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Ionospheres and magnetospheres of comets
Adv. Space Res. Vol. 20, No. 2, pp. 255-266.1997 8 1997 COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain
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IONOSPHERES AND MAGNETOSPHERES OF COMETS A. J. Coates Mullard Space Science Laboratory, University College London, Holmbury St. Mat-v,Dorkinn RH.5 6hT, U.K.
ABSTRACT As a comet’s orbit takes it near to the Sun, neutral gas and dust are driven away from the nucleus. The gas ionizes due to photoionization and charge exchange. A dense ‘ionosphere’ forms near to the comet, bounded by a contact surface. New, ‘pickup’ ions are produced on a much larger distance scale of -lo6 km, forming an enormous region over which the comet-solar wind interaction occurs. The new ions form an unstable population in the flowing solar wind. The solar wind is slowed, forming a bow shock and several other features including the cometary plasma tail. The solar wind interaction is quite different to that of a magnetized planet, where a magnetopause forms a blunt obstacle in the supersonic solar wind flow. In this paper we review our knowledge of the plasma environment of comets, particularly in the light of spacecraft data from the three comets visited so far. 0 1997COSPAR.Published by Elsevier Science Ltd INTRODUCTION Since the suggestion of a solar wind based on observations of comet tails (Biermann, 1951), the interaction mechanism of comets with the solar wind via pickup ions has attracted much interest. The process of ion pickup and implantation into the solar wind flow, which loads the flow with additional mass, was expected to be the key to the comet-solar wind interaction (e.g., Wallis, 1973). Prior to the spacecraft encounters with comets GiacobiniZinner, Halley and Grigg-Skjellerup, two boundaries in the mass-loaded flow were anticipated, separating plasmas with different properties. These were the bow shock and the contact surface (e.g., Schmidt and Wegmann, 1982, Ip and Axford, 1982). Following the spacecraft encounters, the importance of pickup ions has been confirmed and much work has been done on the wave-particle interactions, particle distributions and wave properties which are key features of the deceleration of the solar wind in the cometary environment. However, the encounters have raised significant questions regarding the permanence of some of the boundaries and features seen in the various data sets, mainly in the region between the bow shock and the contact surface. Although Giotto crossed the contact surface, the cometary ionosphere is little explored as yet. The tail formation region is completely unexplored. Thus there remain significant gaps in our knowledge of the comet-solar wind interaction which future missions such as Rosetta can address. A general review of the comet-solar wind interaction was published by Neugebauer (1990). As well as the properties of the solar wind itself, the gas production rate of the comet is a vital parameter which determines the size and nature of the comet-solar wind interaction region. During the approach of a comet towards the Sun, the gas production rate increases and the interaction changes from that of a bare nucleus to the fully developed coma structure. A schematic of the two extremes is shown in Figures I (a) and l(b). The Figure illustrates the wake-type interaction to be expected at large heliospheric distances, similar to the solar wind interaction with an asteroid or the Moon, and the various features of the developed interaction to be expected at an active comet near to 1AU. 255
256
A. I. Codes CIal.
LOW ACTIVITY
HIGH ACTIVITY Bav shodc @G?Au)
t2nltmarurfsce or bnopluae (de UU)
Fig. 1. Comet-solar wind interaction, (a) at large heliospheric distances (low activity), and (b) closer to the Sun (high activity).
In this paper we review our current knowledge of the comet-solar wind interaction, boundary formation and suggest some questions to be addressed in future studies. PICKUP IONS, ACCELERATION
the pickup process
and
AND MASS LOADING
When a cometary neutral atom or molecule is ionized, it immediately responds to the electric and magnetic fields in the solar wind. In real space its path follows a cycloid, the exact form depends on the relative orientation between the solar wind velocity and the magnetic field. In velocity space this corresponds to a ring distribution. Because the ring distribution is unstable, plasma waves are produced. Prior to the spacecraft encounters, it was expected that particles would scatter in pitch angle to form a shell distribution from the ring as the instabilities developed (e.g., Wu and Davidson, 1972), and a first look at the data confirmed this (e.g., Mukai et al, 1986, Gringauz et al, 1986a, Coates et al, 1989). For a ring distribution, the maximum energy of a pickup ion in the comet frame is 2mv,,2sin2a, where m is the pickup ion mass in amu, v,, is the solar wind speed and (x is the angle between the solar wind and magnetic field vectors. This compares to a maximum of 2mv,,’ for a shell distribution. Note that for an average solar wind energy of lkeV, and for ions of mass 18 (HsO+), the maximum shell energy is 72keV. Fig. 2. Velocity space diagram illustrating the ion pickup process The Giotto Implanted Ion Sensor (Wilken et al, ‘articles scatter from the initial ring RR’ onto the bispherical sheli 1987) covered from 70eV to 85keV in order to 3D, due to interactions with upstream (B) and downstream (D: fully cover the bulk of the pickup ion xopagating waves. distribution. Sensors on Suisei, Vega and ICE could only measure part of the pickup ion
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in the latter case only the high energy tail.
