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COMETARY MAGNETOSPHERES: A TUTORIAL

T. E. Cravens1 and T. I. Gombosi2

1University of Kansas, Dept. of Physics and Astronomy, Malott Hall, 1251 Wescoe Hall Dr.,

Lawrence, KS 66045 USA 2 University of Michigan, Space Physics Research Lab, 2455 Hayward, Ann Arbor, MI 48105 USA

ABSTRACT 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. INTRODUCTION TO THE COMETARY PLASMA ENVIRONMENT

The first indications that the solar wind interacts with comets and that comets have a plasma environment came with remote observations of cometary ion tails (Biermann, 1951; Alfvén, 1957; Biermann et al., 1967; see reviews by Ip and Axford, 1982, and Mendis et al., 1985). Solar wind particles have a low probability of colliding with cometary neutral species at distances far from the nucleus. However, the neutral atoms and molecules can be ionized by solar extreme ultraviolet photons, producing cometary ions such as H2O+ or O+. Once ionized, the newly-born ions, which are almost at rest with respect to the fast solar wind flow, respond to the electric and magnetic fields associated with the solar wind. The cometary ions get “picked-up” by the solar wind flow, more or less efficiently depending on the conditions and time-scales. The distinctive nature of the solar wind interaction with a comet and the subsequent creation of an induced “magnetosphere” follows from the extremely extensive cometary neutral atmosphere which is due to the small size of the nucleus. Ion pick-up due to ionization of these neutrals and the resulting mass-loading controls the density and dynamics of the plasma. The interaction is primarily collisionless far from the comet but becomes collisional in the inner coma. A review of ion pickup at comets can be found in Coates (2003 – this issue). A general tutorial on induced magnetospheres throughout the solar system can be found in Luhmann et al. (2003 – this issue).

Most of our knowledge about the cometary plasma environment has come from a few spacecraft encounters with comets. These include (Table 1) encounters of the NASA International Cometary Explorer (ICE) with comet Giacobini-Zinner (G-Z) in 1985, several encounters with comet Halley in

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1986, including the European Space Agency (ESA) Giotto mission, and a Giotto spacecraft encounter with comet Grigg-Skjellerup (G-S) in 1992. The Sakigake mission to comet Halley was not included in Table 1 since it remained very far from the comet and did not encounter the bow shock. The most recent encounter was of the Deep Space 1 (DS1) spacecraft with comet Borrelly in September 2001 (Young et al., 2003). The DS1 plasma instrumentation was minimal - just one instrument - but a very capable one -- an electron and ion plasma spectrometer (PEPE – Plasma Experiment for Planetary Exploration; Young et al., 2003). In addition, numerous remote observations have been made of comets over the years in all regions of the electromagnetic spectrum including x-rays.

Table 1. Spacecraft encounters with comets Spacecraft/Year Comet Heliocentric Distance Gas Production Bow Shock (AU) Rate Q (s-1) (1000 km) ICE Giacobini- 1.03 2 - 4 x 1028 (a) 130 (b) Sept. 11, 1985 Zinner 110 Suisei Halley 0.81 ≈ 1030 450 (c ) Mar. 8, 1986 VEGA1 Halley 0.79 1.3 x 1030 (d) 1100 Mar. 6, 1986 520 VEGA2 Halley 0.83 9 x 1029 (d) 1350 Mar. 9,1986 Giotto Halley 0.89 6.9 x 1029 (e) 1160 Mar. 14, 1986 760 Giotto Grigg- 1.01 7.5 x 1027 (f) 19.9 July 10, 1992 Skjellerup 25.5 (b) DS1 Borrelly ≈1 3.5 x 1028 (g) 150 Sept. 21, 2001 330 (b) (a) Mendis et al. (1985) (b) really a broad bow wave rather than a “shock” (c) more subsolar than other measured shock positions which were in the flank (d) Reme (1991) (e) Krankowsky et al. (1986) (f) Johnstone et al. (1993); also see Szabo et al. (2002) (g) Young et al. (2003) Notes: References for the bow shock (or wave) positions can be found in the review papers listed in the text. Both inbound and outbound shock crossings are listed in most cases. ______________________________________________________________________________

A schematic of the cometary plasma environment is shown in Figure 1. The mass-loaded solar wind

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undergoes a weak (Mach 2) shock upstream of the comet at a distance that increases with gas production rate of the comet (for Halley in 1986 this distance, scaled to the subsolar point, was ≈ 0.4 million km). The shocked solar wind continues to slow down and collisions start to become important near the cometopause (probably a gradual transition although shown in the schematic as a sharp boundary). The magnetic field “piles up” into a magnetic barrier in the slowly flowing, heavily mass-loaded plasma just sunward of the nucleus. The draped field lines (discussed later) also form a plasma (or ion) tail, which can be seen in optical images (Brandt and Niedner, 1987; Slavin et al., 1986). A diamagnetic (or field-free) cavity surrounds the nucleus.

