Giotto Observations Research Articles

Magnetic pileup boundary and field draping at Comet Halley

The approach of the Rosetta S/C to Comet Churyumov–Gerasimenko in 2014 reactivates the interest in the plasma interaction of the solar wind with the cometary coma. In preparation for the upcoming S/C observations and the start of outgassing of the cometary nucleus, we reinvestigate the magnetic field data from the Vega-1 S/C at the flyby of Comet Halley (1986), in search of the magnetic pileup boundary and increase of field line draping. The magnetic pileup boundary has been identified as a common feature for unmagnetized bodies with an induced magnetosphere. This boundary marks the outer edge of the magnetic pileup region, also known as the magnetic barrier region, in which the magnetic field is strong and highly draped. Initially, the magnetic field draping around Comet Halley was clearly identified from the Vega-1 magnetometer data through reversal of the field component in direction to the Sun at closest approach.Here, a detailed analysis is performed in regions further upstream in the magnetosheath. The Vega-1 high resolution magnetometer data on the in- and outbound leg but inside the bow wave are reinvestigated in search for the magnetic pileup boundary as an indicator for the outer edge of the magnetic barrier. The magnetic field pileup region is studied using the correlation between the field component towards the Sun and the radial component in an aberrated cometocentric frame; this technique proved very successful for Mars and also for comets Giacobini–Zinner and Halley in the case of Giotto observations. We can clearly identify the different regimes in the magnetic field data, on the in- and outbound leg of the orbit. Waves just within the newly determined magnetic pileup region have properties different from mirror mode waves, whereas waves observed out of the magnetic pileup boundary are confirmed as mirror mode. The boundaries found at Comet Halley prove that also the detailed structure of the interaction of unmagnetized bodies with an atmosphere with the solar wind is valid for active comets, but with larger space scale.

COMET 1P/HALLEY MULTIFLUID MHD MODEL FOR THE GIOTTO FLY-BY

The interaction of comets with the solar wind has been the focus of many studies including numerical modeling. We compare the results of our multifluid MHD simulation of comet 1P/Halley to data obtained during the flyby of the European Space Agency's Giotto spacecraft in 1986. The model solves the full set of MHD equations for the individual fluids representing the solar wind protons, the cometary light and heavy ions, and the electrons. The mass loading, charge-exchange, dissociative ion-electron recombination, and collisional interactions between the fluids are taken into account. The computational domain spans over several million kilometers, and the close vicinity of the comet is resolved to the details of the magnetic cavity. The model is validated by comparison to the corresponding Giotto observations obtained by the Ion Mass Spectrometer, the Neutral Mass Spectrometer, the Giotto magnetometer experiment, and the Johnstone Plasma Analyzer instrument. The model shows the formation of the bow shock, the ion pile-up, and the diamagnetic cavity and is able to reproduce the observed temperature differences between the pick-up ion populations and the solar wind protons. We give an overview of the global interaction of the comet with the solar wind and then show the effects of the Lorentz force interaction between the different plasma populations.

New experimental results on the degradation of polyoxymethylene: Application to the origin of the formaldehyde extended source in comets

The formaldehyde (H2CO) observed in cometary atmospheres presents a so‐called “extended source,” meaning that its distribution in the coma cannot be explained by sublimation from the nucleus alone; a production inside the coma has to be included. Polyoxymethylene (formaldehyde polymers: (‐CH2‐O‐)n, also called POM) is sometimes evoked as a parent molecule for this extended source. This solid polymer on cometary grains could release gaseous formaldehyde through thermal and photolytic degradation. We have developed an experimental program in order to study the chemical reactions of degradation of POM by UV photolysis and heating. It provides identification of the degradation products and the determination of the photo degradation quantum yields or the thermal degradation kinetics. In this paper we present the improvements of our experimental setup and new measurements on the kinetics of gaseous formaldehyde production by thermal degradation of two types of commercial POM on a greater temperature range than the previous ones. These new data are then included in a model of the outer coma in order to interpret observations. This model takes into account the production of gaseous molecules from solid material present on grains and leads to a very good agreement with Giotto observations of comet 1P/Halley, if we assume that the cometary grains contain 1–16% of POM by mass. Thus, without being final evidence of this polymer presence on comets, we conclude that the degradation of solid POM‐like polymers on cometary grains is to date the best explanation of the H2CO extended source.

