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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 107, NO. A8, 1170, 10.1029/2001JA000125, 2002 Pickup ions near Mars associated with escaping oxygen atoms T. E. Cravens, A. Hoppe, and S. A. Ledvina1 Department of Physics and Astronomy, University of Kansas, Lawrence, Kansas, USA S. McKenna-Lawlor Space Technology Ireland, National University of Ireland, Maynooth, Co. Kildare, Ireland Received 3 May 2001; revised 4 December 2001; accepted 7 December 2001; published 7 August 2002. [1] Ions produced by ionization of Martian neutral atoms or molecules and picked up by the solar wind flow are expected to be an important ingredient of the Martian plasma environment. Significant fluxes of energetic (55–72 keV) oxygen ions were recorded in the wake of Mars and near the bow shock by the solar low-energy detector (SLED) charged particle detector onboard the Phobos 2 spacecraft. Also, copious fluxes of oxygen ions in the ranges 0.5–25 and 0.01–6 keV/q were detected in the Martian wake by the Automatic Space Plasma Experiment with Rotating Analyzer (ASPERA) instrument on Phobos 2. This paper provides a quantitative analysis of the SLED energetic ion data using a test particle model in which one million ion trajectories were numerically calculated. These trajectories were used to determine the ion flux as a function of energy in the vicinity of Mars for conditions appropriate for Circular Orbit 42 of Phobos 2. The electric and magnetic fields required by the test particle model were taken from a threedimensional magnetohydrodynamic (MHD) model of the solar wind interaction with Mars. The ions were started at rest with a probability proportional to the density expected for exospheric hot oxygen. The test particle model supports the identification of the ions observed in channel 1 of the SLED instrument as pick-up oxygen ions that are created by the ionization of oxygen atoms in the distant part of the exosphere. The flux of 55–72 keV oxygen ions near the orbit of the Phobos 2 should be proportional to the oxygen density at radial distances from Mars of about 10 Rm (Martian radii) and hence proportional to the direct oxygen escape rate from Mars that is an important part of the overall oxygen loss rate at Mars. The modeled energetic oxygen fluxes also exhibit a spin INDEX TERMS: 2780 modulation as did the SLED fluxes during Circular Orbit 42. Magnetospheric Physics: Solar wind interactions with unmagnetized bodies; 2152 Interplanetary Physics: Pickup ions; 5421 Planetology: Solid Surface Planets: Interactions with particles and fields; 6215 Planetology: Solar System Objects: Extraterrestrial materials; KEYWORDS: solar wind interaction, pickup ions, oxygen corona, Mars, ion acceleration 1. Introduction [2] Mars lacks a global magnetic field so that the solar wind interacts directly with that planet’s ionosphere and atmosphere [Riedler et al., 1990]. However, the magnetometer aboard the Mars Global Surveyor (MGS) has observed large (hundreds of nanoteslas), albeit localized, magnetic fields [Acuna et al., 1998], which clearly must have dramatic effects on the local plasma environment. Also, the Electron Reflectometer experiment onboard MGS has made interesting measurements relevant to the solar wind interaction with Mars [e.g., Cloutier et al., 1999; Crider et al., 2000]. Mars like Venus possesses a hot 1 Now at Space Sciences Laboratory, University of California at Berkeley, Berkeley, California, USA. Copyright 2002 by the American Geophysical Union. 0148-0227/02/2001JA000125$09.00 SMP oxygen corona [Nagy and Cravens, 1988; Ip, 1990; Kim et al., 1998; Hodges, 2000], which strongly affects the solar wind interaction with those planets [e.g., Luhmann, 1991; Luhmann and Schwingenschuh, 1990; Brecht, 1997; Kallio and Koskinen, 1999; Kotova et al., 1997] by allowing heavy pickup ions to be created within the solar wind flow. [3] Fluxes of both hydrogen and oxygen ions with energies in the ranges 0.5– 25 and 0.01– 6 keV/q have been recorded in the wake of Mars by the Automatic Space Plasma Experiment with Rotating Analyzer (ASPERA) Ion Composition Experiment aboard the Phobos Mission to Mars and its Moons [Lundin et al., 1989; Verigin et al., 1991; Dubinin et al., 1993]. Kallio and Koskinen [1999] used a test particle model to interpret these observations and concluded that the ions are accelerated by the solar wind convective electric field in the vicinity of Mars. Barabash et al. [1991] also studied hydrogen pickup ions measured by the ASPERA instrument onboard Phobos. The energetic particle telescope 7-1 SMP 7-2 CRAVENS ET AL.: PICKUP IONS NEAR MARS solar low-energy detector (SLED) [McKenna-Lawlor et al., 1990], onboard Phobos 2, observed significant fluxes of ions with energies in excess of 50 keV (if oxygen) or 30 keV (if protons) in the vicinity of Mars on a large number (albeit not all) of Phobos 2 orbital passes [Afonin et al., 1989, 1991; Kirsch et al. 1991; McKenna-Lawlor et al., 1993]. These ion fluxes were most frequently found in the wake and bow shock regions. Kirsch et al. [1991] and McKenna-Lawlor et al. [1993] suggested that the observed ions were pickup ions associated with the ionization of atoms in the hot oxygen corona. To quote from the conclusions of Kirsch et al. [1991, p. 767]: ‘‘Pickup ions, most likely constituting O+ and O2+ ions of E > 55 keV, were observed in channels 1 and 2, especially during periods when Phobos 2 was spin stabilized and the solar wind speed was >500 km/s.’’ It was also suggested in this paper that ‘‘the pickup ions detected by SLED should have their origin 5 – 6 104 km upstream of Mars’’ (this is due to the very large gyroradius of a pickup oxygen ion). [4] Pickup ions are created by the ionization of neutrals and are initially almost at rest in comparison with the solar wind. In the absence of waves these ions follow cycloidal trajectories. This behavior of pickup ions is well known from studies of the solar wind interaction with comets [cf. Galeev et al., 1991; Kecskemety et al., 1989]. The cycloidal motion is a combination of E B drift and gyration. The maximum ion speed is vmax = 2u sin VB, where u is the solar wind speed and VB is the angle between the solar wind velocity and the magnetic field direction. The maximum energy for O+ ions can easily exceed 100 keV (and can be detected by SLED) if u and VB are sufficiently large (also see Kirsch et al. [1991]). The distribution function associated with pickup ions is a ring beam distribution. As has been shown for comets (see, for example, Johnstone et al. [1991]), this distribution is unstable against the growth of very low frequency waves, which then can modify the distribution via pitch angle and energy diffusion. However, in the case of Mars where a pickup O+ gyroradius is several Martian radii, waves will not have sufficient time to grow significantly and thus modify the ion distribution function. Consequently, the distribution will be nongyrotropic. This scenario is similar to the ion pickup process at Pluto as described in the test particle model of Kecskemety and Cravens [1993]. Note that the gyroradius of a pickup O+ ion in the unshocked solar wind is several Martian radii near Mars [Luhmann, 1991], so that in order to be accelerated to an energy in excess of tens of keV, as observed by SLED, such an ion would have to be born at least several Martian radii upstream of the planet, or the observation point. [5] In this paper, we present the first quantitative analysis of the SLED energetic (up to 100 keV) oxygen fluxes using test particle calculations of ion pick up in the vicinity of Mars. Ion fluxes were calculated at four locations for Circular Orbit 42 of the Phobos 2 spacecraft and compared with fluxes measured by the SLED instrument. Note that Phobos 2 executed 114 circular orbits about Mars with period 8 hours and at a radial distance of 2.8 Rm. The radius of Mars is denoted 1 Rm = 3398 km. The model predictions for the ion fluxes observed by SLED are directly proportional to the neutral oxygen density at distances upstream of the planet of roughly 5 – 15 Rm. This oxygen population should consist almost entirely of escaping atoms; hence the ion fluxes Figure 1. Schematic of Phobos 2 orbit, orientation, and solar low-energy detector (SLED) look direction and view cone. The spin axis of the spacecraft points toward the Sun (to the left). The asterisks indicate the centers of three of the four sampling bins (really planes): bins at the inbound shock, the wake, and the upstream region. measured by SLED should be proportional to the ‘‘direct’’ component of the escape flux of oxygen atoms from Mars. This direct component derives from the hot oxygen produced by dissociative recombination of O2+ ions in the ionosphere [Nagy and Cravens, 1988; Fox and Hac, 1997]. Other contributions to the overall oxygen loss from the planet can come from the ionization of exospheric oxygen atoms on ballistic trajectories closer to the planet or due to escape of oxygen ions directly from the ionosphere in the tail. Good agreement can be obtained between our predicted energetic oxygen ion fluxes and the SLED fluxes for reasonable values of the neutral oxygen densities. 