Icarus. Ion composition and chemistry in the coma of Comet 1P/Halley A comparison between Giotto s Ion Mass Spectrometer and our ion-chemical network

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1 Icarus 199 (2009) Contents lists available at ScienceDirect Icarus Ion composition and chemistry in the coma of Comet 1P/Halley A comparison between Giotto s Ion Mass Spectrometer and our ion-chemical network Martin Rubin a,, Kenneth C. Hansen a, Tamas I. Gombosi a, Michael R. Combi a, Kathrin Altwegg b, Hans Balsiger b a University of Michigan, Space Research Building, 2455 Hayward Street, Ann Arbor, MI 48109, USA b University of Bern, Physikalisches Institut, Sidlerstrasse 5, 3012 Bern, Switzerland article info abstract Article history: Received 9 May 2008 Revised 3 October 2008 Accepted 18 October 2008 Available online 17 November 2008 Keywords: Comet Halley Comets, coma Comets, composition Ionospheres Photochemistry In order to understand the cometary plasma environment it is important to track the closely linked chemical reactions that dominate ion evolution. We used a coupled MHD ion-chemistry model to analyze previously unpublished Giotto High Intensity Ion Mass Spectrometer (HIS-IMS) data. In this way we study the major species, but we also try to match some minor species like the CH x and the NH x groups. Crucial for this match is the model used for the electrons since they are important for ion electron recombination. To further improve our results we included an enhanced density of supersonic electrons in the ion pile-up region which increases the local electron impact ionization. In this paper we discuss the results for the following important ions: C +, CH +, CH + 2, CH+ 3, N+, NH +, NH + 2, NH+ 3, NH+ 4, O+, OH +,H 2 O +,H 3 O +,CO +,HCO +,H 3 CO +,andch 3 OH + 2. We also address the inner shock which is very distinctive in our MHD model as well as in the IMS data. It is located just inside the contact surface at approximately 4550 km. Comparisons of the ion bulk flow directions and velocities from our MHD model with the data measured by the HIS-IMS give indication for a solar wind magnetic field direction different from the standard Parker angle at Halley s position. Our ion-chemical network model results are in a good agreement with the experimental data. In order to achieve the presented results we included an additional short lived inner source for the C +,CH +,andch + 2 ions. Furthermore we performed our simulations with two different production rates to better match the measurements which is an indication for a change and/or an asymmetric pattern (e.g. jets) in the production rate during Giotto s fly-by at Halley s comet Elsevier Inc. All rights reserved. 1. Introduction Neutral gas, produced by the comet as it approaches the Sun, moves along ballistic trajectories with typical speeds of 1 kms 1 until it is ionized by photoionization, electron impact ionization, or charge exchange. For a well-developed cometary atmosphere, the length scale over which these processes take place can be orders of magnitude larger than the nucleus itself. In this so-called mass loading process the newly ionized atoms and molecules are picked-up by the magnetized solar wind plasma. This leads to a continuous deceleration of the solar wind until a weak shock is formed (bow shock) and the flow speeds become subsonic. For Comet 1P/Halley this occurs at roughly 400,000 km from the subsolar point. Giotto however, given by its trajectory, crossed the bow shock further out at roughly 1.1 Mkm. In the following the distances are always given along Giotto s flight path. Close to the comet, crossed by Giotto at approximately 4500 km from the nucleus, an inner shock is formed. From the nucleus to * Corresponding author. Fax: +1 (734) address: rubinmar@umich.edu (M. Rubin). the inner shock the ions move slowly due to the collisional coupling with the dense neutral gas ( 1 kms 1 ) but remain supersonic because of the low temperature and therefore low pressure of the gas. Crossing the inner shock the plasma decelerates to subsonic speeds and is then deflected tailward which leads to an enhanced plasma density. The outer boundary of this narrow increase in ion density is the contact surface which separates the ions coming from the comet and those generated and picked-up further away from the comet. The outer magnetic field is prevented from penetrating and hence the so-called diamagnetic cavity around the nucleus is formed. Outside the contact surface the plasma is moving subsonic due to the higher ion temperature and pressure. Between the bow shock and the contact surface the so-called ion pile-up region can be observed. This increase in ion density is a result of the rise in temperature of the thermal electrons. Starting at roughly a few thousand kilometers from the nucleus, the electron temperature grows, reducing the ion electron recombination rate. The result is a rise in the number density of the ions, peaking around 10,000 km from the comet. Since the electrons cannot be simulated self-consistently in our ion-chemical network, we use a model previously presented by Häberli et al. (1996) as in /$ see front matter 2008 Elsevier Inc. All rights reserved. doi: /j.icarus

2 506 M. Rubin et al. / Icarus 199 (2009) put. It shows that the enhanced temperature of thermal electrons is also colocated with an enhanced density of high energy electrons 1 caused by the draping of the magnetic field-lines on the dayside of the comet. In our model we track the most dominant species along the streamlines extracted from a magnetohydrodynamics (MHD) simulation of Comet 1P/Halley, originally developed by Gombosi et al. (1996). The chemistry code, designed by Häberli et al. (1995), integrates a few hundred chemical reactions (photodissociation, photoionization, electron impact ionization, ion molecule reactions, charge exchange, and dissociative ion electron recombination) including 24 species along these lines. Häberli et al. (1997) validated this code by comparing column densities of the water group ions to measurements performed by DiSanti et al. (1990) with a longslit spectrograph and a narrow-band filter on a CCD camera. We use data collected by the High Intensity Ion Mass Spectrometer (HIS-IMS) from ESA s Giotto mission to Comet Halley in 1986 (Balsiger et al., 1986 and Altwegg et al., 1993) inordertorefine and adjust our chemistry code. Thus we not only look at column densities but at compositions along the Giotto trajectory. Although we are limited to Giotto s inbound trajectory, this comparison to the in situ data gives a more detailed insight into the physical and chemical processes in the coma of a comet and leads to useful information about the gas production rate and the composition of a comet s nucleus. 