Polyoxymethylene as Parent Molecule for the Formaldehyde Extended Source in Comet Halley

Among unsolved questions raised by observations of comets is the origin of extended sources, i.e., the distribution of molecules in the coma which cannot be explained by a direct sublimation from the nucleus. Polyoxymethylene [formaldehyde polymer: also called POM] is sometimes ([CH 2 -O[) n , invoked as a potential parent compound, the degradation of which could produce the required amount of across the coma, but no quantitative study has ever been undertaken with relevant parameters. H 2 CO From new experimental data, we are now able to consider multiphase chemistry: POM in the solid state on cometary grains slowly degrades by solar photons and heat and produces in the gaseous H 2 CO phase. This is a new approach to cometary organic chemistry. We show, by considering simple assumptions about the cometary environment, that the hypothesis of POM on grains leads to a very good agreement with Giotto observations if we assume that the cometary grains are D7% POM by mass at a temperature of 330 K.

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Absolute measurements have been made of single-electron charge-exchange cross sections of H+, He+, and He2+ in H2O and CO2 in the energy range 0.3-7.5 keV amu-1. Collisions of this type occur in the interaction of solar wind ions with cometary gases and have been observed by the Giotto spacecraft using the Ion Mass Spectrometer/High Energy Range Spectrometer (IMS/HERS) during a close encounter with comet Halley in 1986. Increases in the He+ ion density, and in the He2+ to H+ density ratio were reported by Shelley et al. and Fuselier et al. and were explained by charge exchange. However, the lack of reliable cross sections for this process made interpretation of the data difficult. New cross sections are presented and discussed in relation to the Giotto observations.

Laboratory simulation of the physical processes occurring on and near the surfaces of comet nuclei

Abstract— Laboratory comet simulation experiments are discussed in the context of theoretical models and recent ground‐based and spacecraft observations, especially the Giotto observations of P/Halley. The set‐up of various comet simulation experiments is reviewed. A number of small‐scale experiments have been performed in many laboratories since the early 1960s. However, the largest and most ambitious series of experiments were the comet simulation experiments known as KOSI (German = Kometen Simulation). These experiments were prompted by the appearance of Comet P/Halley in 1986 and in planning for the European Space Agency's Rossetta mission that was originally scheduled to return samples. They were performed between 1987 and 1993 using the German Space Agency's (DLR) space hardware testing facilities in Cologne. As with attempts to reproduce any natural phenomenon in the laboratory, there are deficiencies in such experiments while there are major new insights to be gained. Simulation experiments have enabled the development of methods for making comet analogues and for exploring the properties of such materials in detail. These experiments have provided new insights into the morphology and physical behavior of aggregates formed from silicate grains likely to exist in comets. Formation of a dust mantle on the surfaces and a system of ice layers below the mantle caused by chemical differentiation have been identified after the insolation of the artificial comet. The mechanisms for heat transfer between the comet's surface and its interior, the associated gas diffusion from the interior of the surface, and compositional, structural, and isotopic changes that occur near the surface have been described by modeling the experimental results. The mechanisms of the ejection of dust and ice grains from the surface and the importance of gas‐drag in propelling grains have also been explored.

A new look at the nature of comet Halley's LF electromagnetic waves: Giotto observations

All of the comet Halley high‐time resolution magnetic field data have been examined to determine the nature of the “turbulence” and its difference from that of comets Giacobini‐Zinner and Grigg‐Skjellerup. Although much of the wave appears unpolarized, occasionally there are intervals of clear order. We find several interesting new wave polarizations: arc‐, left‐hand/arc‐ (sunglass) and left‐hand circular polarized waves (in the spacecraft frame). The former two types have separations of ∼120 s between individual pulses, indicating that the waves are generated from pickup of the H2O group cometary ions. The third type of waves occurs in a wave‐train and may be a detached whistler packet. The unusual polarizations could be caused by wave refraction in the highly turbulent (and high β) Halley environment or by nonlinear evolution due to strong growth rates. It is noted that some of the large amplitude waves are non‐planar. These results are further details of the “linear polarization” of Halley waves determined by previous coherency analyses, and may explain some of the evolution leading to its plasma turbulence.

