the lengthwise and transverse ion diffusion coefficients are estimated.     

Large cometary grains – their destruction and changes in the luminosity of comets

The catastrophic thermodynamic destruction of large cometary heterogeneous grains lying on the surface of a comet nucleus is examined. The core–mantle grain-structure model is assumed. Grain fragmentation as an explanation of sudden changes in cometary brightness is proposed. The approach presented to the problem of cometary outbursts is a development of a previous author's paper. The proposed mechanism is based on the idea of thermodynamical destruction of heterogeneous cometary grains. Numerical simulations have been carried out for a wide range of values of physical characteristics of cometary material. The results obtained are consistent with observational data. The main conclusion of this paper is that thermodynamical fragmentation of large grains can explain variations in brightness and also outbursts of comets.     

离子彗星的稳定性

介绍了彗星等离子体的结构和稳定性的研究情况，着重于离子彗尾，更着重于介绍在这方面国内的工作。     

哈雷彗星主尾流的研究

哈雷彗星在日彗距较大时出现长而直的主彗尾（尾流），这是很有趣的。尾流一般是指等离子体尾流；但是，当地球接近彗星轨道面时，尘埃尾流可能叠加到主彗尾上。在一般感光波段宽的彗星底片上很难区分这两种尾流。本文选取哈雷彗星在不同日彗距的5条主尾流，作了光度测量和比较分析。得出沿各尾轴及其垂直方向几个截面的亮度分布、亮度半极大全宽、尾轴的视风差角和真风差角及彗尾长度。在所分析的蓝敏底片上，过近日点前的2个尾流肯定是等离子体尾流，而5个尾流的相似性以及其他证据说明它们主要都是等离子体尾流，尘埃彗尾的污染是次要的。     

Simulation of cometary magnetic tails

In order to simulate a cometary tail in a laboratory the flow of hydrogen collisionless supersonic plasma with the magnetic field frozen in was used. The wax ball served as a model of the cometary nucleus. The experimental conditions met the principle of limiting simulation. Field lines enveloped the nucleus at the day side and stretched along the flow at the night side. Tension of field lines in the magnetic tail provided the acceleration of ionized products of wax evaporation up to about 10⁶ cm s^–1. The control experiments showed that the magnetic tail is caused by currents due to the Lorentz electric field.     

Instability of the cometary plasma tail—the instability in a plasma sheet

We consider the instability of the cometary plasma tail which is composed of a neutral sheet,two lobes of the ion tail and solar wind.The plasma is assumed to be highly conductive and incompressible.The unstable state yields a magnetic field which is perpendicular to the tail axis.Our result is consistent with findings about plasma from the International Cometary Explorer(ICE).     

The stability of the cometary plasma tail and rays

The stability of both the main cometary plasma tail and the tail rays is considered, taking into account the coupling between the plasma and the neutrals that flow out radially from the nucleus. It is shown that this coupling has a negligible effect on wave damping. Rather, we found that the neutral wind tends to destabilize the flanks of the main tail. On the other hand, the cometary rays are subject to both stabilizing and destabilizing effects because of the ion-neutrals drag. As a result, helical perturbations should become azimuthally asymmetric. Our study predicts that the folding rays may become wavy while approaching the tail axis, whereas they should remain straight far away from the tail axis.     

Multi-fluid model of comet 1P/Halley

A new three-dimensional magnetohydrodynamic model of the coma of a comet has been developed and applied to simulations of a Halley-class coma using the solar-wind conditions of the Giotto flyby of Halley in 1986. The code developed for high-performance parallel processing computers, combines the high spatial resolution of smaller than 1 km grid spacing near the nucleus, with a large computational domain that enables structures nearly 10 million km down the comet tail to be modeled. Ions, neutrals, and electrons are considered as separate interacting fluids. Significant physical processes treated by the model include both photo and electron impact ionization of neutrals, recombination of ions, charge exchange between solar-wind ions and cometary neutrals, and frictional interactions between the three fluids considered in the model. A variety of plasma structures and physical parameters that are the output of this model are compared with relevant Giotto data from the 1986 Halley flyby.     

Nucleus and tail studies of comet P/Swift-tuttle

CCD images of comet P/Swift-Tuttle, obtained in April 1994 with the 2.2m telescope at ESO La Silla/Chile, showed a comaless stellar nucleus. From absolute photometry we estimated the equivalent radius of the cometary nucleus to be about 11 km (assuming an albedo of 0.04 as for P/Halley) for two rotation phase angles which differ by about 75 deg. From that we conclude that the nucleus is either of rather spherical shape or that the viewing geometry was almost pole-on during our observations.An analysis of the plasma tail and inner coma of the comet by means of photographic plates and CCD images through IHW and BVR filters, obtained with the 80cm Schmidt camera and the 1.2m telescope at Calar Alto/Spain in November 1992, revealed several tail rays, head streamers and substructures in brightness excess areas in the coma. While some of the tail rays extended to several million km nuclear distance, most of them can be traced to starting points which lie in a region just 20000–35000 km projected distance tailward from the nucleus.     

The destruction of cometary grains and changes in the luminosity of comets

One explanation of the sudden changes in the brightness of comets is proposed based on the author's earlier suggestions involving the fragmentation of cometary grains. Within the inner coma, a core‐mantle model of the structure of grains is assumed. The proposed mechanism is a combination of electrostatic stress and thermodynamical fragmentation of the cometary grains water‐ice mantle. It has been shown that the vapour pressure of volatile inclusions placed in the waterice mantle of grains can increase sufficiently to cause their fragmentation. It takes place before grains can completely sublime into the vacuum away. Numerical calculations have been carried out for a large range of values of probable physical characteristics of cometary material. The proposed approach yields increases in cometary brightness consistent with observations of typical cometary outbursts. It is concluded that this approach can provide an explanation of the sudden change in activity of comets for a wide range of heliocentric distances (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Wide Field Imaging of Ion Tail of Comet C/Hale–Bopp

A sequential imaging observation of the ion tail of Comet C/Hale-Bopp 1995O1 was carried out in February–March 1997 with a wide-field CCD imaging camera using narrow band filters for two ion species; CO⁺ and H₂O⁺ along with those for blue and red continuum. From the surface photometry of the ion tail of two species, we derived a relationship between plasma density and distance from the nucleus. The local velocity of the ion flow as a function of the distance from the nucleus was also estimated on the basis of some assumptions. We report preliminary results of our analysis, and discuss some characteristics of cometary plasma and its interaction with interplanetary magnetic field (IMF). May the source be with you! This revised version was published online in July 2006 with corrections to the Cover Date.