Pickup ions near Mars associated with escaping oxygen atoms
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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 three-
dimensional 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
modulation as did the SLED fluxes during Circular Orbit 42. INDEX TERMS: 2780
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 oxygen corona [Nagy and Cravens, 1988; Ip, 1990; Kim
et al., 1998; Hodges, 2000], which strongly affects the solar
[2] Mars lacks a global magnetic field so that the solar
wind interaction with those planets [e.g., Luhmann, 1991;
wind interacts directly with that planet’s ionosphere and
Luhmann and Schwingenschuh, 1990; Brecht, 1997; Kallio
atmosphere [Riedler et al., 1990]. However, the magneto-
and Koskinen, 1999; Kotova et al., 1997] by allowing heavy
meter aboard the Mars Global Surveyor (MGS) has
pickup ions to be created within the solar wind flow.
observed large (hundreds of nanoteslas), albeit localized,
[3] Fluxes of both hydrogen and oxygen ions with ener-
magnetic fields [Acuna et al., 1998], which clearly must
gies in the ranges 0.5– 25 and 0.01– 6 keV/q have been
have dramatic effects on the local plasma environment.
recorded in the wake of Mars by the Automatic Space Plasma
Also, the Electron Reflectometer experiment onboard
Experiment with Rotating Analyzer (ASPERA) Ion Compo-
MGS has made interesting measurements relevant to the
sition Experiment aboard the Phobos Mission to Mars and its
solar wind interaction with Mars [e.g., Cloutier et al., 1999;
Moons [Lundin et al., 1989; Verigin et al., 1991; Dubinin et
Crider et al., 2000]. Mars like Venus possesses a hot
al., 1993]. Kallio and Koskinen [1999] used a test particle
1
model to interpret these observations and concluded that the
Now at Space Sciences Laboratory, University of California at
Berkeley, Berkeley, California, USA.
ions are accelerated by the solar wind convective electric
field in the vicinity of Mars. Barabash et al. [1991] also
Copyright 2002 by the American Geophysical Union. studied hydrogen pickup ions measured by the ASPERA
0148-0227/02/2001JA000125$09.00 instrument onboard Phobos. The energetic particle telescope
SMP 7-1SMP 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 Figure 1. Schematic of Phobos 2 orbit, orientation, and from studies of the solar wind interaction with comets [cf. solar low-energy detector (SLED) look direction and view Galeev et al., 1991; Kecskemety et al., 1989]. The cycloidal cone. The spin axis of the spacecraft points toward the Sun motion is a combination of E B drift and gyration. The (to the left). The asterisks indicate the centers of three of the maximum ion speed is vmax = 2u sin VB, where u is the four sampling bins (really planes): bins at the inbound solar wind speed and VB is the angle between the solar shock, the wake, and the upstream region. wind velocity and the magnetic field direction. The max- imum energy for O+ ions can easily exceed 100 keV (and can be detected by SLED) if u and VB are sufficiently large measured by SLED should be proportional to the ‘‘direct’’ (also see Kirsch et al. [1991]). The distribution function component of the escape flux of oxygen atoms from Mars. associated with pickup ions is a ring beam distribution. As This direct component derives from the hot oxygen produced has been shown for comets (see, for example, Johnstone et by dissociative recombination of O2+ ions in the ionosphere al. [1991]), this distribution is unstable against the growth [Nagy and Cravens, 1988; Fox and Hac, 1997]. Other of very low frequency waves, which then can modify the contributions to the overall oxygen loss from the planet can distribution via pitch angle and energy diffusion. However, come from the ionization of exospheric oxygen atoms on in the case of Mars where a pickup O+ gyroradius is several ballistic trajectories closer to the planet or due to escape of Martian radii, waves will not have sufficient time to grow oxygen ions directly from the ionosphere in the tail. Good significantly and thus modify the ion distribution function. agreement can be obtained between our predicted energetic Consequently, the distribution will be nongyrotropic. This oxygen ion fluxes and the SLED fluxes for reasonable values scenario is similar to the ion pickup process at Pluto as of the neutral oxygen densities. 