Three‐dimensional multiscale MHD model of cometary plasma environments

First results of a three‐dimensional multiscale MHD model of the interaction of an expanding cometary atmosphere with the magnetized solar wind are presented. The model starts with a supersonic and super‐Alfvènic solar wind far upstream of the comet (25 Gm upstream of the nucleus) with arbitrary interplanetary magnetic field orientation. The solar wind is continuously mass loaded with cometary ions originating from a 10‐km size nucleus. The effects of photoionization, electron impact ionization, recombination, and ion‐neutral frictional drag are taken into account in the model. The governing equations are solved on an adaptively refined unstructured Cartesian grid using our new multiscale upwind scalar conservation laws‐type numerical technique (MUSCL). We have named this the multiscale adaptive upwind scheme for MHD (MAUS‐MHD). The combination of the adaptive refinement with the MUSCL‐scheme allows the entire cometary atmosphere to be modeled, while still resolving both the shock and the diamagnetic cavity of the comet. The main findings are the following: (1) Mass loading decelerates the solar wind flow upstream of the weak cometary shock wave (M ≈ 2, MA ≈ 2), which forms at a subsolar standoff distance of about 0.35 Gm. (2) A cometary plasma cavity is formed at around 3 × 103 km from the nucleus. Inside this cavity the plasma expands outward due to the frictional interaction between ions and neutrals. On the nightside this plasma cavity considerably narrows and a relatively fast and dense cometary plasma beam is ejected into the tail. (3) Inside the plasma cavity a teardrop‐shaped inner shock is formed, which is terminated by a Mach disk on the nightside. Only the region inside the inner shock is the “true” diamagnetic cavity. (4) The model predicts four distinct current systems in the inner coma: the density peak current, the cavity boundary current, the inner shock current, and finally the cross‐tail current. (5) The calculated plasma parameters (magnetic field, plasma density, speed, and temperature) are in very good agreement with published Giotto observations.

Observation and interpretation of cometary low frequency waves

The first observations of cometary wave activity were carried out in 1985/1986 by several space missions (ICE, VEGAs 1 and 2, Suisei, Sakigake, Giotto) in the environments of comets Giacobini-Zinner and Halley. The interpretation of thesein situ field (and particle) measurements fostered investigations on (among other topics) wave generation that, leaving aside the inherently nonlinear (but related) problem of the eventual formation of a cometary bow shock wave, explored the free energy available in two specific features of the velocity distributions of the newborn particle populations: their parallel (with respect to the IMF direction) drift in the solar wind frame and perpendicular ring-like organization. Analytical and simulation works looked into the influence of the solar wind and cometary newborn parameters on the instabilities and the ensuing, or associated (as evidenced by wave observations), nonlinear phenomenology. Comprehensive reviews have described the experimental and theoretical results obtained in this cometary wave research until 1992 and identified outstanding problems warranting further attention. Here, only a cursory revisit to the Giacobini-Zinner/Halley era of low frequency wave observation and interpretation shall be made: rather, attention shall be predominantly focussed on the new implications to cometary wave research of the recent Giotto encounter with comet Grigg-Skjellerup on July 10 of 1992. The three visited comets, starting with their gas production rates, had different characteristics that showed up in thein situ observations. Yet, with the important exception of the Grigg-Skjellerup encounter, the interpretation of the wave activity measurements could be made in terms of common basic generation mechanisms adapted to the relevant properties of the appropriate plasma environment. New aspects emerged in the last Giotto cometary mission: the smaller gas production rates yield a scale length for the neutral gas density that is not (much) larger than the gyration distance of a heavy newborn ion (estimated by the product of the solar wind speed and the ion cyclotron period). As a consequence of this inhomogeneity, the velocity distribution of the heavy newborn ions exhibits gyrophase organization, i.e. nongyrotropy. This new source of free energy, albeit briefly mentioned in a few studies preceding the Grigg-Skjellerup mission, was not investigated in the context of the Giacobini-Zinner and Halley encounters. Since the last Giotto observations strongly suggest that nongyrotropy plays a prominent role in wave generation as the comet Gigg-Skjellerup nucleus is approached and its stability characteristics have only seldomly been analyzed, the review shall emphasize the wave generation capabilities of particle populations with gyrophase organization.