From the shell, velocity diffusion was expected to broaden the shell in energy. Given enough time, a Maxwellian would finally be produced. Results from Giotto show that this latter stage may only be approached in the very inner regions of comet Halley (Schwenn et al, 1987, Goldstein et al, 1992.). From theoretical considerations (Galeev and Sagdeev, 1988) it is clear that simple shells are an approximation because the energy of the waves and the energy lost by the particles must be taken into account. This led to the idea of a ‘bispherical shell’ distribution for the cometary ions, which was subsequently confirmed in the data from Halley (Coates et al, 1990b, Huddleston et al, 1993a). The situation is illustrated in Figure 2, where the scattering of particles in pitch angle from the ring R onto the spherical arcs RB and RD can be seen. In these cases, energy from the particle distribution goes into plasma waves. These ideas have been developed further by Huddleston et al (1992) who presented a simulation of the pitch angle and energy diffusion at comet Halley including the bispherical effects, and by Huddleston and Johnstone (1992) who used a bispherical model to successfully predict the wave spectrum and to compare the energy available from the ring distribution to the energy released and the energy in the waves. From the Giotto data at comet Halley, Coates et al (1990b) presented a study of scattering in pitch angle and energy. They found that well upstream of Halley the cometary ions follow the ring prediction, while closer to the comet the distributions are more shell like. The velocity distributions in the solar wind frame show a small amount of acceleration in the upstream region, significant enough to be seen by the energetic particle instrument (McKenna-Lawlor et al, 1986), but the most significant acceleration was seen downstream of the cometary bow shock. Coates et al (1990b) also showed that the bulk velocity of the pickup heavy ion population was consistent with bispherical distributions in the region of the shock, despite the particle acceleration seen there. A more detailed review of the results at GZ and Halley was given by Coates (1990a). In the case of comet Grigg-Skjellerup, Coates et al (1993) showed that the ion distributions were ring-like almost up to the bow shock, except that one sided distributions were seen for a time outbound. They also discovered nongyrotropic distributions particularly in the inner regions at GS, due to the different ion implantation rates between adjacent cusps of the cycloid in those regions. This can cause additional instabilities to operate (Neubauer et al, 1993, Motschmann and Glassmeier, 1993, Cao et al, 1995). Further results on non-gyrotropy were presented recently (Coates et al, 1996). The pickup process at Halley and Grigg-Skjellerup has been compared by Johnstone (1995). Mass loading by cometary ions was predicted and observed at comets. Models for the deceleration were presented by Huddleston et al (1990) for comet Halley and Huddleston et al (1993b) for Grigg-Skjellerup. Figure 9 of Coates et al (1990b) compares the model to the measurements at comet Halley and good agreement is found. An alternative approach of predicting the cometary ion density for observed solar wind velocity has been tried by Coates et al (1987, 1990a and 1997), again with reasonable results. Energetic ions had not been expected at comets, but they were seen at all of the comets visited so far (e.g., Hynds et al, 1986, Somogyi et al, 1986, McKenna-Lawlor et al, 1986, 1993) and mechanisms were suggested for their acceleration. These included adiabatic acceleration, the interaction of ions with waves across a sharp gradient in solar wind flow speed (Fermi I) or the interaction withupstream and downstream propagating waves (Fermi II). The form of the bulk of the cometary ion distribution function depends on the point in the mass loaded flow at which the observation is made (see Coates, 1991 b for a review). Well upstream of the comet the one-dimensional distribution function is a delta function modified on the low energy side due to the bispherical distributions. Closer to the comet, where the solar wind speed is lower and some mass loading has occurred, the distribution is modified (Galeev et al, 1985; see Figure 3 of Coates, 1991b) both by the slowing solar wind and by adiabatic acceleration. Downstream of the bow shock, two populations are found due to the velocity jump across the shock (Galeev et al, 1985, Thomsen et al, 1987). All of these distributions are further modified by the Fermi I or II processes. Comparison of models with data showed that Fermi II alone may not be sufficient to explain the Halley observation (Huddleston et al, 1992 Fig. 7). The proportion of downstream propagating waves was found to be
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larger than expected, which may account for some additional acceleration Coates et al (1996) showed that additional energy may go into particle deceleration BOUNDARIES
IN THE COMETARY
ENVIRONMENT
In Figure 3, we have adapted a schematic produced by Neubauer (1990) to observations of all the cometary encounters so far. As mentioned above, the the only boundaries predicted to form prior to the spacecraft encounters. additional features: the mystery boundary, the cometopause, the magnetic boundary. The observations are summarized in Table 1. We will now discuss Table 1. Summary
(Johnstone et al, 1996). Recently, acceleration from the solar wind
of Observed Boundary Crossings
include the trajectories and boundary bow shock and cavity boundary were The data have added at least four pileup boundary and the ion pileup each of the boundaries in turn.
at Comets (Y=Yes, N=No)
HALLEY * GS .. ..Comet .... ..............................................._._......_....... I............. ....GZ ........ ............................ .. ....c= ......................... I._.........” ....” ..... _._.“_._.” ......” .............. _.._............................................................................ ICE Vl v2 Suisei Giotto Giotto Bow shock Y Y Y Y Y Y Mystery boundary ? ? ? ? Y Y Cometopause Y Magnetic pileup boundary N N Y Y Discontinuities X, X’, Y, Z Y Ion pileup boundary Y Y Y ? Cavityboundary _____1__1_--
Bow Shock A review of the theory and observations of the cometary bow shock was given by Coates (1995). Briefly, the bow shock was one of the predicted boundaries in the comet-solar wind interaction region. In the predictions, the mean molecular weight of the flow reached a critical value of 413 BW?t ? SW in the one dimensional case (Biermann et al, 1967) before a singularity, the bow shock, occurred. In more realistic two and three dimensional simulations a weak bow shock forms at a value lower than 4/3, dependent on the gas production rate (e.g., Schmidt and Wegmann, 1982, Gombosi et al, 1996). I
I
Observations at comets showed that this boundary was present at all spacecraft of the encounters but its properties changed dramatically between crossings at the same comet depending on the interplanetary magnetic field orientation. In all cases the cometary ions dominated the dynamics in