2. Giotto Ion Mass Spectrometer (IMS) In 1986 ESA s Giotto spacecraft passed on the sunward side of Comet 1P/Halley down to a minimal distance of almost 600 km (more trajectory details given in Reinhard, 1986). On board was the Ion Mass Spectrometer (IMS) with its two sensors, namely the High Energy Range Spectrometer (HERS), optimized for solar wind and hot pickup ions, and the High Intensity Spectrometer (HIS), optimized for the cold, slow part of the ion distribution (see Altwegg et al., 1993). Balsiger et al. (1987) discuss both sensors of the IMS in detail, however, in this work we focus on HIS-IMS. It uses a combination of electric and magnetic analyzers to map out the three-dimensional velocity distribution as a function of the ion s mass per charge ratio over the range from 12 to 56 amu/e. The instrument was aligned along the spin axis which was oriented roughly 600 km in front of the comet and had an intrinsic field of view of This led, together with the spin of the spacecraft and the position of the sensor with respect to the spin axis, to a conical field of view with a half angle of 12 including a 3 region of overlap in the center. The velocity distribution was measured in a limited range around the Giotto/Halley relative velocity of km s 1,bothwithrespecttoabsolutespeed and angles of incidence. The composition of these ions was integrated over one spin period of approximately 4 s inside 40,000 km and over five spin periods outside this distance. When taking the high relative velocity into account, this leads to a spatial resolution of 275 km inside 40,000 km cometocentric distance and 1375 km outside. The Giotto IMS performed continuous measurements during the inbound flight to Comet 1P/Halley. 3. MHD model for Comet 1P/Halley Our magnetohydrodynamic (MHD) model of Comet 1P/Halley is based on that of Gombosi et al. (1996). This model conserves mass, momentum, and energy and self-consistently computes the magnetic field, the plasma mass density, the plasma flow, and the plasma pressure in every cell within the simulation domain. The vast extent of a cometary atmosphere compared to the size of the nucleus demands a highly adaptive refinement of the simulation volume in order to resolve the different sized features including the bow shock as well as the inner shock. In our simulation the resolution varies from km far from the comet down to 60 km in the vicinity of the comet s nucleus in order to resolve the structures described above. The whole simulation domain extends km along the comet sun line 2 and km for both perpendicular directions. The model includes the effects of photoionization, electron impact ionization, recombination, and ion-neutral frictional drag. This is achieved through additional source terms in the MHD equations. The source terms describe the change in mass density, momentum, and pressure caused by these processes as detailed by Gombosi et al. (1996). We numerically solve the consequent system of eight scalar partial differential equations, including mass density, pressure, velocity, as well as magnetic field with a second order (in space as well as in time) parallel scheme. It is based on a HLLE-type Riemann-solver developed by Linde (1998). Additionally, the condition B = 0 is enforced everywhere at all times, which is important especially in shock-capturing codes (see Powell et al., 2001). The electrons play an important part in a comet s chemistry; namely, they directly influence the ion electron (dissociative) recombination and ionization by electron impact. Because the electron temperature cannot be calculated self-consistently in our MHD code we use the model described in the next section. It consists of two parts, one describing the temperature, and the other describing an additional suprathermal electron population. Both are used as input for the MHD, as well as the chemistry code. Table 2 lists the input parameters used in this study including different interplanetary magnetic field (IMF) parameters as discussed in Section 5. Our results indicate that a change in production rate may have occurred during and/or before the Giotto fly-by. Therefore, we used two distinct neutral gas production rates to better our fit of the HIS-IMS data which is also discussed in the same section. For both of these production rates we ran our MHD code until the result reached steady state Electron model Ground-based observations and in situ measurements of Comet 1P/Halley s ion densities showed distinct discrepancies between the measured data and the computer models developed preceding the Giotto mission (see comparison in Altwegg et al., 1993). This is true especially for the ion pile-up region (between 8000 and 15,000 km) which had not been predicted by models at that time. The temperature profile of the thermal electrons has been identified to be crucial for this effect and was originally derived by Ip (1985). The main source of electrons is photoionization of the neutral gas by solar EUV. The electrons carry away the excess energy which is typically between 12 to 15 ev. Furthermore, solar wind electrons and secondary electrons contribute to the population of electrons. Inside the contact surface, the electrons are efficiently cooled by inelastic collisions with the abundant water molecules. Häberli et al. (1996) later proposed another temperature profile of the thermal electrons, based on an additional energy source for the thermal electron gas. The compression of the magnetic field on the dayside of the comet leads to an enhanced flux of solar wind electrons which provide the missing heat to the thermal electron gas in the vicinity of the nucleus. 1 High energy in terms of enough energy for electron impact ionization, typically >15 ev. 2 This distance is equally split into upstream and downstream direction.