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 2. Data From the SLED Instrument on Phobos 2 Mars [Luhmann, 1991], so that in order to be accelerated to [6] The Phobos 2 spacecraft was sometimes spinning and an energy in excess of tens of keV, as observed by SLED, at other times was spin-stabilized. During Circular Orbit 42 such an ion would have to be born at least several Martian the spacecraft spin axis was pointed toward the Sun (see radii upstream of the planet, or the observation point. Figure 1). The SLED particle detector onboard Phobos 2, [5] In this paper, we present the first quantitative analysis described by McKenna-Lawlor et al. [1990], consisted of of the SLED energetic (up to 100 keV) oxygen fluxes using solid-state detectors in two different telescopes which were test particle calculations of ion pick up in the vicinity of directed at an angle 55 with respect to the spacecraft-Sun Mars. Ion fluxes were calculated at four locations for Circular line (in order to avoid the direct impact of sunlight). This Orbit 42 of the Phobos 2 spacecraft and compared with fluxes line-of-sight direction is approximately in the direction of measured by the SLED instrument. Note that Phobos 2 the nominal interplanetary magnetic field once per spin. executed 114 circular orbits about Mars with period 8 [7] The front detector of telescope 1 (Te1) was covered hours and at a radial distance of 2.8 Rm. The radius of Mars with 15 mg/cm2 aluminum, whereas telescope 2 (Te2) used is denoted 1 Rm = 3398 km. The model predictions for the ion an additional aluminum foil of 500 mg/cm2, which absorbed fluxes observed by SLED are directly proportional to the protons of energy
CRAVENS ET AL.: PICKUP IONS NEAR MARS 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
Figure 2. Simultaneous magnetic field and energetic ion is less than one percent.
measurements made by the Phobos 2 (Revolution 42) SLED [10] The solar wind conditions appropriate for the test
and MAGMA (magnetometer) instruments during Circular particle simulation for Circular Orbit 42 were determined
Orbit 42 on 2 March 1989. (top panels): Sun-spacecraft line from Phobos 2 data monitored by the TAUS experiment
component of the magnetic field Bx and the magnetic field [Rosenbauer et al., 1989] for a time just prior to the inbound
magnitude B. (bottom) Fluxes of energetic ions recorded by shock crossing. We used here average values of the meas-
Te1 in the lowest-energy channel of SLED (34 –51 keV for ured solar wind parameters since these parameters displayed
protons and 55– 72 keV for oxygen ions). The time period some variability. The average solar wind speed was 600 km
includes a passage through the Martian wake. The neutral s1 and the magnetic field components were Bx = 2 nT,
sheet (i.e., the center of the tail) is located where the x By = 4.8 nT, and Bz = 0.0 nT. The angle between the solar
component of the magnetic field changes sign). The bow wind velocity and the magnetic field was VB = 67 on
shock crossings are also evident in the magnetic field data, average. The coordinate system used in analyzing the
where the field strength changes abruptly. Adapted from Afonin et al. [1991] data by Kirsch et al. [1991] is Mars
Afonin et al. [1991]. 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
fluxes. In each case the opening angle of the telescope used a coordinate system in which the x axis is directed
apertures was ±20. The geometric factor of both telescopes antisunward. Hence the magnetic field in our model points
was 0.25 cm2 sr, and the time resolution was 240 s. The 180 away from the actual direction. The E B drift
lowest two energy channels of Te1 are relevant to our study direction is not changed by this transformation. The only
and utilized energy ranges of 34 – 51 and 51– 202 keV, consequence for our calculations is that the vz velocity
respectively, for protons and, owing to pulse height defect component changes sign, which does not affect any
effects, 55– 72 and 72– 223 keV for O+ ions, respectively model-data comparisons we carry out in this paper. For
[McKenna-Lawlor et al., 1990; Kirsch et al., 1991]. The the pertinent solar wind conditions the gyroradius of a
instrumental geometry was such that particles reached the pickup O+ ion is rc = 1.76 104 km = 5.2 Rm and the
detectors if they were within a cone with half-angle 20. maximum pickup ion speed is 1100 km s1, which corre-
The differential flux is obtained for a given channel from sponds to a maximum kinetic energy of 100.3 keV.