2. Data From the SLED Instrument on Phobos 2 [6] The Phobos 2 spacecraft was sometimes spinning and at other times was spin-stabilized. During Circular Orbit 42 the spacecraft spin axis was pointed toward the Sun (see Figure 1). The SLED particle detector onboard Phobos 2, described by McKenna-Lawlor et al. [1990], consisted of solid-state detectors in two different telescopes which were directed at an angle 55 with respect to the spacecraft-Sun line (in order to avoid the direct impact of sunlight). This line-of-sight direction is approximately in the direction of the nominal interplanetary magnetic field once per spin. [7] The front detector of telescope 1 (Te1) was covered with 15 mg/cm2 aluminum, whereas telescope 2 (Te2) used an additional aluminum foil of 500 mg/cm2, which absorbed protons of energy <350 keV but allowed the detection of 35 – 350 keV electrons. Thus the count rate difference between the ‘‘open’’ telescope Te1 and the ‘‘foil’’ telescope Te2 allows differentiation between proton and electron CRAVENS ET AL.: PICKUP IONS NEAR MARS Figure 2. Simultaneous magnetic field and energetic ion measurements made by the Phobos 2 (Revolution 42) SLED and MAGMA (magnetometer) instruments during Circular Orbit 42 on 2 March 1989. (top panels): Sun-spacecraft line component of the magnetic field Bx and the magnetic field magnitude B. (bottom) Fluxes of energetic ions recorded by Te1 in the lowest-energy channel of SLED (34 –51 keV for protons and 55– 72 keV for oxygen ions). The time period includes a passage through the Martian wake. The neutral sheet (i.e., the center of the tail) is located where the x component of the magnetic field changes sign). The bow shock crossings are also evident in the magnetic field data, where the field strength changes abruptly. Adapted from Afonin et al. [1991]. fluxes. In each case the opening angle of the telescope apertures was ±20. The geometric factor of both telescopes was 0.25 cm2 sr, and the time resolution was 240 s. The lowest two energy channels of Te1 are relevant to our study and utilized energy ranges of 34 – 51 and 51– 202 keV, respectively, for protons and, owing to pulse height defect effects, 55– 72 and 72– 223 keV for O+ ions, respectively [McKenna-Lawlor et al., 1990; Kirsch et al., 1991]. The instrumental geometry was such that particles reached the detectors if they were within a cone with half-angle 20. The differential flux is obtained for a given channel from the raw number of counts over the integration period by dividing by the geometric factor, the integration period, and the energy channel width. This procedure gives the flux averaged over the solid angle of the aperture, which underestimates the actual differential flux (or intensity) for highly anisotropic distributions. [8] The electronic thresholds of SLED 1 were set by using a calibrated pulse generator. The energy thresholds for protons were then calibrated from the energy loss curves in silicon. McKenna-Lawlor et al. [1993] made a calculation of the energy thresholds similar to an earlier calculation reported on in the original instrument description paper [McKenna-Lawlor et al., 1990]. Two SLED instruments were, in fact, originally launched on the two spacecraft of the Phobos mission (SLED 1 on Phobos 2 and SLED 2 on Phobos 1). Phobos 1 and 2 were launched on 7 and 12 July SMP 7-3 1988, respectively. SLED 2 was activated on 19 July and SLED 1 on 23 July. Before ground contact with Phobos 1 was lost at the end of August 1988, both SLED instruments were operating and obtained ‘‘cruise’’ data. The fluxes both instruments recorded were closely related, indicating a similar performance. [9] Figure 2 shows SLED data for Phobos 2 Circular Orbit 42 for channel 1 [from Afonin et al., 1991]. The fluxes in channel 2 were about a factor of 10 less than the channel 1 fluxes near the outbound bow shock crossing. Ion fluxes are observed both near the bow shock crossings and in the Martian wake (at the distance of the Phobos 2 orbit, 2.8 Rm). The bow shock crossings are evident in the magnetic field strength jumps in Figure 2. The SLED ion fluxes show a modulation at the spacecraft spin rate, which indicates that the ion distributions were anisotropic. Typical peak ion fluxes in channel 1 were 1000 cm2 s1 sr1 near the inbound shock crossing and 3000 cm2 s1 sr1 near the outbound shock. The statistical uncertainty associated with these fluxes is less than one percent. [10] The solar wind conditions appropriate for the test particle simulation for Circular Orbit 42 were determined from Phobos 2 data monitored by the TAUS experiment [Rosenbauer et al., 1989] for a time just prior to the inbound shock crossing. We used here average values of the measured solar wind parameters since these parameters displayed some variability. The average solar wind speed was 600 km s1 and the magnetic field components were Bx = 2 nT, By = 4.8 nT, and Bz = 0.0 nT. The angle between the solar wind velocity and the magnetic field was VB = 67 on average. The coordinate system used in analyzing the Afonin et al. [1991] data by Kirsch et al. [1991] is Mars centered with the x axis directed sunward, the z axis directed normal to