3 Ion composition and chemistry in the coma of Comet 1P/Halley 507 Both effects, the temperature of the thermal electrons as well as the population of suprathermal electrons, need to be taken into account for an accurate plasma chemistry model of Comet 1P/Halley. The electron temperature influences the dissociative ion electron recombination whereas the suprathermal electrons are important due to their energy being above the threshold of electron impact ionization. The temperature profile of thermal electrons we have used can be seen in Fig. 1. For the density of suprathermal electrons we introduced a simplified model consisting of a spherically symmetric profile of monoenergetic electrons at a temperature of 220,000 K. Although this simplification cannot be applied to the nightside of the comet, it is an adequate assumption for Giotto s trajectory, passing the ion-pile up region on the day side of the nucleus. The applied radial density profile up to 100,000 km can be seen in Fig. 2. Similar to the work done by Häberli et al. (1997), we set the density of suprathermal electrons in the undisturbed solar wind to be equal to the solar wind density and, in the region inside the bow shock down to a distance of 100,000 km from the comet s nucleus, to a number density of 20 cm 3. Accordingly, we also assume that there is no electron impact ionization inside the contact surface which is consistent with the fact that the electrons are efficiently cooled by collisions with the dense water molecules in the vicinity of the comet. However, in between 5000 km and 100,000 km we fitted the data from Häberli et al. (1996) which has not been incorporated in the previous study. 4. Chemistry model for Comet 1P/Halley We use the plasma flow field computed by our MHD model (shown later on in Fig. 5) as input for our chemistry model (CHI model). First, the chemistry code follows the streamlines in order to determine their origin which can be either cometary or solar. This is important since the boundary conditions are different for both cases. At the surface of the comet the ion densities are very low due to the long ionization scale-lengths. In the latter case the initial composition is set to be equal with the values of the solar wind which is mostly protons and a small fraction of doubly charged helium ions in our model. Along these streamlines we integrate the continuity equations of the main ions in the coma to derive the corresponding ion densities. This allows for a close study also farther away from the comet and extends the previous detailed work by Haider and Bhardwaj (2005) which is limited to the region of radial outflow of the ions inside the magnetic cavity. Our modeled species and corresponding main production and loss mechanisms can be found in Table 1. Fig. 1. The temperature profile of the thermal electrons used for the Halley MHD as well as the chemistry code simulations (Häberli et al., 1996 and Gombosi et al., 1996). Fig. 2. The suprathermal electron density profile (electron energy 19 ev) used for the Halley chemistry code simulations. The arrows in this plot indicate that only lower limits are given for these three points in the work by Häberli et al. (1996). Inside the contact surface there are no suprathermal electrons due to collisional cooling with the water group molecules. Table 1 Modeled ion species. This table lists the ion species tracked in our chemistry model of Comet 1P/Halley. Ion Major production mechanism Major destruction mechanism See Fig. H + Sun H + + H 2 O H 2 O + + H He + He ++ + H 2 O He + + H 2 O + He + + H 2 O H 2 O + + He He ++ Sun He ++ + H 2 O He + + H 2 O + C + C + hv C + + e C + + H 2 O HCO + + C Fig. 8 CH + CH 2 + hv CH + + H + e CH + + H 2 O H 3 O + + C Fig. 9 CH + 2 CH 2 + hv CH e CH+ 2 + H 2O H 3 CO + + H Fig. 10 CH + 3 CH 2 + H 3 O + CH H 2O CH e neutral products Fig. 11 N + N + hv N + + e N + + H 2 O H 2 O + + N Fig. 10 NH + NH + hv NH + + e NH + + H 2 O H 2 O + + NH Fig. 11 NH + 2 NH 2 + hv NH e NH+ 2 + H 2O H 3 O + + NH Fig. 12 NH + 3 NH 2 + H 3 O + NH H 2O NH e neutral products Fig. 13 NH + 4 NH 3 + H 3 O + NH H 2O NH e neutral products Fig. 14 O + O + hv O + + e O + + H 2 O H 2 O + + O Fig. 12 OH + OH + hv OH + + e OH + + H 2 O H 2 O + + OH Fig. 13 H 2 O + H 2 O + hv H 2 O + + e H 2 O + + H 2 O H 3 O + + OH Fig. 14 H 3 O + H 2 O + + H 2 O H 3 O + + OH H 3 O + + e neutral products Fig. 15 CO + CO + hv CO + + e CO + + H 2 O H 2 O + + CO Fig. 16 HCO + H 2 O + + CO HCO + + OH HCO + + H 2 O H 3 O + + CO Fig. 17 H 3 CO + H 2 CO + H 3 O + H 3 CO + + H 2 O H 3 CO + + e neutral products Fig. 18 CH 3 OH + 2 CH 3 OH + H 3 O + CH 3 OH H 2O CH 3 OH e neutral products Fig. 19

4 508 M. Rubin et al. / Icarus 199 (2009) Neutral coma We use a spherically symmetric model for the neutral coma as described by Haser (1957) with the neutral gas production rates listed in Tables 2 and 3. The model includes daughter species, as listed in Table 3, which result from photodissociation of the parent neutrals. The hydrogen number density is derived from the work of Combi (1996) by scaling to the water production rates indicated in Table 2. From the same work, we obtain a model for the velocities of the cometary neutral gas. A selected list of reaction frequencies for the photodissociation are shown in Table 4. Fig. 3 shows the measured neutral number density profile of mass/charge = 28 amu/e, corresponding mainly to carbon monoxide CO, measured by the NMS (neutral mode), Giotto s neutral mass spectrometer (see Krankowsky et al., 1986, and Reber, 1995). The measurements are corrected for the CO yield given in Meier (1992) assuming that CO is the major neutral at mass/charge = 28 amu/e. Also shown in this figure is our neutral model for two different production rates as indicated by the solid line for the lower production rate Q 1 and the dotted line for the higher production rate Q 2 (Q 1 = 60% Q 2 ). The figure clearly shows that neither profile fits the entire fly-by, but together the two models are a good fit. We used two point sources which both differ by only their production rates. This temporal variation of the production rate also serves as an alternative explanation to the model discussed by Eberhardt et