Fig. 1. Schematic of the plasma environment of an active comet. The various plasma boundaries and regions are discussed in the text. A typical charge transfer collision of a heavy solar wind ion with a neutral and leading to the emission of an x-ray photon is also shown.

The topic of cometary plasma physics is now very extensive so that a broad review is difficult to undertake in a concise manner. This paper will take a tutorial approach and will be brief. The reader is referred to the many reviews for more complete coverage of this topic (Coates, 1997; Galeev, 1991; Cravens, 1991a; Flammer, 1991; Tsurutani, 1991). The plasma regions noted in Figure 1 will be briefly discussed in different sections of the paper. A section of the paper will also review cometary x-rays and the last section of the paper will mention future prospects for cometary exploration. ION PICK-UP AND MASS-LOADING

The nucleus of a comet is typically several kilometers across and consists of a mixture of frozen volatiles (that is, ice) and dust. The most abundant ice species is water (H2O), but CO2, CO, NH3, etc., are also present (Krankowsky et al., 1986; Altwegg et al., 1999). When a comet approaches close enough to the Sun, the ice sublimates, producing gas which flows out into space carrying some of the dust along with it. The neutral gas coma of a comet extends large distances from the nucleus with a neutral density that varies approximately as:

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!

nn(r) =

Q

4"un

1

r2exp # r $( ) (1)

where Q is the gas production rate, un is the neutral gas speed (un ≈ 1 km/s), r is the heliocentric distance, and λ = un τion is an ionization length scale. τion is the lifetime against ionization and is about 106 s for a heliocentric distance of about 1 AU. A typical neutral atom reaches a cometocentric distance of about 106 km before being lost to ionization. Ionization is mainly caused by photoionization by solar radiation, but charge transfer collision with solar wind protons, although not a net source of ions, does produce heavy cometary ions.

Newly-born cometary ions are almost at rest with respect to the solar wind flow (u >> un where u ≈ 400 km/s is a typical solar wind speed). These ions are subject to the Lorentz force in the comet frame of reference and subject to the solar wind motional electric field (E = - u x B) and the interplanetary magnetic field B. The ions are “picked-up” by the solar wind by these fields, although initially they are not fully assimilated into the flow. This cometary ion pick-up process has a very extensive literature, and readers should consult one of the review papers for a more thorough treatment of this topic (e.g., Galeev et al., 1985; Galeev, 1991; Lee, 1989; Terasawa, 1989; Johnstone et al., 1991; Johnstone, 1995; Coates, 1997). The ions both gyrate about the magnetic field and E x B drift, such that the ion trajectories are cycloidal and the associated ion distribution function is a ring-beam distribution in the solar wind reference frame. This type of distribution is highly unstable against the growth of ultra-low frequency (or ULF) electromagnetic waves (i.e., frequencies near the heavy ion gyrofrequency, or ≈ 10 mHz). The wave spectrum tends to have a strong monochromatic component for less active comets or at large cometocentric distances (i.e., comet G-Z far upstream) but tends to be turbulent-like with power spread over a large range of wavenumbers for regions closer to the comet (Sagdeev et al., 1986; Lee, 1989; Tsurutani, 1991; Mazelle et al., 1997).