the ecliptic plane, and the y axis completing the right-handed coordinate system. However, for this paper we used a coordinate system in which the x axis is directed antisunward. Hence the magnetic field in our model points 180 away from the actual direction. The E B drift direction is not changed by this transformation. The only consequence for our calculations is that the vz velocity component changes sign, which does not affect any model-data comparisons we carry out in this paper. For the pertinent solar wind conditions the gyroradius of a pickup O+ ion is rc = 1.76 104 km = 5.2 Rm and the maximum pickup ion speed is 1100 km s1, which corresponds to a maximum kinetic energy of 100.3 keV. 3. Model of Pickup Oxygen Ions Near Mars [11] The model consists of two parts: (1) a test particle (or Monte Carlo) part in which the trajectories of 106 randomly created ions are numerically determined and (2) a magnetohydrodynamic (MHD) model part that is needed to generate the electric and magnetic fields required by the test particle code. The test particle model requires a distribution of birth points in space. The locations of these birth points were chosen randomly with a probability proportional to the ionization rate and hence to the neutral density (assuming a constant ionization frequency). Because we are mainly concerned with ions born more than 5 Rm from Mars (well outside the bow shock), photoionization is the main ionization source although charge transfer collision of SMP 7-4 CRAVENS ET AL.: PICKUP IONS NEAR MARS Figure 3. The trajectory of an O+ ion born a distance of 105 km from Mars is shown. The inner box is the region within which the results of the MHD calculation are used for the E and B fields. Mars is depicted to scale and the 4 sampling planes used are shown. The centers of the planes are at a radial distance from Mars of 2.8 Rm, corresponding to the distance of the Phobos 2 spacecraft in its circular orbit. The sample ion trajectory shown is cycloidal upstream of the shock, but the trajectory is not a simple cycloid downstream where the ion encounters a larger field. solar wind protons with neutral oxygen makes some contribution. The ionization frequencies for both of these sources should be independent of distance from Mars outside the shock. We adopt a total ionization frequency, per neutral, of 4.5 107 s1, appropriate for the period of rising solar activity and for the heliocentric distance of Mars. 3.1 Neutral Density [12] The ions observed by SLED, with energies in excess of 55 keV, must be born at distances in excess of several Rm from Mars. The relevant neutral oxygen atoms belong to the hot oxygen corona that is created by the dissociative recombination of ionospheric O2+ [cf. Nagy and Cravens, 1988; Lammer and Bauer, 1991; Kim et al., 1998; Hodges, 2000]. Most of the oxygen reaching the distances we are interested in will be escaping oxygen. The calculations of Fox and Hac [1997] demonstrated that about half of the O atoms thus produced do indeed exceed the escape speed at the exobase. Unfortunately, numerical models of the Martian exosphere reported on in the literature do not extend out to distances of several Martian radii, which we need. We instead extend out to larger distances the results of exospheric hot oxygen models for closer distances. We adopt the neutral atomic oxygen profile presented in Figure 3 (high solar activity case) of Kim et al. [1998] for the altitude range of 200 to 6200 km. The Hodges [2000] oxygen profile for noon and the Kim et al. [1998] profile are consistent. We found that the following expression fits the oxygen density profile rather well for the 3000- to 6200-km altitude region: nO ¼ nexo ðRm =rÞ2 ; 3 ð1Þ where nexo = 4000 cm is a density at the exobase and r is the radial distance from Mars. We employ equation (1) for altitudes greater than 6200 km. Our calculated ion fluxes are directly proportional to nexo. Our calculated results are shown for this value of nexo, but it will turn out that better agreement of the calculated ion fluxes with the SLED data is obtained if nexo 800 cm3 rather than 4000 cm3. [13] Exospheric densities at large distances from a planet vary with radial distance r differently depending whether the responsible particles are on ballistic, satellite, or escape trajectories [Chamberlin, 1978]. Neglecting satellite orbits, the escape component should dominate at radial distances of many planetary radii. In this case the neutral density for large radial distances (r Rm) should be roughly given by an expression like equation (1) with nexo Fesc / huni, where huni is a typical neutral speed. Hot oxygen produced by the more energetic reaction channels of dissociative recombination have speeds of 6– 7 km/s [Fox and Hac, 1997]. Using these speeds and the Hodges