al. (1987) including a point and an extended source for carbon monoxide. As later discussed in Section 5, the transition region for the neutrals around 10,000 km from the nucleus is closer than observed for the ions: Outside the contact surface the ions are decoupled from the radial outflowing neutrals and have a very distinct flow pattern from the neutrals. As a result of the interaction of the solar wind with the cometary plasma, the ions form a complex and asymmetric profile compared to the radially expanding neutral coma. Since ions are not only produced locally but also transported along one of those two sets of streamlines with either cometary or upstream solar wind starting points, this transition region can be in a different place for the ions compared to the neutrals Source of ions The cometary coma can be assumed to be optically thin, therefore the photoionization rates are constant throughout the simulation domain. We use the frequency integrated values given by Huebner et al. (1992). For a few minor species the reaction rates have been estimated by comparison with known rates from similar molecules. The values can be found in Table 5. Table 4 Photodissociation. Frequencies for selected photodissociation reactions. Reaction Frequency [s 1 ] Reference H 2 O + hv OH + H Huebner et al. (1992) H 2 O + hv O + H Huebner et al. (1992) OH + hv O + H Schleicher and A Hearn (1988) CO + hv C + O Huebner et al. (1992) CO 2 + hv CO + O Huebner et al. (1992) H 2 CO + hv products Meier et al. (1993) NH 3 + hv NH 2 + H Huebner et al. (1992) NH 2 + hv NH + H Huebner et al. (1992) NH 2 + hv N + H Huebner et al. (1992) NH + hv N + H Fink et al. (1991) CH 4 + hv products Huebner et al. (1992) CH 3 OH products Huebner et al. (1992) Table 2 MHD model input parameters. This table lists the parameters we used for the MHD model of Comet 1P/Halley for the Giotto fly-by on March 14, Quantity Symbol Value Distance from the Sun R h 0.90 AU Gas production rate Q s 1 High gas production rate Q s 1 Solar wind velocity u SW 371 km s 1 Solar wind density n SW 8.0 cm 3 Interplanetary magnetic field (IMF) magnitude B 4.81 nt IMF longitude a B long 20.9 IMF latitude a B lat 40.8 Radial neutral gas velocity u n 1kms 1 Ionization scale length b λ b 10 6 km Ion neutral momentum transfer collision rate k in cm 3 s 1 Mean molecular mass of cometary ions m c 17 amu a Regarding to the Halley Sun Ecliptic (HSE) coordinate system with the negative x-axis pointing at the Sun (see Fig. 7b). b In free streaming solar wind. Fig. 3. Mass/charge = 28 amu/e density profile (mainly carbon monoxide) as measured by Giotto s neutral mass spectrometer NMS in neutral mode (see Reber, 1995) as well as used as input in the neutral coma model of the chemistry code. The lower production rate Q 1 is represented by the solid line whereas the dotted line shows the higher production rate Q 2 (see Tables 2 and 3). The NMS measurements are corrected for the CO yield (see Meier, 1992) assuming CO being the dominant species with mass/charge = 28 amu/e. Table 3 Neutral coma model. This table lists the parameters we used for the neutral coma model as input for the chemistry model (see also Eberhardt, 1999). Furthermore we ve added an extended source for C +,CH +,andch + 2 ions with a short scale length of only 220 km to fit the IMS data close to the nucleus. Molecule Abundance rel. [%] Reference Included daughter species H 2 O 100 Definition OH, O, H CO 13 This work (see Fig. 3) O, C CO 2 3 Krankowsky et al. (1986) CO,O,C H 2 CO 3.8 Geiss et al. (1991), Meier et al. (1993) CO,O,C CH 3 OH 1 Geiss et al. (1991) CH Altwegg et al. (1994) CH 3,CH 2 C 2 H Eberhardt (1999) CH 3,CH 2 NH Meier et al. (1994) NH 2,NH,N HCN 0.2 Eberhardt (1999) N

5 Ion composition and chemistry in the coma of Comet 1P/Halley 509 The electron impact ionization rates used can be found in Table 6. The cross sections are taken from measurements made by Hwang et al. (1996) and Orient and Srivastava (1987) or have been calculated according to the Binary-Encounter-Bethe (BEB) model (see e.g. Kim and Desclaux, 2002). As for photoionization, we have estimated a few reaction rates from rates for similar molecules. These values have then been multiplied by the electron velocity assuming a constant electron energy of 19 ev from our previously discussed electron model. With this rough approximation we simplify the actual, much more complex energy distribution shown in Rème et al. (1987). Furthermore, it also reduces the amount of electron energy dependent cross sections to be estimated. We have added some new processes that were not previously included in the work done by Häberli et al. (1997): the effect of electron impact dissociation and photodissociation of molecular ions. Among estimated rate constants, we used the values given by Lecointre et al. (2006), Vane et al. (2007), Bannister et al. (2003), and Schulz et al. (1986). Since our modeled ion densities for C + and O + ions are too low in the ion pile-up region we ve studied Table 5 Photoionization. Frequencies for selected photoionization reactions. Reaction Frequency [s 1 ] Reference H 2 O + hv H 2 O + + e Huebner et al. (1992) H 2 O + hv OH + + H + e Huebner et al. (1992) H 2 O + hv O + + H 2 + e Huebner et al. (1992) H 2 O + hv H + + OH + e Huebner et al. (1992) OH + hv OH + + e Huebner et al. (1992) OH + hv O + + H + e Estimate OH + hv H + + O + e Estimate O + hv O + + e Huebner et al. (1992) H + hv H + + e Huebner et al. (1992) C + hv C + + e Huebner et al. (1992) N + hv N + + e Huebner et al. (1992) CO + hv CO + + e Huebner et al. (1992) CO + hv C + + O + e Huebner et al. (1992) CO + hv O + + C + e Huebner et al. (1992) CO 2 + hv CO + + O + e Huebner et al. (1992) CO 2 + hv O + + CO + e Huebner et al. (1992) CO 2 + hv C + + O 2 + e Huebner et al. (1992) H 2 CO + hv H 2 CO + + e Huebner et al. (1992) H 2 CO + hv HCO + + H + e Huebner et al. (1992) H 2 CO + hv CO + + H 2 + e Huebner et al. (1992) CH 3 OH + hv CH 3 OH + + e Huebner et al. (1992) NH 3 + hv NH e Huebner et al. (1992) NH 3 + hv NH H + e Huebner et al. (1992) NH 3 + hv NH + + H 2 + e Huebner et al. (1992) NH 3 + hv N + + H 2 + H + e Huebner et al. (1992) NH 2 + hv NH e Estimate NH 2 + hv NH + + H + e Estimate NH + hv NH + + e Estimate NH + hv N + + H + e Estimate processes that could provide the missing ions. Dissociation processes would lead to an enhanced density of atomic ions instead of molecular ions. However, due to the fact that the densities of the neutral species are higher by several orders of magnitude compared to the corresponding ions these reactions introduced only marginal changes in the modeled ion densities Charge exchange and ion molecule reactions The most important charge exchange rates can be found in Table 7. We use the cross sections given by Banks and Kockarts Table 6 Electron impact ionization. Rates for selected electron impact ionization reactions. The corresponding electron energy is assumed to be 18 ev ( 200,000 K). Reaction Rate constant [cm 3 s 1 ] Reference H 2 O + e H 2 O + + 2e Rao et al. (1995) H 2 O + e OH + + H + 2e Rao et al. (1995) H 2 O + e O + + H 2 + 2e Rao et al. (1995) H 2 O + e H + + OH + 2e Rao et al. (1995) OH + e OH + + 2e Estimate OH + e O + + H + 2e Estimate OH + e H + + O + 2e Estimate O + e O + + 2e Kim and Desclaux (2002) H + e H + + 2e Kim and Rudd (1994) C + e C + + 2e Kim and Desclaux (2002) N + e N + + 2e Kim and Desclaux (2002) CO + e CO + + 2e Orient and Srivastava (1987) CO + e C + + O + 2e Orient and Srivastava (1987) CO + e O + + C + 2e Orient and Srivastava (1987) CO 2 + e CO + + O + 2e Orient and Srivastava (1987) CO 2 + e O + + CO + 2e Orient and Srivastava (1987) CO 2 + e C + + O 2 + 2e Orient and Srivastava (1987) H 2 CO + e H 2 CO + + 2e Kim and Irikura (2000) H 2 CO + e HCO + + H + 2e Estimate H 2 CO + e CO + + H 2 + 2e Estimate CH 3 OH + e CH 3 OH + + 2e Estimate NH 3 + e NH e Kim et al. (1997b) NH 3 + e NH H + 2e Estimate NH 3 + e NH + + H 2 + 2e Estimate NH 3 + e N + + H 2 + H + 2e Estimate NH 2 + e NH e Estimate NH 2 + e NH + + H + 2e Estimate NH + e NH + + 2e Estimate NH + e N + + H + 2e Estimate CH 4 + e CH H + 2e Kim et al. (1997b) CH 3 + e CH e Kim et al. (1997b) CH 3 + e CH H + 2e Estimate CH 2 + e CH e Hwang et al. (1996) CH 2 + e CH + + H + 2e Estimate CH + e CH + + 2e Kim et al. (1997a) Table 7 Charge exchange. This table lists selected charge exchange cross sections in our chemistry model of Comet 1P/Halley. Reaction Cross section [10 16 cm 2 ] Reference H + + O O + + H ( ln v) 2 Banks and Kockarts (1973) H + + H 2 O H 2 O + + H ( ln v) 2 Tawara (1978) H + + OH OH + + H ( ln v) 2 Estimate H + + NH NH + + H ( ln v) 2 Estimate H + + CO 2 CO H ( ln v)2 Tawara (1978) H + + CO CO + + H ( ln v) 2 v cm s 1 Tawara (1978) [ v 2 exp( v)] 2 v < cm s 1 Tawara (1978) H + + N N + + H ( ln v) 2 v cm s 1 Rapp and Francis (1962) [ v exp( v)] 2 v < cm s 1 Rapp and Francis (1962) H + + C C + + H ( ln v) 2 v cm s 1 Rapp and Francis (1962) [ v exp( v)] 2 v < cm s 1 Rapp and Francis (1962) OH + + H 2 O H 2 O + + OH ( ln v) 2 Estimate CO + + H 2 O H 2 O + + CO ( ln v) 2 Estimate CO + + OH OH + + CO ( ln v) 2 Estimate CO + + CO 2 CO CO ( ln v)2 Estimate

6 510 M. Rubin et al. / Icarus 199 (2009) Table 8 Ion molecule reactions. Rates for selected ion molecule reactions. Reaction Rate constant [cm 3 s 1 ] Reference H 2 O + + H 2 O H 3 O + + OH Huntress (1977) H 2 O + + NH 2 NH OH Millar et al. (1991) H 2 O + + NH 3 NH H Anicich (1993) H 2 O + + H 3 NH OH Anicich (1993) H 2 O + + H 2 CO H 3 CO + + OH Anicich (1993) H 2 O + + H 2 CO H 2 CO + + H 2 O Anicich (1993) H 2 O + + CO HCO + + OH Anicich (1993) H 2 O + + CH 3 OH CH 3 OH OH Estimate H 3 O + + NH 2 NH H 2O Estimate H 3 O + + NH 3 NH H 2O Anicich (1993) H 3 O + + H 2 CO H 3 CO + + H 2 O Anicich (1993) H 3 O + + CH 3 OH CH 3 OH H 2O Anicich (1993) OH + + H 2 O H 3 O + + O Anicich (1993) OH + + H 2 O H 2 O + + OH Anicich (1993) OH + + CO HCO + + O Anicich (1993) OH + + H 2 CO H 2 CO + + OH Anicich (1993) OH + + H 2 CO H 3 CO + + O Anicich (1993) OH + + CO 2 HCO O Anicich (1993) OH + + NH 3 NH OH Anicich (1993) OH + + NH 3 NH O Anicich (1993) O + + H 2 O H 2 O + + O Anicich (1993) O + + OH OH + + O Estimate O + + H 2 CO HCO + + OH Anicich (1993) O + + H 2 CO H 2 CO + + O Anicich (1993) O + + NH 3 NH O Anicich (1993) O + + CO 2 O CO Anicich (1993) O + + CH 3 OH H 3 CO + + OH Anicich (1993) H + + H 2 O H 2 O + + H Anicich (1993) H + + OH OH + + H Anicich (1993) H + + CO 2 HCO + + O Anicich (1993) H + + O O + + H Anicich (1993) H + + NH 3 NH H Anicich (1993) H + + NH NH + + H Estimate CO + + H 2 O H 2 O + + CO Anicich (1993) CO + + H 2 O HCO + + OH Anicich (1993) CO + + NH 3 NH CO Anicich (1993) CO + + H 2 CO H 2 CO + + CO Anicich (1993) CO + + H 2 CO HCO + + HCO Anicich (1993) CO + + OH OH + + CO Anicich (1993) CO + + CO 2 CO CO Anicich (1993) CO + + CH 2 CH CO Estimate NH H 2O NH OH Anicich (1993) NH NH 3 NH NH Anicich (1993) NH H 2CO NH HCO Anicich (1993) NH CH 3OH NH CH 3O Anicich (1993) NH CH 2 CH NH Estimate NH H 2O H 3 O + + NH Anicich (1993) NH NH 3 NH NH Anicich (1993) NH NH 3 NH NH Anicich (1993) NH H 2CO H 3 CO + + NH Anicich (1993) NH CH 3OH CH 3 OH NH Anicich (1993) NH CH 2 CH NH Estimate H 3 CO + + NH 3 NH H 2CO Anicich (1993) H 3 CO + + NH 2 NH H 2CO Estimate H 3 CO + + CH 2 CH H 2CO Estimate H 3 CO + + CH 3 OH CH 3 OH H 2CO Anicich (1993) CH 3 OH NH 3 NH CH 3OH Estimate CH 3 OH NH 2 NH CH 3OH Estimate CH 3 OH CH 2 CH CH 3OH Estimate N + + H 2 O H 2 O + + N Anicich (1993) N + + CO CO + + N Anicich (1993) N + + CO 2 CO N Anicich (1993) N + + H 2 CO H 2 CO + + N Anicich (1993) N + + CH 3 OH products Anicich (1993) N + + NH 3 NH N Anicich (1993) C + + H 2 O HCO + + H Anicich (1993) C + + H 2 CO CH CO Anicich (1993) C + + CH 3 OH CH HCO Anicich (1993) C + + CO 2 CO + + CO Anicich (1993) (1973) and Tawara (1978) and the theory described by Rapp and Francis (1962) in order to cover the missing data. In contrast to the charge exchange reactions, where no direct collision is needed in order to exchange an electron, exothermic ion molecule reactions are often directly linked to the collision rate. Thus, if no activation energy is needed these reactions can take place at very low temperatures (T < 150 K) where the relevant reduced ion-neutral temperature is given by T r = (m i T n + m n T i )/(m n + m i ). There are further adjustments for the collision rates in case the molecule has a permanent dipole (see