Fig. 3. Schematic of the comet-solar wind interaction, with spacecraft trajectories anr boundary crossings (Adapted from Neubauer, 1990)
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terms of pressure (see Coates et al, 1997). The width of the feature varied from a few water group ion gyroradii in the quasi-perpendicular case to many gyroradii in the quasi-parallel case (Neubauer et al, 1990). Where sufficient data were available, the bow shock was shown to occur at the predicted point in the mass-loaded flow (see Staines et al, 1991; Coates et al, 1990a; Coates et al, 1997). Also the Mach number of the features supported the interpretation of a bow shock (Smith et al, 1986; Coates et al, 1990a; Coates et al, 1997). The various particle species also increased in temperature across the boundary (Neugebauer et al, 1987; Staines et al, 1991; Coates et al, 1990a). While there is some evidence that the GZ data can be explained in terms of a reforming shocklet model (Omidi and Winske, 1990), and the Suisei and Giotto-Halley inbound data could have been affected by interplanetary disturbances (Tatrallyay et al, 1993, Kessel et al, 1994), there is now broad agreement that the boundary seen in the various cases was a bow shock. Mystery Boundary One of the dramatic plasma discoveries at comet Halley was the clear bifurcation of the cometary ion peak in the ‘cometosheath’ region between the bow shock and contact surface (Johnstone et al, 1986, Thomsen et al, 1987). The explanation for the bifurcation has already been alluded to, namely the different velocity either side of the bow shock which gives different pickup shell radii. However, this does not explain the very suddenly enhanced splitting of the water group ion population which occurs at approximately 500,000 km from the nucleus caused by a change in solar wind velocity there. This boundary, called the mystery boundary as its origin is still not understood, is also characterized as the end of a region of significantly enhanced densities of hot (0.8-3.6 keV) electrons (e.g., Reme et al, 1987, Reme 1991). The mystery boundary also ends a region of higher solar wind density and velocity, and following the boundary the number of ions in the ram direction increases sharply. There is no strong effect in the magnetic field to match these sharp and significant features in the plasma data. The boundary is still present on the outbound pass (although suitable electron data are not available) and also appears to be at comet GS (Johnstone et al, 1993, Mazelle et al, 1995, Jones and Coates, 1996). Also, it has been argued (Reme, 1991) that comparable boundaries were also observed by Vega, Suisei and ICE. However it is not present in any of the MHD or multi-fluid models and its origin has still not been explained. Although the boundary itself is still a mystery, it is conceivable that the increased numbers of hot electrons in the region it bounds could play a role in the recently discovered X-ray emission from comets (Lisse et al, 1996). Cometonause Analysis of the data from Vega-2 has shown the existence of a relatively sharp boundary some 160,000 km from according to measurements in the ram comet Halley separating regions of different chemical composition, direction (Gringauz et al, 1986b). In these data the protons showed a sudden decrease in density and the cometary ion density also suddenly increased, as seen in the ram direction. In addition, the proton velocity and water group velocity was quite different each side of the transition (Tatrallyay et al, 1995). It has been claimed that the cometopause is a permanent boundary in the comet-solar wind interaction (Gringauz and Verigin, 1991). The analysis of Giotto data from a similar region revealed a much broader transition between a mass-loaded solar wind region and a heavy ion dominated region with no significant velocity difference, although some authors claim to have evidence for a cometopause in the JPA (Johnstone Plasma Analyser) data at 180,000 km or 23:19 SCET (Amata et al 1986, Formisano et al 1990). This interpretation as a ‘cometopause’ has been attacked by Reme (1991), Reme et al (1994) and Neugebauer et al. (1992). The latter pointed out, probably correctly, that some artefacts appear in the Formisano et al analysis due to field of view limitations. In Figure 4 we show a direct comparison between the JPA-FIS (Fast Ion Sensor) and IIS (Implanted Ion Sensor) data in the cometopause region for the instrument angular bins closest to the ram direction. The top panel shows ITS data in mass group 1 (O-l.7 amu/q) from the sensor l.Y-25” from ram, the middle panel shows FIS data in the bin 35”-72” from ram, and the bottom panel shows IIS mass group 3 (6.5-20 amu/q) in the 15”-25” from ram bin. Note that the high energy population in the middle panel, ending at
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panel). At this same time, the water group ions in the low energy branch (lower panel) are becoming more intense, and the high energy branch are gradually disappearing. There is no strong discontinuity seen at the ‘cometopause’ in any of this raw data. We conclude that a gradual change of plasma conditions was in progress at this time during the Giotto flyby. In this region it is clear that charge exchange collisions are becoming more important, which would give a shift from a solar wind dominated flow to cometary ion domination. This is the physics which has been put forward to explain the cometopause transition and it remains to be explained why the transition was seen to occur more rapidly on Vega than Giotto. A sharp boundary is not predicted by models and one possibility is that a convected change of interplanetary conditions could explain the Vega observations. Recently, Tatrallyay et al (1995) have suggested that the velocity change between the protons on the upstream side and the cometary ions downstream may be important in determining the sharpness of the transition region. Magnetic Pileup Boundary (MPB) In the Giotto data, it is more widely accepted that a separate magnetic pileup boundary is seen some 45,000 km or 11 minutes later than the cometopause, at 23:30 SCET (e.g., Neubauer et al, 1986, Reme et al, 1986,‘Mazelle et al,
10' lBO
10'
lo-' 100
10-z
10-i 10'
100 10-1
10'
10" IO-'
10_'
10" lBQ 10'
10'
10-z lQB lo-'
10-a
I
22: 40
'
’
23 130
Time
23140
23120
1986
Day 072
to
Fig. 4. Data from JPA bins closest to the ram direction, between 340,000 and fhe top and bottom panels are from IIS mass groups 1 and 3 respectively; the eale). No strong cometopause is seen. A reduction in proton density (top two group branch (lower panel) are seen at the magnetic pileup boundary (23:30). ,een as proton density decreases, and transition Y (23:56) is visible as the end
I
00; 00
073 12,000 km from the nucleus of comet Halley. middle panel is from FW (different energy panels) and a reduction in the upper water Discontinuities X (23:42) and X’ (23:49) are of the lower water group line(bottom panel).