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 3. Model of Pickup Oxygen Ions Near Mars
averaged over the solid angle of the aperture, which under- [11] The model consists of two parts: (1) a test particle
estimates the actual differential flux (or intensity) for highly (or Monte Carlo) part in which the trajectories of 106
anisotropic distributions. randomly created ions are numerically determined and (2)
[8] The electronic thresholds of SLED 1 were set by a magnetohydrodynamic (MHD) model part that is needed
using a calibrated pulse generator. The energy thresholds for to generate the electric and magnetic fields required by the
protons were then calibrated from the energy loss curves in test particle code. The test particle model requires a distri-
silicon. McKenna-Lawlor et al. [1993] made a calculation bution of birth points in space. The locations of these birth
of the energy thresholds similar to an earlier calculation points were chosen randomly with a probability proportional
reported on in the original instrument description paper to the ionization rate and hence to the neutral density
[McKenna-Lawlor et al., 1990]. Two SLED instruments (assuming a constant ionization frequency). Because we
were, in fact, originally launched on the two spacecraft of are mainly concerned with ions born more than 5 Rm from
the Phobos mission (SLED 1 on Phobos 2 and SLED 2 on Mars (well outside the bow shock), photoionization is the
Phobos 1). Phobos 1 and 2 were launched on 7 and 12 July main ionization source although charge transfer collision ofSMP 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 con- altitudes greater than 6200 km. Our calculated ion fluxes are
tribution. The ionization frequencies for both of these directly proportional to nexo. Our calculated results are
sources should be independent of distance from Mars out- shown for this value of nexo, but it will turn out that better
side the shock. We adopt a total ionization frequency, per agreement of the calculated ion fluxes with the SLED data
neutral, of 4.5 107 s1, appropriate for the period of is obtained if nexo 800 cm3 rather than 4000 cm3.
rising solar activity and for the heliocentric distance of Mars. [13] Exospheric densities at large distances from a planet
vary with radial distance r differently depending whether
3.1 Neutral Density the responsible particles are on ballistic, satellite, or escape
[12] The ions observed by SLED, with energies in excess trajectories [Chamberlin, 1978]. Neglecting satellite orbits,
of 55 keV, must be born at distances in excess of several Rm the escape component should dominate at radial distances of
from Mars. The relevant neutral oxygen atoms belong to the many planetary radii. In this case the neutral density for
hot oxygen corona that is created by the dissociative recom- large radial distances (r Rm) should be roughly given by
bination of ionospheric O2+ [cf. Nagy and Cravens, 1988; an expression like equation (1) with nexo Fesc / huni,
Lammer and Bauer, 1991; Kim et al., 1998; Hodges, 2000]. where huni is a typical neutral speed. Hot oxygen produced
Most of the oxygen reaching the distances we are interested by the more energetic reaction channels of dissociative
in will be escaping oxygen. The calculations of Fox and Hac recombination have speeds of 6– 7 km/s [Fox and Hac,
[1997] demonstrated that about half of the O atoms thus 1997]. Using these speeds and the Hodges escape flux gives
produced do indeed exceed the escape speed at the exobase. nexo 300 cm3, which is about a factor of 10– 15 less than
Unfortunately, numerical models of the Martian exosphere the nexo value we used in equation (1). This discrepancy
reported on in the literature do not extend out to distances of suggests that a significant fraction of the oxygen in the
several Martian radii, which we need. We instead extend out 3000- to 6000-km altitude region (shown by Kim et al. or
to larger distances the results of exospheric hot oxygen by Hodges) might consist of oxygen atoms on bound
models for closer distances. We adopt the neutral atomic ballistic trajectories. If both satellite and ballistic orbits
oxygen profile presented in Figure 3 (high solar activity were also included, the asymptotic radial dependence of
case) of Kim et al. [1998] for the altitude range of 200 to the exospheric density would be r3/2 rather than r2
6200 km. The Hodges [2000] oxygen profile for noon and [Chamberlin, 1978]), and the oxygen density near 10 Rm
the Kim et al. [1998] profile are consistent. We found that the would be about a factor of 2 greater than the values given by
following expression fits the oxygen density profile rather this expression. However, it is not obvious that satellite
well for the 3000- to 6200-km altitude region: orbits are able to be populated at Mars. This aspect of the
Martian exosphere needs further study.