Average Dipole Orientation (ADO) theory by Moran and Hamill, 1963). The ion molecule reaction cross sections we use can be found in Table Loss of ions Dissociative ion electron recombination, which is the main loss process of cometary ions, varies with the temperature of the thermal electrons by typically Te 0.5 for T e < 1000 K (see McGowan and Mitchell, 1984). Some polyatomic ions, however, have a steeper decrease in their recombination rate with temperature (see e.g. Larson et al., 1998). For the two most abundant ions, H 3 O + and H 2 O +, we use the values measured and modeled for different temperature ranges up to 30,000 K by Mul et al. (1983). For most other polyatomic ions where no experimental data were available we estimate the ion electron recombination rates being similar to the one of H 3 O +. Table 9 lists a selected set of dissociative ion electron dissociation rates. It is obvious that the actual reaction rates depend directly on the temperature profile used for the thermal electrons discussed above. 5. Comparison with observations Fig. 4 shows the total mass density around 1P/Halley modeled with our MHD code. The bow-shock is clearly visible at a subsolar distance of roughly 400,000 km. This figure also shows Giotto s inbound trajectory along which the HIS-IMS collected the data presented in this work. Fig. 5 isaclose-up,fromthesamesimulation, showing the cometary nucleus, the ion pile-up at roughly 10,000 km, and the inner shock at 4500 km. Modeled ion bulk flow vectors along the Giotto trajectory can be seen as well as the velocity streamlines in the Halley Sun Ecliptic (HSE) plane. This figure also shows two separated plasma populations along the Giotto trajectory: Close to the nucleus the plasma flows radially outward due to the collisional coupling with the radial outflowing neutrals and is then decelerated and deflected towards the tail at the inner shock whereas the ions picked-up upstream also move tailward but are deflected around the contact surface. As outlined above, two different production rates seem to be necessary to match the neutral measurements. We find that using these rates is also required to better match the ion measurements performed with the HIS-IMS. Fig. 6 shows a comparison of the modeled total ion density profile (mass/charge = amu/e) and the measurements performed by HIS-IMS. The small bump in the simulated density corresponds to the inner shock at roughly 4550 km, just inside the contact surface (CS) which itself is denoted by the black vertical line. As seen in Fig. 5 the plasma populations inside and outside the contact surface are separated, and the flow velocity directions are different, however the densities inside and outside the shock are alike. Furthermore, the ion pile up region around 10,000 km can also be seen even though it is not as prominent as measured by the Giotto IMS. The main effect for this rise in ion density is the reduction in recombination of H 3 O + with thermal electrons caused by the rising electron temperature given in Fig. 1. The second effect responsible is the rise in electron impact ionization frequency caused by the higher number density

7 Ion composition and chemistry in the coma of Comet 1P/Halley 511 Fig. 4. The total mass density [amu/cm 3 ] in the Sun Halley Giotto plane is plotted from the MHD model of Comet 1P/Halley. The Sun is on the left side and the plotted domain km km. The bow shock at roughly 400,000 km upstream from the comet s subsolar point is clearly visible. The thick black line shows Giotto s inbound trajectory whereas the thin grid shows the regions of different resolution (the edge length of the cells is cut in half at each boundary) allowing to resolve the different sized structures. Fig. 5. Closer view into the total mass density [amu/cm 3 ] in the Sun Halley Giotto plane from the MHD model of Comet 1P/Halley. The thick black line shows Giotto s inbound trajectory including the ion bulk velocities in different positions. Furthermore, on the outbound side, several streamlines which are used for the integration in the chemistrycodeareplottedinred.thesunisontheleftsideandtheplotteddomain km km. The ion pile-up region (a) which intersects with Giotto s trajectory at 10,000 km from the nucleus and the inner shock (b) at 4500 km surrounding the diamagnetic cavity (c) are clearly visible. The contact surface at the outer edge of the inner shock separates the ions generated inside the contact surface from those generated outside, which can be clearly seen in the pattern of the plotted streamlines. of suprathermal electrons discussed before and presented in Fig. 2. Beyond a cometocentric distance of 20,000 km, we ve also plotted the results with a higher total production rate Q 2 (Q 1 = 60% Q 2 ; Table 2 with all species scaled by the same ratio). The cometocentric distance of the apparent transition from the lower to the higher production rate is different for the individual species due to the different ion velocities (see Figs. 8 19); A fact we are not able to reproduce with the single fluid MHD model used as input for the chemistry code. Nonetheless the apparent transition for the ions is always further out than for the neutrals as discussed in Section 4.1. We present two possible explanations for the apparent transition from low to high production rates. The first is an asymmetric production rate due to regions of different activity on the nucleus combined with the comet s rotation, as for instance observed in images of Comet 1P/Halley taken in the light of the CN