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1989). The main characteristics of this boundary on -the inbound pass were a sudden magnetic field jump (tangential discontinuity), a slowing and reduction of electron density, and a decrease in the electron perpendicular pressure. A dramatic increase in the rate of charge exchange has also been reported (Fuselier et al, 1991). Also, a proton density decrease was seen both by IMS (Goldstein et al, 1987) and by JPA. Referring to Figure 4, the decrease in proton density is seen in the top two panels and a reduction in the density of water group ions in the upper branch is seen in the lower panel. There are signs of a similar boundary outbound in the magnetometer data but it was not as marked (Neubauer et al, 1986, 1987). This boundary, inside which the magnetic field drapes strongly around the contact surface, was not predicted by models and, interestingly, was absent in the Vega data at comet Halley. This, and the observation that the cometary ion density shows no significant discontinuity, has led some authors to speculate a solar wind origin (e.g., Neugebauer et al, 1991), while others have postulated that on the night side it becomes the plasma tail boundary (Neubauer et al, 1987). A magnetic pileup boundary was also observed at GS (Neubauer et al, 1993, Reme et al, 1993, Mazelle et al, 1995). The IIS time resolution was not ideal for diagnosing the plasma in this region, although a decrease in proton density is not inconsistent with the data; also, a split of the proton population into beams in this region has been noted recently (Jones and Coates, 1996). Discontinuities
X, X’. Y and Z
Three additional boundaries have been pointed out in the Giotto data between the MPB and the IPB, called X, Y and Z by the IMS team. Discontinuity X is at 23:42 SCET, and is in a region where cometary ions are gradually being lost by charge exchange (Johnstone et al, 1986); there is also a sudden, significant drop in proton density there (Goldstein et al, 1987). This drop in proton density is also seen in the JPA data, see Figure 4. A similar boundary can be seen after this time at 23:49 SCET, where the proton density decreases (seen in FIS and IIS) and the water group population also starts a density decrease. This is a new feature, which we will call X’, not noticed up to now, and deserves some further study. Transition Y is the start of a sharp decrease in ion temperature, at 2356. This can also be seen by JPA as the end of the lower pickup ion line in the bottom panel of Figure 4. Discontinuity Z, at 2359, marks the transition from radial outward flow of cometary ions away from the nucleus to stagnant plasma (Balsiger et al, 1986, Schwenn et al, 1987). Ion Pileup Boundary (IPB) In the Giotto data at approximately 10,000 km, just after midnight, a significant peak in counts was found in the IMS data (Balsiger et al, 1986). The gradient of the log count rate changed from i’ (inside) to i* (outside). This can be interpreted as the limit of ionospheric plasma (e.g., Neugebauer, 1990). A magnetic field maximum also appeared at this point. In this region, significant chemical changes are occurrng, for example the fraction of the H,O+ ion became dominant close to this point (Balsiger et al, 1986). This boundary was also seen by the Vega probes at comet Halley (Vaisberg et al, 1987). It was not predicted in MHD models and it remains the most significant deviation from these models in the inner regions (Altwegg et al, 1993). The early interpretations of this region, including temporal effects in the neutral gas, change of upstream solar wind parameters and local electron impact ionization, were all rejected (Ip et al, 1987) in favour of increased electron temperature causing the reduction of the rate of recombination of ions and electrons. Recently, this region has been modelled in detail (Haberli et al, 1995), and it was shown quantitatively that the electron temperature increase with increasing distance inferred from NMS data (Eberhardt and Krankowsky, 1995) can cause a reduction in ion recombination and thus enhances the H30+. However an additional mechanism for heating the electrons is required, and this could be provided by magnetic field compression (Haberli et al, 1996). Cavitv Boundary Giotto was the only spacecraft to cross the expected cavity boundary, nevertheless it is reasonable to suppose that this is one of the permanent boundaries in the comet-solar wind interaction. The Giotto observations showed that the magnetic field was excluded from the cavity, and the boundary itself was approximately 25km wide (Neubauer
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1988). Near the cavity boundary, bursts of ions were seen by JPA (Johnstone et al, 1986) and by I&IS (Goldstein et al, 1989). The position of the boundary can be understood by a balance between inward magnetic gradient forces and outward ion-neutral drag (see review by Cravens, 1991). The pileup of ions just inside the cavity boundary was predicted by Cravens (1990) and this perhaps plays the role of the inner shock, a long predicted structure in the inner coma. Inside the cavity boundary the cometary plasma is stagnant. One interesting feature discovered in the inner regions was the existence of negative ions (Chaizy et al, 1991). The density variation with distance is steeper inside the cavity boundary compared to outside, and the existence of these ions was not predicted by models. What is now clear is that their existence is an important clue to the photochemical processes at work. CONCLUSIONS It is clear from this brief review that the solar wind interaction with a comet is radically different to that with a magnetized planet. An induced magnetosphere forms, due to the implanted cometary ions loading the flow over vast regions of space and causing the draping of magnetic field within the extended ‘obstacle’. The unstable implanted ion distributions cause plasma waves and scatter to form a bispherical distribution in velocity space. Particle acceleration is caused due to wave-particle interactions in the disturbed cometary environment. Nongyrotropy of the ion distributions is significant where the ion implantation rate varies significantly over the cycloid. This is particularly relevant at small, or weakly outgassing, comets. Of the various boundaries observed in the interaction region, only the bow shock and contact surface were predicted. These, were confirmed as permanent features of the developed solar wind interaction. Another permanent boundary is the ion pileup boundary. Other boundaries, which may not be permanent, are the mystery boundary, the cometopause and the magnetic pileup boundaries. Although the latter was seen both at Halley and GS it was absent during the Vega flybys of Halley and hence it may not be permanent. The encounters thus far have left us with a number of intriguing questions about the areas they visited, including the following. How important is the Fermi I mechanism in particle acceleration? Is the inner shock really a layer just inside the cavity boundary? How permanent are the boundaries in the comet-solar wind interaction, and what is their formation mechanism? How does the cometary plasma tail form and what is the importance of tail rays? Could the mystery region hot electrons cause X-ray emission in the cometary environment? There are also significant questions about the regions not visited, or only fleetingly sampled, so far. These include many features of the cometary ionosphere, photochemical interactions and the formation of negative ions, the exploration of the tail formation region, the dependance of features on gas production rate as the comet approaches the Sun, the role of reconnection in the interaction processes, and the dependence on different solar wind conditions. Some of these questions should be answered by the Rosetta and other cometary missions planned for the future. ACKNOWLEDGMENTS AJC acknowledges
the support of the Royal Society, UK.
REFERENCES Altwegg, K., H. Balsiger, J. Geiss, R. Goldstein, W.-H. Ip, et al, The ion population between 1300 km and 230000 km in the coma of comet P/Halley, Astron. Astrophys., 279(l), 260-266, 1993. Amata, E., V. Formisano, R. Cerulli-Ireili, P. Torrente, A.D. Johnstone, et al, The cometopause region at comet Halley, ProceedingsZOth ESLAB symposium on the Exploration of Halley’s Comet, Eur. .Space Agency Spec. Publ., SP-250(I), pp2 13-2 18, 1986. Balsiger, H., K. Altwegg, F. Buhler, J. Geiss, A.G. Ghielmetti, et al, Ion composition anddynamics at comet Halley, Nature, 321,330-334, 1986.