The ULF waves interact with the cometary ions via pitch-angle scattering, and the ring-beam distribution is gradually transformed into a shell distribution. The velocity space shell is formed initially at the local solar wind speed, but the shell thickens as the solar wind slows down with decreasing cometocentric distance and also due to energy diffusion associated with the second order Fermi wave-particle process. Collisions also affect the evolution of the cometary distribution function in the regions close (within the cometopause shown in Figure 1) to the comet (cf. Puhl et al., 1993; Cravens, 1991b). The actual distribution more closely resembles a bi-spherical distribution than a simple shell, but discussion of this process is best left to other reviews (Johnstone, 1995; Coates, 1997). The above evolutionary process takes place more thoroughly and at greater distances for more active comets like Halley than for weaker comets such as Grigg-Skellerup. Figure 2 shows measured velocity space distributions for comet G-S from the implanted ion sensor (IIS) of the Johnstone plasma analyzer (JPA) onboard Giotto. The top distribution has clearly not evolved far from a ring-beam and even the distribution measured closer to the comet (but still outside the shock, located at about half this distance) has started to become shell-like but not completely. In order to model this pick-up process, particularly for weak comets, kinetic methods such as test particle methods (McKenzie et al., 1994), linear wave theory (e.g., Lee, 1989), and hybrid simulations (e.g., Lipatov et al., 1997) are needed.

The addition of heavy cometary ions to the solar wind flow alters the dynamics due to the mass-addition itself and also because the pick-up ions have a large pressure. The net effect is to slow down the solar wind. This slow-down process takes place continuously until a critical mass flux is reached, at which point a shock forms (see Galeev, 1991, for a review of this topic). The shock distance (rshock) thus depends on the ionization rate and on the activity of the comet (that is, on Q). For a strong comet like Halley the shock distance was observed at large distances (rshock≈ 1 million km in the flanks), whereas for comet G-S, it was seen at rshock ≈ 25,000 km (see Table 1). The shock is weak (Mach number of ≈ 2) because the flow has already been slowed down by the upstream mass-loading. The flow downstream of the shock continues to slow down until it almost stagnates near the nucleus for an active comet. Figure 3 shows solar wind flow vectors measured by the Suisei mission near comet Halley. The solar wind speed

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Fig. 2. Examples of water group ion distributions observed by the Giotto IIS upstream of the bow shock. The star marks the location of the expected ring-beam (and birth point of new ions) and the shell is marked as the dashed line. From Huddleston et al. (J. Geophys. Res., 1993) and Coates et al. (Geophys. Res. Lett., 1993); copyright 1993 American Geophysical Union.

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Fig. 3. Solar wind flow vectors measured by the plasma analyzer onboard the Suisei spacecraft during its encounter with comet Halley in 1986. From Mukai et al. (Nature, 1986); copyright Nature Publishing Group; used with permission. clearly decreases from upstream to downstream of the bow shock, although it has been pointed out that the Suisei shock apparently was anomalously strong (Tatrallyay et al., 1993). A useful review on the cometary bow shock is provided by Coates (1995). The shock microphysics differs from that of other collisionless shocks due to the presence of the heavy ions (Galeev, 1991). The shock front has structure with a length scale comparable to the heavy ion gyroradius (i.e., Δr ≈ 104 km). For comet Halley, Δr / rshock ≈ 10-2, whereas for comets G-Z or G-S this ratio was of order unity, so that the shock crossing was not very obvious and the term “bow wave” was in some cases thought to be more appropriate than shock. COLLISIONAL PROCESSES AT COMETS

The plasma flow downstream of the bow shock in the magnetosheath (or “cometosheath”) region gradually slows down and contains some features, such as the “mystery” boundary, which remain poorly understood (e.g., Reme, 1991). In a region surrounding the nucleus, the flow eventually becomes very slow, or stagnant (i.e., flow speeds less than a few km/s) for a comet like Halley. The magnetic field strength increases in this region and a magnetic barrier, or pile-up region, forms. The field lines associated with this barrier wrap around the comet and form a magnetotail. In essence, this is the cometary “magnetosphere” or induced magnetosphere. Figure 4 shows some results for the inner coma from a global MHD model of the plasma environment of comet Halley (Gombosi et al., 1996). The inner part of the magnetic barrier and the part of the magnetotail near the nucleus are evident in this figure. The cavity region is discussed later.

In the next couple of sections we briefly review the physical and chemical processes in this magnetosphere. The cometary plasma concentrated in the tail region is visible with telescopic observations (the “ion tail”). A composition change in the plasma was also observed by spacecraft near comet Halley at the cometopause boundary (or transition since this often does not appear as a sharp “boundary”) (rcom ≈ 105 km). The fluxes of solar wind protons and He++ ions were observed to decrease

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rapidly, and the densities of cometary species increase, as illustrated in Figure 5. In particular, notice the decrease of solar wind protons (denoted H+

sw). O+ ions able to be measured by the HERS sensor start to be picked up by the solar wind downstream of the shock, and their density also decreases at roughly the cometopause distance. The O+ ions measured by HIS are lower in energy. The cometopause transition appears to be sharper and more boundary-like in the VEGA data (Gringauz et al., 1986) than in the Giotto data reproduced here. The comet Borrelly cometopause was observed by DS1 (Young et al., 2003) to be located at distances of about 15,000 km and 9000 km, inbound and outbound, respectively.