Comment on “Comparison of observed and calculated implanted ion distributions outside comet Halley's bow shock” by T. I. Gombosi, M. Neugebauer, A. D. Johnstone, A. J. Coates, and D. E. Huddleston

It is argued that the formulation used by Gombosi (1991) does not distinguish between two fundamental types of resonant interactions (those between cometary pickup ions and LF electromagnetic waves). Two problems arise from using their overly simplified expressions. The wave polarization of the solar wind Alfven wave is not purely right-handed, and values from alpha sub com and alpha sub sw are not well known, nor are the 'polarizations' of the 'backward'-going waves in either case. Gombosi et al. discuss the antisunward vs the sunward propagating solar wind Alfven wave, and assume a reasonable value of 0.2 for alpha sub sw. However, alpha sub com is totally unknown. It is concluded that a more correct analysis of wave polarization can lead to significant changes in the formulation and results from Gombosi et al. In his reply Gombosi argues that his simplified wave-particle interaction model is consistent with Giotto observations and plasma kinetic simulations and it leads to a reasonable approximation of the implanted ion transport coefficients. The more complicated approach suggested by Tsurutani & Thorne (1993) requires the introduction of several ad hoc parameters and it does not lead to any change of the energy diffusion coefficient.

Pickup protons and water ions at comet Halley: Comparisons with Giotto observations

A quasi‐linear diffusion model including both pitch angle and energy diffusion, adiabatic compression and convective motion with the solar wind flow has been used to investigate the cometary ion pickup process along the Sun‐comet line at comet Halley. The total pickup ion densities and magnetic turbulence spectrum levels observed by Giotto were used to constrain the quasi‐linear model. Comparisons of the model results were made with energetic ion distributions observed by instruments onboard the Giotto spacecraft. The observed power spectrum index of magnetic turbulence γ is about 2–2.5. However, our simulation shows that when γ was 2, the calculated proton distributions were much more isotropic than the observed ones; hence we have chosen γ = 2.5 in our study. Furthermore, we assumed that only about 5% of the total low frequency wave power propagates away from the comet. The numerical solutions of the quasi‐linear diffusion equations show that the isotropization of the pickup ion distribution, particularly at the pickup velocity, is not complete even close to the bow shock (but upstream), which agrees with the observations. The calculated and observed proton distributions are somewhat more isotropic than the water group ion distributions in the region far upstream of the shock, but the opposite is true near the bow shock. We find that given the observed turbulence level, quasi‐linear theory gives pickup ion energy distributions that agree with the observed ones quite well and easily produces energetic ions with energies up to hundreds of keV.

Quasi‐linear velocity space diffusion of heavy cometary pickup ions on bispherical diffusion characteristics

We consider an ion transport equation describing the source, convection, adiabatic acceleration and quasi‐linear velocity diffusion of cometary pickup ions. The equation is solved numerically along solar wind plasma flow lines for the environment of comet Halley, to obtain distributions at positions on the Giotto spacecraft trajectory which may be compared with observations. We use realistic input models for the observed spectrum of upstream and downstream propagating wave turbulence and for the ion source and mass loading of the solar wind. The ion resonance condition is not approximated in the diffusion term and we obtain full two‐dimensional (pitch angle and velocity) numerical distributions which show pitch angle scattering about the wave scattering centers at ± VA where VA is the Alfvén wave speed in the solar wind frame. Peak ion phase‐space densities approximately follow the bispherical shell geometry. The energy distributions F(υ) agree well with Giotto observations at lower υ but in the high‐energy tail there is evidence possibly for the first‐order Fermi acceleration mechanism taking place in the comet Halley foreshock region.