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ionization
in
0273-Ir77/91$17.00+ 0.00
PII: SO273-1177(97)00543-7
IONOSPHERES AND MAGNETOSPHERES OF COMETS A. J. Coates Mullard Space Science Laboratory, University College London, Holmbury St. Mat-v,Dorkinn RH.5 6hT, U.K.
ABSTRACT As a comet’s orbit takes it near to the Sun, neutral gas and dust are driven away from the nucleus. The gas ionizes due to photoionization and charge exchange. A dense ‘ionosphere’ forms near to the comet, bounded by a contact surface. New, ‘pickup’ ions are produced on a much larger distance scale of -lo6 km, forming an enormous region over which the comet-solar wind interaction occurs. The new ions form an unstable population in the flowing solar wind. The solar wind is slowed, forming a bow shock and several other features including the cometary plasma tail. The solar wind interaction is quite different to that of a magnetized planet, where a magnetopause forms a blunt obstacle in the supersonic solar wind flow. In this paper we review our knowledge of the plasma environment of comets, particularly in the light of spacecraft data from the three comets visited so far. 0 1997COSPAR.Published by Elsevier Science Ltd INTRODUCTION Since the suggestion of a solar wind based on observations of comet tails (Biermann, 1951), the interaction mechanism of comets with the solar wind via pickup ions has attracted much interest. The process of ion pickup and implantation into the solar wind flow, which loads the flow with additional mass, was expected to be the key to the comet-solar wind interaction (e.g., Wallis, 1973). Prior to the spacecraft encounters with comets GiacobiniZinner, Halley and Grigg-Skjellerup, two boundaries in the mass-loaded flow were anticipated, separating plasmas with different properties. These were the bow shock and the contact surface (e.g., Schmidt and Wegmann, 1982, Ip and Axford, 1982). Following the spacecraft encounters, the importance of pickup ions has been confirmed and much work has been done on the wave-particle interactions, particle distributions and wave properties which are key features of the deceleration of the solar wind in the cometary environment. However, the encounters have raised significant questions regarding the permanence of some of the boundaries and features seen in the various data sets, mainly in the region between the bow shock and the contact surface. Although Giotto crossed the contact surface, the cometary ionosphere is little explored as yet. The tail formation region is completely unexplored. Thus there remain significant gaps in our knowledge of the comet-solar wind interaction which future missions such as Rosetta can address. A general review of the comet-solar wind interaction was published by Neugebauer (1990). As well as the properties of the solar wind itself, the gas production rate of the comet is a vital parameter which determines the size and nature of the comet-solar wind interaction region. During the approach of a comet towards the Sun, the gas production rate increases and the interaction changes from that of a bare nucleus to the fully developed coma structure. A schematic of the two extremes is shown in Figures I (a) and l(b). The Figure illustrates the wake-type interaction to be expected at large heliospheric distances, similar to the solar wind interaction with an asteroid or the Moon, and the various features of the developed interaction to be expected at an active comet near to 1AU. 255
256
A. I. Codes CIal.
LOW ACTIVITY
HIGH ACTIVITY Bav shodc @G?Au)
t2nltmarurfsce or bnopluae (de UU)
Fig. 1. Comet-solar wind interaction, (a) at large heliospheric distances (low activity), and (b) closer to the Sun (high activity).
In this paper we review our current knowledge of the comet-solar wind interaction, boundary formation and suggest some questions to be addressed in future studies. PICKUP IONS, ACCELERATION
the pickup process
and
AND MASS LOADING
When a cometary neutral atom or molecule is ionized, it immediately responds to the electric and magnetic fields in the solar wind. In real space its path follows a cycloid, the exact form depends on the relative orientation between the solar wind velocity and the magnetic field. In velocity space this corresponds to a ring distribution. Because the ring distribution is unstable, plasma waves are produced. Prior to the spacecraft encounters, it was expected that particles would scatter in pitch angle to form a shell distribution from the ring as the instabilities developed (e.g., Wu and Davidson, 1972), and a first look at the data confirmed this (e.g., Mukai et al, 1986, Gringauz et al, 1986a, Coates et al, 1989). For a ring distribution, the maximum energy of a pickup ion in the comet frame is 2mv,,2sin2a, where m is the pickup ion mass in amu, v,, is the solar wind speed and (x is the angle between the solar wind and magnetic field vectors. This compares to a maximum of 2mv,,’ for a shell distribution. Note that for an average solar wind energy of lkeV, and for ions of mass 18 (HsO+), the maximum shell energy is 72keV. Fig. 2. Velocity space diagram illustrating the ion pickup process The Giotto Implanted Ion Sensor (Wilken et al, ‘articles scatter from the initial ring RR’ onto the bispherical sheli 1987) covered from 70eV to 85keV in order to 3D, due to interactions with upstream (B) and downstream (D: fully cover the bulk of the pickup ion xopagating waves. distribution. Sensors on Suisei, Vega and ICE could only measure part of the pickup ion
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in the latter case only the high energy tail.