Fig. 4. Magnetic field strength (color scale shown in figure) and field lines in the inner coma of comet Halley from a numerical global 3-dimensional MHD model. From Gombosi et al. (J. Geophys. Res., 1996); copyright 1996 American Geophysical Union.

Why do the solar wind ions, and even the more energetic cometary ions, start to disappear near the cometopause? As the plasma flows closer to the nucleus, where the neutral density is higher, collisional phenomena start to become important (Cravens, 1991b). The process thought to be responsible for the cometopause is charge transfer (Gombosi, 1987; Ip, 1989). In particular, solar wind protons encountering a cometary neutral atom or molecule can remove an electron from a target molecule, becoming a fast neutral H atom in the process (H+

sw + M Hfast + M+, where M can be H2O, OH, O, etc.). The charge transfer cross sections are quite large at solar wind energies, so that this process first becomes important further from the nucleus than for other collisional processes. Obviously, the distance (rcom) depends on the comet activity (i.e., on Q), and the theory (e.g., Gombosi, 1987) predicts that the comet Borrelly cometopause distance is much less than the Halley distance, as is observed (Young et al., 2003). Cometary x-ray production is closely related to this cometopause phenomena, but discussion of this topic

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will be postponed until a later section of the paper.

Fig. 5. Densities of various ion species along the Giotto inbound trajectory (but plotted versus cometocentric distance) from the HIS and HERS sensors ion mass spectrometer (Balsiger et al., 1986; Shelley et al., 1987). The HIS sensor measures higher energy ions than the HERS sensor. From Ip (Astrophys. J., 1989; reproduced by permission of the AAS).

Other collisional phenomena include chemistry, collisional cooling of electrons due to rotational and

vibrational excitation of neutral molecules, and momentum-transfer collisions between ions and neutrals. These momentum transfer collisions play a key role in forming the diamagnetic cavity which was observed to surround the nucleus of comet Halley.

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A large number of cometary volatile species exist (cf. Altwegg et al., 1999), and as these species are ionized and dissociated by solar radiation a complex photochemistry ensues (cf. Huebner et al., 1991; Wegmann et al., 1987). The most abundant ion produced in the inner coma is, not surprisingly, H2O+, but this ion species quickly reacts with H2O and produces H3O+. The observed major ion species in the inner coma is indeed H3O+ (Balsiger et al., 1986; Altwegg et al., 1993), as is evident in Figure 5. H3O+ ions are chemically removed by dissociative recombination with electrons. The photochemical equilibrium expression for the electron density in the inner coma (found by setting local production equal to local loss) yields an electron density that varies inversely with r (cf. Mendis et al., 1985; Cravens, 1991a). Indeed, the density in Figure 5 varies in this manner out to a distance of about 104 km, where transport processes also operate. An example of another important ion-neutral reaction is that of NH3

+ ions, formed by ionization of ammonia, with H2O, which creates NH4

+ ions. The existence of H3O+ ions out to distances past 2 x 104 km in Figure 5 is an indication that chemistry

is important out to this distance, although simple photochemical equilibrium clearly does not apply as evidenced by the local plasma density peak located near 104 km. Ip et al. (1986) and Sauer and Baumgartel (1986) were able to generate a density enhancement theoretically with a reduction of the recombination rate. Cravens (1989a) accomplished this in a one-dimensional MHD model by increasing the electron temperature (which reduces the dissociative recombination rate coefficient). This explanation of the plasma enhancement was pursued in more detail in several papers (Eberhardt and Krankowsky, 1995; Haberli et al., 1995; Gombosi et al., 1996). An as yet unresolved difficulty with this explanation is that the combination of high plasma density and electron temperature results in a high localized plasma pressure that has a noticeable effect on the dynamics and on the magnetic field (Lindgren et al., 1997; Israelevich et al., 1997). Why should the electron temperature increase at 104 km? Gan and Cravens (1990) studied the electron energetics in the inner coma using a theoretical model and found that a “thermal collisionopause” is present at roughly this distance. At smaller distances, the electrons and neutral temperatures are closely coupled by efficient collisional cooling (especially rotational and vibrational cooling), whereas at larger distances the cooling does not keep pace with heating from superthermal electrons. THE INNER COMA AND DIAGMAGNETIC CAVITY