Modeling Dust Fragmentation in Comets

AbstractWe developed a 1-D hydrodynamic model of dusty gas flow with dust fragmentation in a cometary atmosphere and performed calculations for a dust-size distribution with radiia =10−4— 10 cm and densities variable with dust size. A comparison was made with Giotto observations of dust jet intensities within 100 km of the nucleus of Comet Halley. We found that dust fragmentation cannot be solely responsible for the flattening of the dust intensity near the nucleus with respect to the 1/R law. We conclude that a combination of geometric effects and grain fragmentation may explain the observed intensity profiles.

Pitch angle diffusion of newborn ions due to intrinsic turbulence in the solar wind

The objective of the present study is to understand the interaction of the solar wind with newborn ions in far upstream regions of a comet where the level of intrinsic turbulence is moderately low. Based on the assumption that quasi‐linear theory is adequate and applicable, we investigate the pitch angle diffusion process and the time evolution of the newborn ion distribution function. Numerical solutions to the quasi‐linear diffusion equation, including the effect of resonance broadening and that of continuous creation of newborn ions due to ionization process, are obtained under several assumptions and approximations. It is found that theoretical results are consistent with the Giotto observations recently reported by Neugebauer et al. (1989) which enable us to understand the formation of partial shells which were frequently observed in far upstream regions. The pitch angle diffusion process is also examined by the hybrid simulation. The results of the simulation are in qualitative agreement with those obtained from the quasi‐linear analysis.

Energetic neutral atoms in cometary comae

A Monte Carlo simulation model is constructed to follow the collisional scattering of the energetic neutral atoms (ENAs) in the cometary coma. It is found that collisional thermalization in the inner coma could lead to the production of an isotropic flux of fast-moving atoms with velocities on the order of 40-60 km/s. This thermalized component may be an important source for the cometary heavy ions detected at large cometocentric distances. Collisional scattering of the ENAs with the neutral coma also results in heating of the expanding gas. The generation of hot ions in the vicinity of the magnetic field is studied by estimating the production rates of secondary ions from impact ionization of the ENAs in the inner coma. The monoenergetic beam of ENAs used here has an initial streaming velocity of 230 km/s. The theoretical prediction so obtained is not consistent with the Giotto observations that the main population of the hot ions should be confined mostly inside the ionospheric contact discontinuity. 24 refs.

On the excitation of cyclotron harmonic waves by newborn heavy ions

Wave measurements in planetary foreshocks and cometary environments show the sporadic occurrence of magnetic spectra with harmonic structure related to ion cyclotron frequencies. Dilute populations of anisotropic and/or drifting charged particles can excite obliquely propagating modes with spacecraft frequencies close to the observed harmonics. We extend previous analyses of this generation mechanism to drifting and nondrifting loss‐cone type distributions of heavy ions in a dense hydrogen magnetoplasma, characterizing the complex (real frequency and growthrate) dispersion, polarization and compressibility of the unstable cyclotron harmonic waves. Solution of the full kinetic dispersion equation shows that it is possible to attain harmonic excitation, both in the drifting and nondrifting regimes. However, the bandwidth inherent to frequency Doppler shifts of obliquely propagating waves might preclude the observation of spectral structure in the spacecraft frame. The Giotto observations in the upstream region of comet Halley provide a reference to discuss the results.

A test particle model of pickup ions at comet Halley

A test particle treatment is used to investigate some of the details of the pickup cometary ions observed at comet Halley. The effects of the large‐scale magnetic and motional electric fields, as described by an MHD model of the comet, produce the characteristic V shape seen in Giotto observations in simulated energy‐time spectrograms. It is demonstrated that scattering produced by the addition of magnetic field fluctuations can obscure the tendency of the large‐scale field to deflect energetic ions picked up in the outer coma from the tail axis. The fact that the V is so clearly observed in the Giotto spectrogram thus suggests that although scattering must be invoked to explain the isotropic pitch angle distributions and highest energy ions, it does not have a major effect on the overall pickup ion spectrum.