From the shell, velocity diffusion was expected to broaden the shell in energy. Given enough time, a Maxwellian would finally be produced. Results from Giotto show that this latter stage may only be approached in the very inner regions of comet Halley (Schwenn et al, 1987, Goldstein et al, 1992.). From theoretical considerations (Galeev and Sagdeev, 1988) it is clear that simple shells are an approximation because the energy of the waves and the energy lost by the particles must be taken into account. This led to the idea of a ‘bispherical shell’ distribution for the cometary ions, which was subsequently confirmed in the data from Halley (Coates et al, 1990b, Huddleston et al, 1993a). The situation is illustrated in Figure 2, where the scattering of particles in pitch angle from the ring R onto the spherical arcs RB and RD can be seen. In these cases, energy from the particle distribution goes into plasma waves. These ideas have been developed further by Huddleston et al (1992) who presented a simulation of the pitch angle and energy diffusion at comet Halley including the bispherical effects, and by Huddleston and Johnstone (1992) who used a bispherical model to successfully predict the wave spectrum and to compare the energy available from the ring distribution to the energy released and the energy in the waves. From the Giotto data at comet Halley, Coates et al (1990b) presented a study of scattering in pitch angle and energy. They found that well upstream of Halley the cometary ions follow the ring prediction, while closer to the comet the distributions are more shell like. The velocity distributions in the solar wind frame show a small amount of acceleration in the upstream region, significant enough to be seen by the energetic particle instrument (McKenna-Lawlor et al, 1986), but the most significant acceleration was seen downstream of the cometary bow shock. Coates et al (1990b) also showed that the bulk velocity of the pickup heavy ion population was consistent with bispherical distributions in the region of the shock, despite the particle acceleration seen there. A more detailed review of the results at GZ and Halley was given by Coates (1990a). In the case of comet Grigg-Skjellerup, Coates et al (1993) showed that the ion distributions were ring-like almost up to the bow shock, except that one sided distributions were seen for a time outbound. They also discovered nongyrotropic distributions particularly in the inner regions at GS, due to the different ion implantation rates between adjacent cusps of the cycloid in those regions. This can cause additional instabilities to operate (Neubauer et al, 1993, Motschmann and Glassmeier, 1993, Cao et al, 1995). Further results on non-gyrotropy were presented recently (Coates et al, 1996). The pickup process at Halley and Grigg-Skjellerup has been compared by Johnstone (1995). Mass loading by cometary ions was predicted and observed at comets. Models for the deceleration were presented by Huddleston et al (1990) for comet Halley and Huddleston et al (1993b) for Grigg-Skjellerup. Figure 9 of Coates et al (1990b) compares the model to the measurements at comet Halley and good agreement is found. An alternative approach of predicting the cometary ion density for observed solar wind velocity has been tried by Coates et al (1987, 1990a and 1997), again with reasonable results. Energetic ions had not been expected at comets, but they were seen at all of the comets visited so far (e.g., Hynds et al, 1986, Somogyi et al, 1986, McKenna-Lawlor et al, 1986, 1993) and mechanisms were suggested for their acceleration. These included adiabatic acceleration, the interaction of ions with waves across a sharp gradient in solar wind flow speed (Fermi I) or the interaction withupstream and downstream propagating waves (Fermi II). The form of the bulk of the cometary ion distribution function depends on the point in the mass loaded flow at which the observation is made (see Coates, 1991 b for a review). Well upstream of the comet the one-dimensional distribution function is a delta function modified on the low energy side due to the bispherical distributions. Closer to the comet, where the solar wind speed is lower and some mass loading has occurred, the distribution is modified (Galeev et al, 1985; see Figure 3 of Coates, 1991b) both by the slowing solar wind and by adiabatic acceleration. Downstream of the bow shock, two populations are found due to the velocity jump across the shock (Galeev et al, 1985, Thomsen et al, 1987). All of these distributions are further modified by the Fermi I or II processes. Comparison of models with data showed that Fermi II alone may not be sufficient to explain the Halley observation (Huddleston et al, 1992 Fig. 7). The proportion of downstream propagating waves was found to be
258
A. J. Gates
et 01.
larger than expected, which may account for some additional acceleration Coates et al (1996) showed that additional energy may go into particle deceleration BOUNDARIES
IN THE COMETARY
ENVIRONMENT
In Figure 3, we have adapted a schematic produced by Neubauer (1990) to observations of all the cometary encounters so far. As mentioned above, the the only boundaries predicted to form prior to the spacecraft encounters. additional features: the mystery boundary, the cometopause, the magnetic boundary. The observations are summarized in Table 1. We will now discuss Table 1. Summary
(Johnstone et al, 1996). Recently, acceleration from the solar wind
of Observed Boundary Crossings
include the trajectories and boundary bow shock and cavity boundary were The data have added at least four pileup boundary and the ion pileup each of the boundaries in turn.
at Comets (Y=Yes, N=No)
HALLEY * GS .. ..Comet .... ..............................................._._......_....... I............. ....GZ ........ ............................ .. ....c= ......................... I._.........” ....” ..... _._.“_._.” ......” .............. _.._............................................................................ ICE Vl v2 Suisei Giotto Giotto Bow shock Y Y Y Y Y Y Mystery boundary ? ? ? ? Y Y Cometopause Y Magnetic pileup boundary N N Y Y Discontinuities X, X’, Y, Z Y Ion pileup boundary Y Y Y ? Cavityboundary _____1__1_--
Bow Shock A review of the theory and observations of the cometary bow shock was given by Coates (1995). Briefly, the bow shock was one of the predicted boundaries in the comet-solar wind interaction region. In the predictions, the mean molecular weight of the flow reached a critical value of 413 BW?t ? SW in the one dimensional case (Biermann et al, 1967) before a singularity, the bow shock, occurred. In more realistic two and three dimensional simulations a weak bow shock forms at a value lower than 4/3, dependent on the gas production rate (e.g., Schmidt and Wegmann, 1982, Gombosi et al, 1996). I
I
Observations at comets showed that this boundary was present at all spacecraft of the encounters but its properties changed dramatically between crossings at the same comet depending on the interplanetary magnetic field orientation. In all cases the cometary ions dominated the dynamics in
