Cometary magnetospheres: a tutorial

The nucleus of an active comet, such as comet Halley near its perihelion, produces large quantities of gas and dust. The resulting cometary atmosphere, or coma, extends more than a million kilometers into space, where it interacts with the solar wind. An “induced” cometary magnetosphere is a consequence of this interaction. Cometary ion pick-up and mass loading of the solar wind starts to take place at very large cometocentric distances. Eventually this mass loading leads to the formation of a weak cometary bow shock. Even closer to the nucleus, collisional processes, such as ion-neutral chemistry, become important. Other features of the magnetosphere of an active comet include a magnetic barrier, a magnetotail, and a diamagnetic cavity near the nucleus. X-ray emission from comets is produced by the interaction of the solar wind with cometary neutrals and this topic is also discussed. A broad review of the cometary magnetosphere will be given in this paper.     

Cometary constraints on the planet forming environment.

Molecular elemental and isotopic abundances of comets provide sensitive diagnostics for models of the primitive solar nebula. New measurements of the N2, NH and NH2 abundances in comets together with the in situ Giotto mass spectrometer and dust analyzer data provide new constraints for models of the comet forming environment in the solar nebula. An inventory of nitrogen-containing species in comet Halley indicates that NH3 and CN are the dominant N carriers observed in the coma gas. The elemental nitrogen abundance in the gas component of the coma is found to be depleted by a factor approximately 75 relative to the solar photosphere. Combined with the Giotto dust analyzer results for the coma dust component, we find for comet Halley Ngas + dust approximately 1/6 the solar value. The measurement of the CN carbon isotope ratio from the bulk coma gas and dust in comet Halley indicates a significantly lower value, 12C/13C = 65 +/- 9 than the solar system value of 89 +/- 2. Because the dominant CN carrier species in comets remains unidentified, it is not yet possible to attribute the low isotope ratio predominantly to the bulk gas or dust components. The large chemical and isotopic inhomogeneities discovered in the Halley dust particles on 1 mu scales are indicative of preserved circumstellar grains which survived processing in the interstellar clouds, and may be related to the presolar silicon carbide, diamond and graphite grains recently discovered in carbonaceous chondrites. Less than 0.1% of the bulk mass in the primitive meteorites studied consists of these cosmically important grains. A larger mass fraction (approximately 5%) of chemically heterogeneous organic grains is found in the nucleus of comet Halley. The isotopic anomalies discovered in the PUMA 1 Giotto data in comet Halley are probably also attributable to preserved circumstellar grains. Thus the extent of grain processing in the interstellar environment is much less than predicted by interstellar grain models, and a significant fraction of comet nuclei (approximately 5%) may be in the form of preserved circumstellar matter. Comet nuclei probably formed in much more benign environments than primitive meteorites.     

Scientific instrumentation of PLANET-A VUV imaging of the hydrogen coma of Halley

The PLANET-A spacecraft to fly by Comet Halley is equipped with a VUV imaging camera which will take pictures of the hydrogen coma of the comet. The camera is composed of a telescopic mirror lens, a VUV image intensifier, two dimensional CCD, and controlling electronic circuits with a microprocessor. In order to eliminate the blur in the image due to the spinning motion of the spacecraft, a special technique called “spinsynchronized charge swift” is used in the CCD driving.     

Consequences of cometary aurora on the carbon chemistry at comet P/Halley.

Various experimental data acquired during the visit of Halley's comet in 1986 have shown that the amount of carbon produced due to photodissociation of parent carbon bearing species is not ample enough to explain the observations. This requires the presence of an additional source of atomic carbon. One of the possible source could be auroral-type activities resulting from the precipitation of high-energy "auroral electrons" of solar wind origin, the evidence of which have been inferred from many observations at comet Halley. We have developed a coupled chemistry-transport model to study the role of auroral and photoelectron impact as well as of chemistry on the modelling of carbon in the inner coma (< or = 10(4) km) of comet Halley. Our study suggest that electron impact dissociation of CO is the major source of carbon production in the inner coma, not the recombination of CO+ as suggested by earlier workers, while transport is the main loss process.     