escape flux gives nexo 300 cm3, which is about a factor of 10– 15 less than the nexo value we used in equation (1). This discrepancy suggests that a significant fraction of the oxygen in the 3000- to 6000-km altitude region (shown by Kim et al. or by Hodges) might consist of oxygen atoms on bound ballistic trajectories. If both satellite and ballistic orbits were also included, the asymptotic radial dependence of the exospheric density would be r3/2 rather than r2 [Chamberlin, 1978]), and the oxygen density near 10 Rm would be about a factor of 2 greater than the values given by this expression. However, it is not obvious that satellite orbits are able to be populated at Mars. This aspect of the Martian exosphere needs further study. [14] A random number generator was used to create 106 ions whose initial radial distance r is greater than 1.5 Rm and with a probability proportional to the neutral density CRAVENS ET AL.: PICKUP IONS NEAR MARS Figure 4. (top) Scatterplot in vx – vz space for the upstream bin 12. The straight lines are projections into this plane of the directions opposite to the modeled SLED look directions indicated. The mark on the line indicates the speed for a 55keV oxygen ion. (bottom) Same as top panel but scatterplot in vx – vy space. profile mentioned above. The ions are created within a cubical box centered at Mars with 4 105 km for each side. Also, these initial ion positions are created with spherical symmetry. This is certainly not entirely accurate due to day-night differences in the hot oxygen distribution, but this assumption should not make much difference for this paper because we are interested in ions born upstream of the planet. 3.2. Test Particle Model [15] The test particle code we use for this paper is essentially the same code used to investigate ion distribution functions near comets [Cravens, 1989; McKenzie et al., 1994], near Titan [Ledvina et al., 2000] and near Pluto [Kecskemety and Cravens, 1993]. The equation of motion is numerically solved for each ion that is created. The only force included is the Lorentz force: F = q(E + v B), where q = e is the charge, v is the velocity vector, and E and B are the electric and magnetic fields, respectively, at the location of the particle. The electric field is assumed to be the motional electric field, E = u B, where u is the bulk plasma (i.e., solar wind) flow velocity. [16] Two types of fields (cases) were investigated: (1) uniform fields everywhere (i.e., the upstream solar wind field values) and (2) fields taken from a three-dimensional global MHD model of the solar wind interaction with Mars SMP 7-5 (which is briefly described later). The results shown in this paper all come from the second (MHD) case, although results from the first case are mentioned. The MHD fields are used only inside a relatively small ‘‘inner’’ region (i.e., a cube centered at Mars and with dimensions 5.4 104 km ). Uniform fields are used outside this inner region. Figure 3 shows the trajectory of a typical pickup O+ ion born upstream of Mars. Note the large size of the ion’s orbit in comparison with the size of Mars. Ions are removed from the simulation if they penetrate past the exobase (at an altitude of 200 km). [17] The ion trajectories are sampled as they cross four planes (or spatial ‘‘bins’’), each with dimensions 4 Rm 4 Rm and each oriented perpendicular to the x axis (that is, the bin surface is oriented perpendicular to the Sun-Mars direction). Two of the bins are centered on the x axis at the distance of the Phobos 2 spacecraft: one upstream of Mars and the other in the wake. The other two bins are located in the flanks and straddle the bow shock. The centers of three of these bins (or planes) are indicated in Figure 1. The sampling bins were made large in order to increase their collection efficiency, but, even so, they collect less than 1% of the total ions created in the overall simulation box. This box was made very large in order not to accidentally introduce any bias into the counting statistics. Particles crossing the planes/bins are sorted in a number of ways such as by their energy and direction or according to their velocity components. In particular, ion fluxes were determined in a way that mimics the Phobos/SLED viewing geometry. We arbitrarily chose 16 angles about the spacecraft spin axis at which we oriented the ‘‘instrument point direction.’’ For each of these look directions, only ions traveling at an angle (with respect to the pointing direction) of less than the SLED semi-acceptance angle of 20 were recorded and included in the flux for this look direction. We were able to calculate ‘‘absolute’’ instrumental ion flux values by estimating the ratio of the number of ‘‘real’’ ions that would be created per second in the simulation box to the number of ions (106) that we randomly generated in the box. 3.3. Electric and Magnetic Fields From the MHD Model [18] We used a global 3-D single-fluid MHD model to specify the electric and magnetic fields in the inner region of our simulation box. The model included mass loading (source) terms with both ion production (with the oxygen densities discussed above) and dissociative recombination. The upstream boundary conditions for this MHD model were the solar wind conditions appropriate for orbit 42 and discussed earlier. The MHD code used was the Zeus-3-D code developed by the Laboratory for Computational Astrophysics at the National Center for Supercomputer Applications [cf. Hawley and Stone, 1995; Stone, 1999]. A variable Cartesian spatial grid was employed with a spatial grid size ranging from 325 km near Mars up to 805 km far from Mars. This grid size was sufficiently small for a magnetic barrier, a draped field magnetotail (or wake), and a bow shock to all appear in the MHD model results. Note that pickup O+ ions born upstream of Mars are insensitive to the finer details of the field pattern due to their very large gyroradii. The field strength in the subsolar magnetic barrier SMP 7-6 CRAVENS ET AL.: PICKUP IONS NEAR MARS produced for uniform fields (we performed this calculation but do not show the results). The distribution is nongyrotropic (more upgoing ions than downgoing ions) because the upgoing ions are born closer to Mars where the neutral density is greater. The outlying scattered ions in Figures 4 and 5, outside the pickup ion ring, result from reflection from the shock and/or magnetic barrier downstream of the shock. [20] In the wake the ion distribution is no longer a simple ring beam distribution. The ion distribution is more spread out and diffuse. Ions, which are moving upward, have their velocity vectors shifted downward as the ions move past the shock and into the wake. This is due to the larger Lorentz force associated with the increase in the magnetic field experienced by the ions as they approach Mars and pass through the magnetic barrier. This change of velocity has the effect of removing ions from the top part of the ring distribution and shifting them to lower vz values. Ions are also deflected away from the E B drift Figure 5. (top) Same as Figure 4 (vx –vz scatterplot) but for a bin in the flanks near the outbound bow shock (bin 2). The lines indicate projections of the directions opposite to the modeled instrument pointing direction for the spin number indicated. The mark on the line indicates the speed corresponding to a 55-keV oxygen ion. (bottom) Same as top panel but scatterplot in vx – vy space. is 21 nT, which is a factor of 4 greater than the measured upstream IMF value of 5.2 nT. The modeled bow shock in the subsolar region is located at a radial distance of 1.9 Rm and in the flanks it is located at 3.1 Rm. These distances are 15– 20% greater than the average subsolar and flank bow shock positions found in Phobos 2 data [Trotignon et al., 1993] but considerable variability exists in measured bow shock positions [Schwingenschuh et al., 1990]. In fact, the distance to the flank bow shock estimated using the location of the shock crossings for Orbit 42 (i.e., from Figure 2) is roughly 2.8 Rm, which is not that different than the distance obtained with the MHD model. 4. Results 4.1. Velocity Space Scatterplots [19] The velocity vector of each ion was recorded only if it crossed one of the four bins/planes described in the model section of the paper. Scatterplots are shown in Figures 4 – 6. Ions are accepted from a full 4p solid angle for these scatterplots. The upstream distribution is a slightly modified nongyrotropic ring beam distribution and looks very similar to the distribution that would be Figure 6. (top) Same as Figure 4 (vx – vz scatterplot) but for the wake bin 15. The lines indicate projections of the directions opposite to the modeled instrument pointing direction for the spin number indicated. The mark on the line indicates the speed corresponding to a 55-keV oxygen ion. (bottom) Same as top panel but scatterplot in vx – vy space. CRAVENS ET AL.: PICKUP IONS NEAR MARS SMP 7-7 Figure 7. O+ fluxes versus energy for the bin located in the flank and straddling the inbound bow shock for a SLED geometry with an acceptance cone with half-angle 20. Fluxes were calculated for 16 look angles in the spin cycle, but for this spatial bin particles were registered only for 8 of these directions, as shown. Peak SLED channel 1 and 2 count rates for the shock (scaled as indicated, see text) are indicated in two of the panels. direction (in the vx – vy plane) and toward the solar wind direction. 