A diamagnetic cavity was observed at comet Halley by the magnetometer onboard the Giotto spacecraft (Neubauer, 1986) with a boundary at a distance of about 5000 km. The existence of a field-free cavity was anticipated prior to Giotto (cf. Mendis et al., 1985) although it was thought that thermal pressure associated with the ionospheric plasma within the cavity withstands the external magnetic pressure, by analogy with the Venus ionopause. However, the data (Figure 5) shows that the plasma density at r ≈ 5000 km (marked CS in the figure) does not undergo an ionopause-type transition. It has been demonstrated that, unlike Venus (for low solar wind dynamic pressure conditions; Luhmann and Cravens, 1991), outward ion-neutral friction is the dominant force responsible for the existence of the cavity (Ip and Axford, 1987; Cravens, 1986). That is, the inward J x B force associated with the barrier is balanced by the outward ion-neutral friction force of neutrals flowing at about 1 km/s past stagnated ions. A simple analytic application of this force balance reproduces the observed magnetic field profile very nicely (Ip and Axford, 1987; Cravens, 1986).

A narrow (50 km) transition layer exists at the cavity boundary at which the plasma density was observed by the Giotto IMS to be enhanced by a factor of about 3 (Goldstein et al., 1989), as shown in Figure 6. The existence of this enhancement was predicted by Cravens (1989a) and was explained as being due to the pile-up of cometary ions which flow outward from the cavity and collide with the magnetic barrier. The “excess” ions in this transition layer dissociatively recombine so that the term of “recombination layer” suggested by Goldstein et al. seems appropriate. The portion of the layer on the cavity-side in the simulations appeared to be shock-like, although the thickness of this “shock” is about the same as the thickness of the whole layer and also about the same as the ion collision mean free path and as the ion gyroradius (Cravens et al., 1995). Nonetheless, following earlier usage this can be called

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Fig. 6. The boundary of the diamagnetic cavity. The top 2 panels are high resolution Giotto ion mass spectrometer abundances in arbitrary units as a function of time (which is the same as distance – the speed of Giotto was about 68 km/s so that the layer is about 50 km across). The bottom panel shows magnetic field strength measured by the Giotto magnetometer which is zero at later times (in the cavity). Notice that the current layer (i.e., steeper slope) has the same location as the density enhancement, in agreement with the models as discussed in the text. From Goldstein et al. (J. Geophys. Res., 1989); copyright 1989 American Geophysical Union.

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the inner shock (Houpis and Mendis, 1980). The microphysics of this layer was also studied with one-dimensional hybrid codes (Cravens et al., 1995; Flammer, 1993; Puhl-Quinn and Cravens, 1995). Collisional processes are incorporated in these codes using Monte Carlo methods. The overall size and nature of the layer was about the same for the fluid (MHD) code and the hybrid code (Puhl-Quinn and Cravens, 1995), but the detailed microstructure differed, not surprisingly. The global shape of the cavity was predicted by the global MHD model results presented by Gombosi et al. (1996) and is elongated in the tailward direction (Figure 4).

Several spacecraft have now encountered comets, but only Giotto saw a diamagnetic cavity. Theory predicts that the distance to the cavity boundary (rcs) increases with gas production rate (see Flammer, 1991). For example, Cravens (1986) predicted that rcs varies as Q3/4. Using this scaling law, the values of rcs for comets G-Z, G-S, and Borrelly are: 580 km, 160 km, 400 km, respectively. None of the respective spacecraft encountering these comets approached the nucleus within these distances; hence, it is not surprising that a cavity was not observed. In any case, it is likely that the nature of the cavity this close to the nucleus of a weak comet might be somewhat different than at comet Halley because the thickness of the boundary layer will be comparable to the distance to the boundary. COMETARY X-RAY EMISSION