Fig. 3. Schematic of the comet-solar wind interaction, with spacecraft trajectories anr boundary crossings (Adapted from Neubauer, 1990)
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terms of pressure (see Coates et al, 1997). The width of the feature varied from a few water group ion gyroradii in the quasi-perpendicular case to many gyroradii in the quasi-parallel case (Neubauer et al, 1990). Where sufficient data were available, the bow shock was shown to occur at the predicted point in the mass-loaded flow (see Staines et al, 1991; Coates et al, 1990a; Coates et al, 1997). Also the Mach number of the features supported the interpretation of a bow shock (Smith et al, 1986; Coates et al, 1990a; Coates et al, 1997). The various particle species also increased in temperature across the boundary (Neugebauer et al, 1987; Staines et al, 1991; Coates et al, 1990a). While there is some evidence that the GZ data can be explained in terms of a reforming shocklet model (Omidi and Winske, 1990), and the Suisei and Giotto-Halley inbound data could have been affected by interplanetary disturbances (Tatrallyay et al, 1993, Kessel et al, 1994), there is now broad agreement that the boundary seen in the various cases was a bow shock. Mystery Boundary One of the dramatic plasma discoveries at comet Halley was the clear bifurcation of the cometary ion peak in the ‘cometosheath’ region between the bow shock and contact surface (Johnstone et al, 1986, Thomsen et al, 1987). The explanation for the bifurcation has already been alluded to, namely the different velocity either side of the bow shock which gives different pickup shell radii. However, this does not explain the very suddenly enhanced splitting of the water group ion population which occurs at approximately 500,000 km from the nucleus caused by a change in solar wind velocity there. This boundary, called the mystery boundary as its origin is still not understood, is also characterized as the end of a region of significantly enhanced densities of hot (0.8-3.6 keV) electrons (e.g., Reme et al, 1987, Reme 1991). The mystery boundary also ends a region of higher solar wind density and velocity, and following the boundary the number of ions in the ram direction increases sharply. There is no strong effect in the magnetic field to match these sharp and significant features in the plasma data. The boundary is still present on the outbound pass (although suitable electron data are not available) and also appears to be at comet GS (Johnstone et al, 1993, Mazelle et al, 1995, Jones and Coates, 1996). Also, it has been argued (Reme, 1991) that comparable boundaries were also observed by Vega, Suisei and ICE. However it is not present in any of the MHD or multi-fluid models and its origin has still not been explained. Although the boundary itself is still a mystery, it is conceivable that the increased numbers of hot electrons in the region it bounds could play a role in the recently discovered X-ray emission from comets (Lisse et al, 1996). Cometonause Analysis of the data from Vega-2 has shown the existence of a relatively sharp boundary some 160,000 km from according to measurements in the ram comet Halley separating regions of different chemical composition, direction (Gringauz et al, 1986b). In these data the protons showed a sudden decrease in density and the cometary ion density also suddenly increased, as seen in the ram direction. In addition, the proton velocity and water group velocity was quite different each side of the transition (Tatrallyay et al, 1995). It has been claimed that the cometopause is a permanent boundary in the comet-solar wind interaction (Gringauz and Verigin, 1991). The analysis of Giotto data from a similar region revealed a much broader transition between a mass-loaded solar wind region and a heavy ion dominated region with no significant velocity difference, although some authors claim to have evidence for a cometopause in the JPA (Johnstone Plasma Analyser) data at 180,000 km or 23:19 SCET (Amata et al 1986, Formisano et al 1990). This interpretation as a ‘cometopause’ has been attacked by Reme (1991), Reme et al (1994) and Neugebauer et al. (1992). The latter pointed out, probably correctly, that some artefacts appear in the Formisano et al analysis due to field of view limitations. In Figure 4 we show a direct comparison between the JPA-FIS (Fast Ion Sensor) and IIS (Implanted Ion Sensor) data in the cometopause region for the instrument angular bins closest to the ram direction. The top panel shows ITS data in mass group 1 (O-l.7 amu/q) from the sensor l.Y-25” from ram, the middle panel shows FIS data in the bin 35”-72” from ram, and the bottom panel shows IIS mass group 3 (6.5-20 amu/q) in the 15”-25” from ram bin. Note that the high energy population in the middle panel, ending at
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etal.
panel). At this same time, the water group ions in the low energy branch (lower panel) are becoming more intense, and the high energy branch are gradually disappearing. There is no strong discontinuity seen at the ‘cometopause’ in any of this raw data. We conclude that a gradual change of plasma conditions was in progress at this time during the Giotto flyby. In this region it is clear that charge exchange collisions are becoming more important, which would give a shift from a solar wind dominated flow to cometary ion domination. This is the physics which has been put forward to explain the cometopause transition and it remains to be explained why the transition was seen to occur more rapidly on Vega than Giotto. A sharp boundary is not predicted by models and one possibility is that a convected change of interplanetary conditions could explain the Vega observations. Recently, Tatrallyay et al (1995) have suggested that the velocity change between the protons on the upstream side and the cometary ions downstream may be important in determining the sharpness of the transition region. Magnetic Pileup Boundary (MPB) In the Giotto data, it is more widely accepted that a separate magnetic pileup boundary is seen some 45,000 km or 11 minutes later than the cometopause, at 23:30 SCET (e.g., Neubauer et al, 1986, Reme et al, 1986,‘Mazelle et al,
10' lBO
10'
lo-' 100
10-z
10-i 10'
100 10-1
10'
10" IO-'
10_'
10" lBQ 10'
10'
10-z lQB lo-'
10-a
I
22: 40
'
’
23 130
Time
23140
23120
1986
Day 072
to
Fig. 4. Data from JPA bins closest to the ram direction, between 340,000 and fhe top and bottom panels are from IIS mass groups 1 and 3 respectively; the eale). No strong cometopause is seen. A reduction in proton density (top two group branch (lower panel) are seen at the magnetic pileup boundary (23:30). ,een as proton density decreases, and transition Y (23:56) is visible as the end
I
00; 00
073 12,000 km from the nucleus of comet Halley. middle panel is from FW (different energy panels) and a reduction in the upper water Discontinuities X (23:42) and X’ (23:49) are of the lower water group line(bottom panel).