8 512 M. Rubin et al. / Icarus 199 (2009) Fig. 6. Plotted is the total number density for mass/charge = amu/e ions along the Giotto trajectory from the chemistry code using two production rates Q 1 and Q 2 (plotted outside 20,000 km) and Giotto s HIS-IMS data (points). Representative for the following pictures, the vertical line just outside the inner shock shows the position of the contact surface (CS) which separates the inner (radially outflowing) from the outer (deflected around the CS) ion population as shown by the streamlines in Fig. 5. radical (A Hearn et al., 1986). Because of the low neutral gas velocity ( 1 kms 1 ) compared to the vast extent of the cometary structures the signature of a jet or a change in production rate is preserved for several hours. Another possible reason is a change in the neutral production rate preceding and during the time of Giotto s fly-by. However Schleicher et al. (1986) reported only a slightly higher production rate of CN of less than 10% a few hours before the fly-by, which is the time needed by the neutral gas to reach the mentioned 20,000 km distance from the comet s nucleus. Doing two steady-state simulations with our ion-chemical model in order to simulate these dynamical and time-depended effects is only an approximation. However, since the ion densities at a certain point are the sum of all precedent reactions along the corresponding streamline, an accurate model including all the dynamics like jets and change in production rate is not feasible without a large number of estimations. Fig. 7 shows a comparison of our modeled flow field (red solid: production rate Q 1 ; red dashed: Q 2 ) with the measurements given in Altwegg et al. (1993) (blue). In the following we will discuss all eight panels shown in this figure. The interplanetary magnetic field (IMF) has a very distinct influence on the plasma flow field but is not very well known for the time of the fly-by. Raeder et al. (1987) presented evidence for IMF rotation by multiple crossings of current layers inside the coma. Israelevich et al. (1994) then used this data to reconstruct the interplanetary magnetic field configuration during the fly-by and found complete reversals of the field. Furthermore VEGA, ICE, and Sakigake were far away from the field lines hitting the comet during Giotto s fly-by and thus their magnetic field data cannot be used as a reference. Our initial simulation used the standard Parker angle at Halley s position, with the field in the Halley Sun Ecliptic plane. We find that if we rotate the IMF vector around the Sun comet axis we can better fit the measured flow directions. Our simulation shows that the flow field is quite different in the plane containing the upstream solar wind IMF and the one perpendicular to it (as shown in the case of comet 67P/Churyumov Gerasimenko in Hansen et al., 2007). The four panels on the left side show the results obtained with the IMF vector in the ecliptic plane centered at Comet Halley (Halley Sun Ecliptic plane) with an angle of roughly 45 (in the HSE coordinate system) corresponding to the nominal Parker angle identical to the original work by Gombosi et al. (1996). The four panels on the right side show the result when turning the magnetic field vector counterclockwise by 67.5 around the Sun comet axis. Figs. 7a and 7b give the modeled ion bulk flow vectors as well as the measurements from HIS-IMS projected into the HSE plane. Included is also the alignment of the magnetic field with respect to the Giotto trajectory in the top right corner of the corresponding panel. It can be seen clearly that the direction of the magnetic field directly influences the direction of the plasma flow. Taking the azimuthal resolution of HIS-IMS of 22.5 into account, the ion bulk flow vectors are in much better agreement in the case of the turned IMF in panel (b) compared to (a). Evidently the lower production rate (red solid vectors) leads to higher plasma velocities when compared to the higher production rate (red dotted vectors) when projected into the HSE plane. This is also shown in panels (c) and (d). The results in both panels indicate that using two different production rates reproduces the measurements better. According to the results we find the transition region being between 20,000 and 30,000 km from the comet. Panels (a) through (d) are given in linear scale and the data is limited to 50,000 km. The total plasma velocity given in (e) and (f) also includes the velocity component vertical to the HSE plane. The profile in panel (f), where the field is rotated, is also in better agreement with the HIS-IMS data as opposed to (e) where the plasma velocity for the higher production rate is too low to fit the HIS-IMS data. Panels (g) and (h) both present the total ion number densities (mass/charge = amu/e ions) with panel (h) equivalent to Fig. 6. There are only minor differences between panels (g) and (h): Turning the interplanetary magnetic field moves the ion pile-up region slightly outwards. This effect is more pronounced when turning the magnetic field by a full 90 into the Sun comet pole plane perpendicular to the HSE plane (not shown here). However, doing so significantly lowers the agreement of the modeled ion number density profile along the Giotto trajectory with the measured data. Obviously there are far more IMF configurations possible than a simple rotation around the Sun comet axis only. The limited amount of data does not allow for a more detailed study, however, while the simple exercise of rotating the field may not uniquely determine the field orientation and strength, it shows that the field orientation can strongly influence the plasma data. Results in the plane of the field and perpendicular to it show very different characteristics. Nevertheless it can be seen that our model is in good agreement within the region of interest, namely along the Giotto trajectory. Farther from the comet, where the angle between the ion flow direction and Giotto s trajectory is large and the ion velocities exceed a certain value, it is possible that the 12 half angle of the HIS-IMS instrument entrance aperture prevents the incoming ions from entering the analyzer section of the sensor (see Balsiger et al., 1987). According to our MHD model, for a mean ion mass of 17 amu this effect becomes important above a cometocentric distance of roughly 80,000 km where a high ion velocity of 17 km s 1 and an incident angle of roughly 60 with respect to Giotto s inbound trajectory cuts off the incoming ions. Since the ion velocities are a function of their mass this effect is more important for the lower mass ions. This is a possible explanation for the ion densities of the lighter ions dropping off faster than predicted by the model as can be seen later on in some of the results Ion mass 12 to 14 The mass/charge = 12 amu/e in our model is represented by C + ions, stemming mainly from photoionization of neutral carbon which itself should come primarily from photodissociation of CO, CO 2, and H 2 CO. The results are plotted in Fig. 8. The main loss process of the C + ion is the reaction with H 2 O since it does not recombine with the thermal electrons. The disagreement between