Thermal emission from cometary dust

The thermal emission from the dust coma of a comet can be analyzed to yield the flux and size distribution of the dust grains and the relative abundance of silicate and absorbing grains.     

Numerical simulations of the radiance from the comet 46P/Wirtanen in the various configurations of the measurements during “Rosetta” mission

The work we present deals with the spectrometric measurements of VIRTIS instrument of the Comet P/Wirtanen planned for the Rosetta mission. This spectrometer can monitor (VIRTIS M channel: 0.250μm – 0.980μm; Δκ=20cm⁻¹; 0.980 – 5.0 μm; Δκ = 5cm⁻¹; VIRTIS H channel: 2.0 μm – 5.0 μm; Δκ=2cm⁻¹) the nucleus and the coma in order to provide a general picture of coma's composition, the production of gas and dust, the relationship of coma production to surface composition and the structure and variation of mineralogy of the nucleus surface. During the mission the observation conditions of the spectroscopic investigation change due to different relative positions spacecraft/comet, and to the different illumination conditions of the surface at various distances of the comet to the Sun. The nucleus surface is continuously modified by the ice sublimation accompanied by gas and dust emission. Consequently the surface also its spectrophotometric properties changes and their monitoring can give a new insight. The important role of simulations is to predict the results of measurements in various experimental condition what, in the future, can help in interpretation of the measured data.

In this paper the first results of our simulation the radiance from the comet in the 0.25–5.0μm spectral range at two distances from the Sun (1AU and 3AU) are shown. The distance between the Rosetta orbiter and the nucleus surface as well as the sun zenith angles are taken into account according to the Rosetta mission phases. In fact the surface and coma properties vary along the comet orbit, and should be taken into account in our calculations. The optical parameters of the dust on the surface (e.g. reflectance) and in the coma (e.g. Q_ext) were calculated from optical constants of possible comet analogues. The thermodynamic parameters of the comet are taken from the models of comet evolution. Through this kind of modelling it is possible to identify the surface characteristics in spectra of the radiation from the surface of nucleus transmitted through the coma loaded with dust and gases.

Even if the “Rosetta mission” is postponed, with the consequence of a target change, we think that our idea and the method used for the simulations can be useful also for the new Rosetta target - the comet 67P/Churyumov Gerasimenko.     

Solar wind interaction with comet Halley

In March 6 and 9, 1986 the spacecrafts ‘Vega-1’ and ‘Vega-2’ have flown through the coma of comet Halley and have carried measurements of plasma, energetic particles, magnetic field and plasma waves along its trajectory. A short review of these measurements and its comparison with theoretical models of solar wind interaction with comets are given.

The spacecrafts ‘Vega-1’ and ‘Vega-2’ have studied the solar wind loading by cometary ions, the structure of cometary bow shock and the processes in the inner coma of comet Halley. Exactly in this sequence we discuss the results of measurements and compare them with the theory.     

The instrument IKS and its calibration

The IKS infrared spectro-photometer will fly on board the VEGA platforms. It is designed to characterize the size, temperature and emissivity of the Comet Halley nucleus, to identify the major gaseous components of the inner coma and to detect the emission of the cometary grains. This paper presents the “calibration” experiments required to reduce the raw data: (i) absolute wavelength calibration of the filter wheels; (ii) modeling of the internal signal, as a function of the temperature of the different sub-systems; (iii) absolute and spectral responsivities of each of the spectrometric and photometric channels, as a function of the wavelength and position of the source in the field of view. Finally, we shall indicate the expected S/N ratios.     

The Comet Halley flyby I.R. sounder “I.K.S.”

An infrared sounder is being developed in France to observe in 1986 Comet Halley from the Soviet “VEGA” flyby probes. The instrument, called “I.K.S.”, has three measuring channels. Two of these channels will provide the spectrum of the comet emission in the spectral intervals 2.5–5.0 μ and 6–12 μ, at a constant resolution λ/Δλ = 50.The third channel analyzes the comet I.R. image at a spatial frequency of about 1 arc minute^?1; two I.R. colours are used in this channel: 7–10 μ and 10–14 μ. From the results expected, it is hoped that (1) most primary simple molecules emitted by the nucleus will be identified; (2) the chemical composition and perhaps crystalline structure of the dust grains and ices released by the comet will be derived; and (3) the diameter of the nucleus and its brightness temperatures will be measured.     