Ionospheres and Magnetospheres of Comets
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1989). The main characteristics of this boundary on -the inbound pass were a sudden magnetic field jump (tangential discontinuity), a slowing and reduction of electron density, and a decrease in the electron perpendicular pressure. A dramatic increase in the rate of charge exchange has also been reported (Fuselier et al, 1991). Also, a proton density decrease was seen both by IMS (Goldstein et al, 1987) and by JPA. Referring to Figure 4, the decrease in proton density is seen in the top two panels and a reduction in the density of water group ions in the upper branch is seen in the lower panel. There are signs of a similar boundary outbound in the magnetometer data but it was not as marked (Neubauer et al, 1986, 1987). This boundary, inside which the magnetic field drapes strongly around the contact surface, was not predicted by models and, interestingly, was absent in the Vega data at comet Halley. This, and the observation that the cometary ion density shows no significant discontinuity, has led some authors to speculate a solar wind origin (e.g., Neugebauer et al, 1991), while others have postulated that on the night side it becomes the plasma tail boundary (Neubauer et al, 1987). A magnetic pileup boundary was also observed at GS (Neubauer et al, 1993, Reme et al, 1993, Mazelle et al, 1995). The IIS time resolution was not ideal for diagnosing the plasma in this region, although a decrease in proton density is not inconsistent with the data; also, a split of the proton population into beams in this region has been noted recently (Jones and Coates, 1996). Discontinuities
X, X’. Y and Z
Three additional boundaries have been pointed out in the Giotto data between the MPB and the IPB, called X, Y and Z by the IMS team. Discontinuity X is at 23:42 SCET, and is in a region where cometary ions are gradually being lost by charge exchange (Johnstone et al, 1986); there is also a sudden, significant drop in proton density there (Goldstein et al, 1987). This drop in proton density is also seen in the JPA data, see Figure 4. A similar boundary can be seen after this time at 23:49 SCET, where the proton density decreases (seen in FIS and IIS) and the water group population also starts a density decrease. This is a new feature, which we will call X’, not noticed up to now, and deserves some further study. Transition Y is the start of a sharp decrease in ion temperature, at 2356. This can also be seen by JPA as the end of the lower pickup ion line in the bottom panel of Figure 4. Discontinuity Z, at 2359, marks the transition from radial outward flow of cometary ions away from the nucleus to stagnant plasma (Balsiger et al, 1986, Schwenn et al, 1987). Ion Pileup Boundary (IPB) In the Giotto data at approximately 10,000 km, just after midnight, a significant peak in counts was found in the IMS data (Balsiger et al, 1986). The gradient of the log count rate changed from i’ (inside) to i* (outside). This can be interpreted as the limit of ionospheric plasma (e.g., Neugebauer, 1990). A magnetic field maximum also appeared at this point. In this region, significant chemical changes are occurrng, for example the fraction of the H,O+ ion became dominant close to this point (Balsiger et al, 1986). This boundary was also seen by the Vega probes at comet Halley (Vaisberg et al, 1987). It was not predicted in MHD models and it remains the most significant deviation from these models in the inner regions (Altwegg et al, 1993). The early interpretations of this region, including temporal effects in the neutral gas, change of upstream solar wind parameters and local electron impact ionization, were all rejected (Ip et al, 1987) in favour of increased electron temperature causing the reduction of the rate of recombination of ions and electrons. Recently, this region has been modelled in detail (Haberli et al, 1995), and it was shown quantitatively that the electron temperature increase with increasing distance inferred from NMS data (Eberhardt and Krankowsky, 1995) can cause a reduction in ion recombination and thus enhances the H30+. However an additional mechanism for heating the electrons is required, and this could be provided by magnetic field compression (Haberli et al, 1996). Cavitv Boundary Giotto was the only spacecraft to cross the expected cavity boundary, nevertheless it is reasonable to suppose that this is one of the permanent boundaries in the comet-solar wind interaction. The Giotto observations showed that the magnetic field was excluded from the cavity, and the boundary itself was approximately 25km wide (Neubauer
A. J. Coates EI al.
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1988). Near the cavity boundary, bursts of ions were seen by JPA (Johnstone et al, 1986) and by I&IS (Goldstein et al, 1989). The position of the boundary can be understood by a balance between inward magnetic gradient forces and outward ion-neutral drag (see review by Cravens, 1991). The pileup of ions just inside the cavity boundary was predicted by Cravens (1990) and this perhaps plays the role of the inner shock, a long predicted structure in the inner coma. Inside the cavity boundary the cometary plasma is stagnant. One interesting feature discovered in the inner regions was the existence of negative ions (Chaizy et al, 1991). The density variation with distance is steeper inside the cavity boundary compared to outside, and the existence of these ions was not predicted by models. What is now clear is that their existence is an important clue to the photochemical processes at work. CONCLUSIONS It is clear from this brief review that the solar wind interaction with a comet is radically different to that with a magnetized planet. An induced magnetosphere forms, due to the implanted cometary ions loading the flow over vast regions of space and causing the draping of magnetic field within the extended ‘obstacle’. The unstable implanted ion distributions cause plasma waves and scatter to form a bispherical distribution in velocity space. Particle acceleration is caused due to wave-particle interactions in the disturbed cometary environment. Nongyrotropy of the ion distributions is significant where the ion implantation rate varies significantly over the cycloid. This is particularly relevant at small, or weakly outgassing, comets. Of the various boundaries observed in the interaction region, only the bow shock and contact surface were predicted. These, were confirmed as permanent features of the developed solar wind interaction. Another permanent boundary is the ion pileup boundary. Other boundaries, which may not be permanent, are the mystery boundary, the cometopause and the magnetic pileup boundaries. Although the latter was seen both at Halley and GS it was absent during the Vega flybys of Halley and hence it may not be permanent. The encounters thus far have left us with a number of intriguing questions about the areas they visited, including the following. How important is the Fermi I mechanism in particle acceleration? Is the inner shock really a layer just inside the cavity boundary? How permanent are the boundaries in the comet-solar wind interaction, and what is their formation mechanism? How does the cometary plasma tail form and what is the importance of tail rays? Could the mystery region hot electrons cause X-ray emission in the cometary environment? There are also significant questions about the regions not visited, or only fleetingly sampled, so far. These include many features of the cometary ionosphere, photochemical interactions and the formation of negative ions, the exploration of the tail formation region, the dependance of features on gas production rate as the comet approaches the Sun, the role of reconnection in the interaction processes, and the dependence on different solar wind conditions. Some of these questions should be answered by the Rosetta and other cometary missions planned for the future. ACKNOWLEDGMENTS AJC acknowledges
the support of the Royal Society, UK.
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