9 Ion composition and chemistry in the coma of Comet 1P/Halley 513 Fig. 7. The left column represents the magnetic field settings used in Gombosi et al. (1996) and the right column the present work. Panels (a) and (b) show the projection of the cometary ion bulk flow vectors into the Halley Sun Ecliptic (HSE) plane (the solar wind magnetic field vector used as boundary condition for the simulation and Giotto s trajectory are shown in the top right corner, the xy-plane being the HSE plane) and panels (c) and (d) the corresponding velocity projected into the HSE plane. Panels (a) through (d) are in linear scale. Panels (e) and (f) show the absolute velocity and the ion number density is given in (g) and (h). Panels (e) through (h) are giveninlogarithmic scale. All panels include both production rates Q 1 (red solid) and Q 2 (red dashed) as well as the HIS measurements (blue; Altwegg et al., 1993). our results and the data in the ion pile-up region is due to the fact that the amount of neutral carbon in our model is a factor of 10 too low when only including the above described processes, a result which has been observed by Woods et al. (1987) based on

10 514 M. Rubin et al. / Icarus 199 (2009) Fig. 8. Mass/charge = 12 amu/e density profile including the C + ion along the Giotto trajectory from the chemistry code with two production rates Q 1 and Q 2 (plotted outside 20,000 km) compared to the Giotto IMS data from the HIS sensor (points). the high abundances of C +,C + 2, and especially C+ 3 measured with the HERS-IMS sensor in between 35,000 and 170,000 km from the comet. Ogilive et al. (1986) proposed large ring carbon clusters (C n with n 4) produced by evaporation from the nuclei as a source of C, C 2,C 3 in Halley s coma. Their laser experiments 3 in the laboratory showed that carbon clusters are broken up and form low allotropes 4 in the shape of chains of less than ten atoms. By dissociation these chains then form C n with n 3. This leads to a significantly higher neutral carbon density in the region of interest and up the streamlines and thereby enhances the density of C + ions in the ion pile-up region. This is also consistent with the fact that the modeled hydrocarbons are a much better fit to the measured data: The disagreement in the model is limited to C +. However, other C + n ions need to be investigated in the future in order to sustain this explanation and be consistent with the observed abundances of neutral C 2 and C 3. Further uncertainties come from the simplified model for the suprathermal electrons or additional dynamic effects such as jets as well as the possible change in total production rate. The results for mass/charge = 13 amu/e, which is represented by CH + and shown in Fig. 9, are very similar, although the agreement in the ion pile-up region is better compared to the mass/charge = 12 amu/e case, consistent with the proposed carbon clusters as an additional source. The major production process is the photoionization of neutral CH and the major loss process the reaction with H 2 OasitisfortheC + ion. Fig. 10 shows the results for N + and CH + 2 as well as the sum of both in order to compare with the mass/charge = 14 amu/e density profile measured by the HIS-IMS. Also here we added, for the sum, the results for a higher production rate. The major production mechanism for both ions is photoionization of their neutral counterparts, N and CH 2, whereas the major loss mechanism for both is the reaction with neutral water H 2 O A source for hydrocarbons in the innermost coma Our model was not able to reproduce the steep decrease in ion density within roughly 3000 km for m/e = 12, 13, and 14 amu/e without including an additional extended source. If this additional 3 The measurements also indicated that this process does not strongly depend on the used laser wavelength of 248 nm ( 5 ev). 4 Pure forms of the same element with different structure (graphite, diamond, amorphous carbon etc.). Fig. 9. Mass/charge = 13 amu/e density profile including the CH + ion along the Giotto trajectory from the chemistry code with two production rates Q 1 and Q 2 (plotted outside 20,000 km) compared to the Giotto IMS data from the HIS sensor (points). Fig. 10. Mass/charge = 14 amu/e density profile including the CH + 2 and N+ ions along the Giotto trajectory from the chemistry code with two production rates Q 1 and Q 2 (plotted outside 20,000 km) compared to the Giotto IMS data from the HIS sensor (points). Both tracked mass/charge = 14 amu/e ions,ch + 2 and N+,arealso plotted individually. source is turned off, the profiles are much flatter inside the contact sheath at roughly 4550 km. The effect of the additional source on the C +,CH +, and CH + 2 ion densities can be seen in all three figures. The main loss process for all three ions is the reaction with H 2 O, the dominant neutral species in this region. In order to fit the steep slope in the HIS-IMS data, the scale length of the source needs to be very short, roughly 220 km as we found. It is assumed that ionized C +, CH +, and CH + 2 is directly produced from this source and not through the dissociation channel of neutral CH 2 CH C with the following ionization, since the combined lifetime of these steps is way too long, especially in the case of the carbon ion. Since we used the same lifetime (scale length) for all the three additional sources (C +,CH +, and CH + 2 ions) we have an indication that the source is the same for all three species. A possible candidate is polyaromatic hydrocarbons (PAHs) stemming from organic CHON particles. However, this alone does not explain the steep drop off in ion density within the first few hundred kilometers of the comet s nucleus. Müller et al. (2002) modeled the incident sunlight and ambient radiation reaching the comet s surface within a cometary dust coma for different comets if the nucleus radius and