Current status and scientific capabilities of the Rosetta lander payload

ESA's cornerstone mission “ROSETTA” to comet 46P/Wirtanen will bring a 100 kg Lander (provided by an international European consortium) with a scientific payload of about 27 kg to the surface of the comet's nucleus. After a first scientific sequence it will operate for a considerable fraction of the cometary orbit around the sun (between 3 AU and 2 AU). The Lander is an autonomous spacecraft, powered with solar cells and using the ROSETTA Orbiter as a telemetry relais to Earth. The main scientific objectives are the in-situ investigation of the chemical, elemental, isotopic and mineralogical composition of the comet, study of the physical properties of the surface material, analyze the internal structure of the nucleus, observe temporal variations (day/night cycle, approach to sun), study the relationship between the comet and the interplanetary matter and provide ground reference data for Orbiter instruments.Ten experiments with a number of sub-experiments are foreseen to fulfil these objectives.In this paper we present the current status of the instrumental development and the scientific capabilities of each of the experiments.     

Chemical interactions of solar wind with cometary nuclei and comae

Solar wind particles, especially H, C, N, O, S, and P-ions, may undergo specific chemical reactions with gaseous or solid matter of comets when in the energy region of a few 10 to some eV. Each component of the solar wind, even if not chemically reactive itself, creates a multiplicity of energetic secondary particles by knock-on processes with the cometary matter. These are responsible for the majority of the so called “hot” chemical processes. Endothermic reactions with high activation energy and atom molecule interactions are possible and may add to the classical exothermic ion-molecule or radical reactions. Other sources of hot atoms or ions in comets are: cosmic rays, acceleration or pick-up processes and turbulences in comae and gas or dust tails, and photon absorption induced dissociation. The products of hot chemical reactions, short period comets experience on their orbits, add to those formed in the individual component ice or dust grains by strong fluxes of energetic particles in times prior to the accretion to a comet.     

The plasma regime at comet Giacobini-Zinner

This review of the plasma regime sampled by the encounter of the International Cometary Explorer spacecraft (ICE) with the comet Giacobini-Zinner, discusses the shock, or bow wave, ion pickup, ionization mechanisms, and the cometary plasma tail.

The observations are consistent with the existence of a weak shock, which may be pulsating, but do not exclude the suggestion by Wallis and Dryer that the shock, though present around the sub-solar point, is in process of decaying to a wave on the flanks.

Pickup of cometary ions provokes, by means of several mechanisms, ion cyclotron, mirror, beam and electrostatic instabilities which cause strong turbulence in the inner coma, as indicated in the power spectra of the magnetic field in the coma and the surrounding volume. Heavy mass loading and consequent slowing down of the solar wind is observed. Acceleration of ions by a stochastic mechanism is indicated.

Ionization of cometary neutrals occurs principally by photoionization and charge exchange. Alfvens critical velocity mechanism, likely operates only in the inner coma not visited by ICE. A steep increase of nearly two orders of magnitude in electron density occurs in the tail, where electron velocity distributions show evidence of entry of electrons from the solar wind. The turbulence there is damped by the high ion density and low temperature.

In general, the vicinity of the comet is filled with plasma phenomena and a rich variety of corresponding atomic and molecular processes can be studied there. Comparison between the ICE, Giotto, and Vega observations forms a most valuable future study.     

Planet-A mission to Halley

Looking at the chance of the next apparition of the Halley comet in 1986, ISAS decided to send a first Japasanese interplanetary spacecraft for the study of cometary hydrogen coma and solar wind. The Planet-A spacecraft which carries VUV imaging camera and solar wind plasma analyser will be launched in August 1985 and flyby the Halley comet in early March 1986 with the distance of several million kilometers from the comet nucleus. This mission is not only self-consistent but collaborative with other space mission as well as earth-bound observations. In the present paper, the Planet-A mission to Halley is described with brief explanation of the spacecraft.     