X-ray emission from a comet was first discovered in 1996 when the Roentgen satellite (ROSAT) observed about 1 GW of power in the soft x-ray part of the spectrum from comet Hyakutake (Lisse et al., 1996). Several mechanisms were first proposed for this emission but most evidence is now in favor of the solar wind charge exchange (or SWCX) mechanism (see the recent review paper by Cravens, 2002). In this mechanism solar wind heavy (Z > 2) ions, which exist in high charge states, undergo charge exchange collisions with cometary neutrals, leaving the product ion highly excited (Cravens, 1997). One example of such a charge transfer reaction is: O7+ + H2O O6+* + H2O + (2) The product O6+* ion is left in an excited state (principal quantum number typically of 4 to 5) and in the process of de-exciting emits an x-ray photon (that is, O6+* O6+ + hν). For reaction (2), one of the more important transitions is 1s2p 1s2 at an energy of 568.4 eV (Kharchenko and Dalgarno, 2001). Recently, high-resolution spectra of comets LINEAR and McNaught-Hartley have been obtained by the Chandra X-ray Observatory and do indeed show a number of lines including one near 560 eV (Lisse et al., 2001; Krasnopolsky et al., 2002). Such spectra clearly contain information pertinent to solar wind composition.

Figure 7 shows images of comet LINEAR in soft x-rays, in EUV, and in the optical parts of the spectrum. X-ray emission calculated with an MHD model is also shown. Cometary EUV and x-ray observations coupled with modeling can tell us something about the solar wind, but can it tell us anything about comets? As noted by Cravens (1997), the morphology of the x-ray emission (e.g., Figure 7) relates to the cometopause phenomenon discussed earlier in this review. The observed x-ray and EUV emission is, in effect, remotely sensing the disappearance of solar wind ions via charge transfer. The peak emission is located upstream of the nucleus and should be associated with the cometopause region, although this requires further quantification. The x-ray and EUV images are very different from the optical image in which the emission stretches tailward and is due to cometary ions. The location of the peak emission should depend strongly on cometary gas production rate (just as the cometopause does), and, indeed, qualitatively this is what is observed (Dennerl et al., 1997). Cometary x-ray observations should also be able to detect asymmetrically distributed gas distributions in the coma, perhaps due to jets, but further study will be required to develop such applications.

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Fig. 7. The first 3 panels, from right to left, display the following images of comet LINEAR: a soft x-ray image from the Chandra X-ray Observatory, an extreme ultraviolet image from the EUVE satellite, and an optical image. These images are adapted from Lisse et al. (2001). The rightmost image is a simulated image from an MHD calculation of comet Hyakutake (adapted from Häberli et al., 1997). Reprinted with permission from Cravens, T. E., X-ray emission from comets, Science, 296, 1042, copyright 2002 American Association for the Advancement of Science. FUTURE PROSPECTS

What are the future prospects for enhancing our understanding of the cometary plasma environment? Briefly, the answer is: (1) remote observations, particularly in the visible (comet tails), ultraviolet (gas coma composition), and x-rays (solar wind), (2) Discovery or New Millenium-class spacecraft missions, (3) the Rosetta mission, and (4) theoretical modeling of the cometary plasma environment. None of the currently planned Discovery or New Millenium missions, other than the successful DS1 mission, contain plasma experiments (Huntress, 1999), which leaves ESA’s Rosetta mission as the only spacecraft mission on the books with plasma capabilities.

The Rosetta mission, with both an orbiter and a lander, covers a very wide range of cometary science -- from surface studies of the nucleus to plasma properties far from the nucleus. In particular, the Rosetta Orbiter Plasma Consortium (RPC) includes a large number of very capable plasma and fields instruments such as a Langmuir probe, electron and ion spectrometers, an ion mass spectrometer, a magnetometer, and a plasma wave analyzer (Trotignon et al., 1999). Rosetta was scheduled for launch in January 2003, and arrival at comet 46P/Wirtanen was originally expected in 2011. Unfortunately, the Ariane launch vehicle is having difficulties and the launch has prudently been delayed. The most likely new comet target is comet Churyumov-Geramisenko. ACKNOWLEDGMENTS Support is acknowledged from NASA Planetary Atmospheres grant NAG5-11038 and from NSF grant ATM-9815574 to the University of Kansas and from a NASA grant to the University of Michigan. REFERENCES Alfvén, H., On the theory of comet tails, Tellus, 9, 92, 1957. Altwegg, K., H. Balsiger, J. Geiss, et al., The ion population between 1300 and 230,000 km in the coma

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Email address of T. E. Cravens cravens@ku.edu Manuscript received 28 February 2003; accepted 1 July 2003