4.2. Calculated Ion Fluxes [21] We simulated the ion fluxes that SLED should see at 16 angles around its spin cycle at our four different bin locations. Only ions whose velocity vectors put them within the 40 instrument (full-width) acceptance cone were recorded for each of the 16 look directions. The ions were further sorted according to their energy in 5-keV bins. Figure 7 shows results for the inbound bow shock crossing for all instrument look directions for which any ions were recorded (in this case, 8 of the directions recorded fluxes while the other 8 directions did not record any ions). The largest fluxes of O+ ions with energies near 55 keV (the SLED lower energy limit) were recorded for directions 10 and 11, whereas smaller fluxes were recorded for direction 5 and very little or no flux for the other directions. These directions (i.e., 5, 10, and 11) are indicated by the straight lines in Figures 4 – 6. For the two shock crossings or for the upstream case, directions 10 and 11 intersect the ring distribution at positive vz where the ion flux is large, and direction 5 intersects the ring at negative vz where the ion flux is much smaller. This asymmetry is due to the nongyrotropy of the ion distribution which, in turn, is due to increasing production rate with decreasing distance from the planet. [22] Figure 8 shows flux versus energy for the other three spatial locations but only for the look direction (or spin angle number) at which the flux was a maximum. Some SLED data from Afonin et al. [1991] are also indicated in Figure 8. The Afonin fluxes are multiplied by the solid angle of the instrument (0.39 sr). The SLED fluxes were spin modulated, and we also chose the peak fluxes for these comparisons; however, SLED had lower time/angle resolution than our model did, which introduces a factor of 3 difference in the peak flux. The model ion fluxes are about a factor of 10– 20 greater than the measured fluxes and a scaling factor is included with the data shown in Figure 8 in order to facilitate the comparison. The channel 2 (72 – 223 keV) SLED fluxes were also placed entirely into a single 5-keV interval for comparison purposes, which introduces another scaling factor of 29.6 for this channel. [23] We also integrated the model ion fluxes over energy for the SLED channels 1 and 2 energy ranges (55 –72 and 72– 223 keV, respectively) in order to obtain a ‘‘SLED’’ count rate for each of the 16 look directions (0 through 15) and for each of the four locations around Mars. However, we still used the better angle resolution of the model in order to see how sharp the peaks are for the angular dependence (i.e., time dependence). The results of this procedure are shown in Figure 9. The spin variations look very similar for the two shock locations and for the upstream location; the maximum flux appears near spin numbers 10 and 11. On the other hand, for the wake the maximum flux appears at spin look direction number 5. The model ion fluxes are confined to a rather narrow angular range in Figure 9 (2 spin numbers or 45). The SLED integration period is about a third of a spacecraft spin period, which is 5 to 6 of our spin angular intervals. Consequently, the peak fluxes in a spin cycle from Figure 9 should be divided by a factor of 3 before being compared with the SLED maximum flux (in a spin cycle) (from Figure 2). Table 1 shows these comparisons for the two shock locations and for the wake. To reiterate, both the instrument and SMP 7-8 CRAVENS ET AL.: PICKUP IONS NEAR MARS wake of Mars that are about a factor of 6 times greater (after adjustment for the fraction of spin cycle covered by the measurements versus the model) than the fluxes observed by the SLED energetic particle instrument onboard the Phobos 2 spacecraft during Circular Orbit 42. More accu- Figure 8. Same as Figure 7, but results are only shown for the look direction that has the largest flux near an ion energy of 60 keV. (top) Fluxes for the bin upstream of the bow shock. (middle) Fluxes for the bin straddling the outbound shock. (bottom) Fluxes for the bin in the wake of Mars and peak SLED channel 1 fluxes for the wake are shown. the simulated instrument had 40 wide acceptance cones, which results in lower peak particle intensities (flux per unit solid angle) than the real pickup ion distribution with its narrow ring. The factor of 3 in Table 1 is just the difference in integration period between the SLED data and our simulated SLED ‘‘data.’’ 5. Discussion and Summary [24] Our combined test particle and MHD model (with nexo = 4000 cm3 in equation (1)) predicts ion fluxes in the Figure 9. Model flux for SLED channels 1 and 2 energy bins versus spin orientation for both shock bins, the upstream bin, and the wake bin. Shock 1 is inbound, and shock 2 is outbound. The model fluxes are treated in the same way as the SLED fluxes except that higher time resolution (i.e., angular resolution, 16 spin