A unidimensional model of comet ionosphere structure

We have considered the one‐dimensional continuity and momentum equations for the plasma created in the expanding coma of a comet near the Sun. For an inverse square dependence of neutral density we have solved the continuity equation in the presence of radial plasma flow. For distances large compared to the size of the nucleus, our distribution falls off as r−1 with a velocity dependent coefficient which reduces to the photochemical steady state case in the limit of vanishing radial plasma outflow. This density distribution is used in the treatment of the momentum equation from which an analytic expression for the magnetic field configuration is derived in the presence of outflow, photoionization, dissociative recombination, plasma fluid pressure, and friction between the ions and neutrals. The magnetic field thus derived can be shown to have a maximum value which we fix to be attained at the point corresponding to the point shown by the Giotto observations and find that for a Halley type comet there will be a region sunward of the nucleus from which the magnetic field is excluded, primarily by the effect of ion neutral collisions, which is consistent with the Giotto observations. We have performed the calculations for both Halley and Giacobini‐Zinner type comets. In the field‐free region we find that the dominant terms in the momentum equation balancing the magnetic pressure gradient are the ion neutral friction and the net mass loading momentum gain. For large distances the field reduces to a constant field. The scale size of field diffusion as a result of finite conductivity, brought about by electron neutral collisions, is found to be vanishingly small.

Numerical simulation of the generation of turbulence from cometary ion pick‐up

Observations of magnetic field fluctuations near comet Halley have revealed a rapid development of a Kolmogoroff‐like turbulence spectrum extending from below 10−2 Hz to above 0.1 Hz. Spectra obtained far from the comet have a strong peak in power near the Doppler‐shifted ion‐cyclotron frequency of singly ionized water. Closer to the comet, the spectrum at higher frequencies is enhanced in power level over the background solar wind spectrum by approximately an order of magnitude. We solved the equations of incompressible MHD using a two‐dimensional 256×256 mode spectral method code to simulate this spectral evolution as an inertial range turbulent cascade. The initial conditions contained a constant magnetic field and a single coherent wave mode at a low wave number. The solar wind turbulence was modelled by a background noise spectrum having a Kolmogoroff spectral index. The coherent mode decayed into an inertial range spectrum with Kolmogoroff slope within a few eddy‐turnover times. Both the time scale and the increase in power level of the turbulence seen in the simulation are in accord with the Giotto observations.

Dust density and mass distribution near comet Halley from Giotto observations

The dust impact detection system (DIDSY) carried by the Giotto spacecraft has measured the density and mass spectrum (in the range 10−20–10−8 kg) of the dust near comet Halley. Profiles of the dust spectrum obtained at a distance of 291,000 km from the Halley nucleus show a complex mass spectrum, depleted in intermediate and small masses. Near closest approach (∼600 km from the nucleus), the dust activity increases dramatically and the spectrum is dominated by larger masses. The cumulative mass index (for a mass distribution function of the form m−α) measured at an approach distance of 2,200 km from the nucleus is α = 0.66±0.05, corresponding to α = 0.83±0.05 at the nucleus. The Halley dust surface emission rate is found to be typically 1×10−6 kg m−2 s−1 for masses ≤10−8 kg, corresponding to a total dust production rate of 3.1×103 kg s−1. Particle fragmentation may have occurred in the dust cloud. Most of the mass striking Giotto resides in the few large particles penetrating the dust shield, and the integral mass is estimated to be ≲150 mg. Although the existence of previously unexpected small particles (of mass 10−20 kg) implies that previous infrared dust modelling1 must now be extended, these particles are found to contribute negligible areal cross-section compared with larger (≥10 µm diameter) particles. Momentum balances and energy considerations applied to an observed deceleration suggest that a large mass of the spacecraft was detached by an impact and ejected at low velocity.