In-situ photopolarimetric measurements of dust and gas in the coma of Halley''s comet

The Halley Optical Probe Experiment (HOPE) on board the Giotto spacecraft has provided the first in-situ measurements, both of the dust and of some gaseous species, from inside the coma of the comet. The instrument has already been described /1/, together with first results /2/. The purpose of this note is to show how optical measurements can lead to in-situ information, how those were obtained during the 13–14 march 1986 Halley fly-by, and what is the status of the data analysis.     

Analysis of the nucleus and circumnuclear area of comet Halley with the “I.K.S.” infrared sounder from the “Vega” flyby probes

The “Vega” Soviet flyby probes to comet Halley will carry a French infrared sounder, called “I.K.S.”. In order to assess its observing capabilities, a theoretical model of the comet infrared emission was constructed. We show how the experiment results will be used to derive the nucleus size and radiative properties, and to study the distribution of gas and dust in the inner coma and circumnuclear area. A preliminary discussion is made of the relevance of the data in instances where the cometary phenomena would be more complex than assumed in the model.     

Coma morphology and dust emission pattern of comet Halley

This contribution starts with a short overview on cometary dust modelling and then focuses on the application of coma modelling with respect to in-situ measurements of cometary dust and ground based observations. The fountain model, valid for the dynamics of small cometary dust particles, is discussed. Models using Keplarian theory for the motion of the dust particles are outlined and the ESOC coma model is presented. Some direct applications of this model to analyse the results of the recent spacecraft flybys of comet Halley, as dust flux profiles, particle ground tracks and envelope positions, are shown. To compare the model with ground-based astronomical observations, the utilization of the ESOC coma model for the generation of synthetic images is demonstrated and some future prospects of this technique are outlined.     

An overview of gas phenomena in comet Halley

Ground-based observations and in-situ measurements of the gas activities and coma structures of comet Halley have yielded many new insights to the expansion of atmospheric gas from the central nucleus. The first impression from these results is discussed in terms of present theoretical understanding of the different gas phenomena in the inner coma (r 10⁴ km). The need for developing a new generation of theoretical models capable of describing the observed highly anisotropic coma activities with spin-modulation in the outgassing rate as well as short-term outbursts is stressed.     

Hydrogen coma of comet P/Halley

The main molecular processes to produce the hydrogen comae of comets are now well known: Water, the main constituent of cometary atmospheres, is photodissociated by the solar ultraviolet radiation to form the high (20 km s⁻¹) and low (8 km s⁻¹) velocity components of the atomic hydrogen. The hydrogen clouds of various fresh comets have been observed in 1216Å by a number of spacecrafts. Ultraviolet observations of short period comets are, however, rather rare. Consequently Comet P/Halley in this apparition is a good object to obtain new physics of the hydrogen coma. Strong breathing of the hydrogen coma of this comet found by “Suisei” provides just such an example. The rotational period of Comet Halley's nucleus, its activity in the form of outbursts alone, and the position of jet sources etc. are determined from the breathing phenomena. Atomic hydrogen from organic compounds with a velocity of 11 km s⁻¹ play an important role in that analysis. The time variations of the water production rate of Comet Halley during this apparition observed by various spacecrafts appear to be in agreement with each other and are about 1.5–2 times larger than the standard model. The difficulty of the calibration problem was emphasized.     

A preliminary consideration of the electron impact ionization effect in cometary comas

From an analysis of the electron number densities and velocity distributions as observed at comet Giacobini-Zinner and comet Halley, it appears that in the so-called “transition region” of the coma electron impact ionization can exceed the nominal photoionization rate of H₂O molecules but not by an order of magnitude. It is possible in localized regions where electron heating by ion acoustic waves and lower hybrid waves occurs, electron impact would become the dominant ionization (and dissociation) mechanism. The overall effect as limited by in-situ measurements of the neutral gas density distribution could be shown to be small, however.     

EUV observations of Comet Halley

The observation of EUV emissions of comet Halley and its plasma-gas environment by means of rocket- or satellite-borne resonance absorption cell spectrophotometer devices is planned. The technical outlay of the payload, the estimated EUV intensities, and the scientific objectives of this mission are presented. Due to complete suppression of the geocoronal He I emissions by He I resonance absorption cells, a quantitative identification of the cometary object in the He I 58.4 nm line is possible, if the He/H abundance ratio in the evaporating cometary matter is higher than 4.0 E-4.