positions) is retained for the model. SMP CRAVENS ET AL.: PICKUP IONS NEAR MARS 7-9 Table 1. Model-SLED Comparisonsa Data (maximum in spin cycle) Model (maximum in spin cycle) Model times 1/3 (modified maximum) Model times 1/3 1/6 (modified maximum and for lower O densities) a Shock 1 Shock 2 Wake 1200 33,000 11,000 1830 3000 39,000 13,000 2170 300 3300 1100 180 Oxygen flux 55 – 72 keV (channel 1) (cm2 s1 sr1). rately, this is true only for those time periods when SLED actually did observe any ion flux. An examination of the Orbit 42 data in Figure 2 indicates that for many time periods SLED did not record any oxygen flux. First, let us consider this issue and then later we will discuss the factor of 6 model/data ratio. [25] The model predicts the existence of significant pickup ion fluxes outside the bow shock, whereas SLED only occasionally measured significant ion fluxes outside the shock. As suggested by Afonin et al. [1991], it is difficult for pickup ions to enter the narrow SLED entrance cone at the energies measurable by SLED. Pickup ion distributions, especially upstream of the shock, depend very sensitively on the solar wind speed and on the IMF direction. For the interplanetary conditions pertaining during Circular Orbit 42 (and adopted in the model) the ring beam distribution was intersected by look directions 10 and 11 (within the half-angle of 20), but if the IMF angle with respect to the solar wind direction had been 15 less than the value used (VB = 67) no intersection would have been present for energies in excess of the SLED channel 1 lower limit of 55 keV. The magnetic field values we adopted for Orbit 42 were ‘‘average’’ values estimated from upstream conditions, but an examination of Figure 2 indicates that even during this one orbital pass the IMF direction varied considerably. These variations were probably sufficient to move the ring beam distribution in and out of the SLED acceptance cone at different times. [26] Now we discuss the factor of 6 difference between the model ion fluxes and the measured fluxes near the shocks and in the wake. The model ion fluxes are directly proportional to the hot oxygen density near the birth points of those ions that reach energies of 55 keV near the Phobos 2 orbit. These birth points must be located further from Mars than 10 Rm due to the several Rm gyroradii of pickup O+ ions. Hence the simplest explanation for the factor of 6 is that the neutral oxygen density used in the model (see equation (1)) at such large distances was too large by this factor. That is, closer agreement would have been obtained if our O density at 10 Rm was not 40 cm3 but was only 7 cm3. The oxygen population at this distance primarily consists of escaping oxygen. This is equivalent to using nexo 800 cm3 rather than nexo = 4000 cm3 in equation (1). As discussed earlier, this smaller density is not too different from the 300 cm3 we crudely estimated using the Hodges [2000] escape rate. As mentioned in the model description our O density was based on the Kim et al. [1998] densities near 2 – 3 Rm; it seems that the hot oxygen in this near-Mars region must include a significant fraction of gravitationally bound O atoms as well as escaping atoms. In any case the flux of 55– 72 keV oxygen ions seen at the orbit of the Phobos 2 by SLED should be proportional to the oxygen density in the vicinity of a radial distance from Mars of 10 Rm, and, hence, proportional to the direct escape rate of oxygen produced by the dissociative recombination of molecular oxygen ions in the ionosphere. Oxygen can also be lost from the planet due to ionization of bound oxygen atoms located closer to the planet than 10 Rm yet still in the solar wind or due to direct outflow of ionospheric ions (mainly on the nightside). [27] In summary, our quantitative model supports the identification of the ions observed in channel 1 of the SLED instrument as pickup oxygen ions that are created by the ionization of oxygen atoms in the distant part of the exosphere, as suggested by Kirsch et al. [1991]. Our model also explains the spin modulation exhibited by the SLED data. Furthermore, the flux of energetic oxygen ions observed near the orbit of the Phobos 2 provides new information on the distant part of the hot oxygen corona. [28] Acknowledgments. Support from NASA Planetary Atmospheres grant NAG5-4358 and NSF grant ATM-9815574 as well as financial support for the SLED instrument from the Irish National Board for Science and Technology is gratefully acknowledged. Computational support from the Kansas Center for Advanced Scientific Computing and NSF MRI grant is also acknowledged. [29] Janet G. Luhmann thanks Stas Barabash and Hannu E. J. Koskinen for their assistance in evaluating this paper. 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