nO ¼ nexo ðRm =rÞ2 ; ð1Þ
[14] A random number generator was used to create 106
3
where nexo = 4000 cm is a density at the exobase and r is ions whose initial radial distance r is greater than 1.5 Rm
the radial distance from Mars. We employ equation (1) for and with a probability proportional to the neutral densityCRAVENS ET AL.: PICKUP IONS NEAR 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
Figure 4. (top) Scatterplot in vx – vz space for the upstream determined in a way that mimics the Phobos/SLED view-
bin 12. The straight lines are projections into this plane of ing geometry. We arbitrarily chose 16 angles about the
the directions opposite to the modeled SLED look directions spacecraft spin axis at which we oriented the ‘‘instrument
indicated. The mark on the line indicates the speed for a 55- point direction.’’ For each of these look directions, only
keV oxygen ion. (bottom) Same as top panel but scatterplot ions traveling at an angle (with respect to the pointing
in vx – vy space. 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’’ instrumen-
profile mentioned above. The ions are created within a tal ion flux values by estimating the ratio of the number of
cubical box centered at Mars with 4 105 km for each ‘‘real’’ ions that would be created per second in the
side. Also, these initial ion positions are created with simulation box to the number of ions (106) that we
spherical symmetry. This is certainly not entirely accurate randomly generated in the box.
due to day-night differences in the hot oxygen distribution,
but this assumption should not make much difference for 3.3. Electric and Magnetic Fields From the
this paper because we are interested in ions born upstream MHD Model
of the planet. [18] We used a global 3-D single-fluid MHD model to
specify the electric and magnetic fields in the inner region
3.2. Test Particle Model of our simulation box. The model included mass loading
[15] The test particle code we use for this paper is (source) terms with both ion production (with the oxygen
essentially the same code used to investigate ion distribution densities discussed above) and dissociative recombination.
functions near comets [Cravens, 1989; McKenzie et al., The upstream boundary conditions for this MHD model
1994], near Titan [Ledvina et al., 2000] and near Pluto were the solar wind conditions appropriate for orbit 42 and
[Kecskemety and Cravens, 1993]. The equation of motion is discussed earlier. The MHD code used was the Zeus-3-D
numerically solved for each ion that is created. The only code developed by the Laboratory for Computational Astro-
force included is the Lorentz force: F = q(E + v B), physics at the National Center for Supercomputer Applica-
where q = e is the charge, v is the velocity vector, and E and tions [cf. Hawley and Stone, 1995; Stone, 1999]. A variable
B are the electric and magnetic fields, respectively, at the Cartesian spatial grid was employed with a spatial grid size
location of the particle. The electric field is assumed to be ranging from 325 km near Mars up to 805 km far from
the motional electric field, E = u B, where u is the bulk Mars. This grid size was sufficiently small for a magnetic
plasma (i.e., solar wind) flow velocity. barrier, a draped field magnetotail (or wake), and a bow
[16] Two types of fields (cases) were investigated: (1) shock to all appear in the MHD model results. Note that
uniform fields everywhere (i.e., the upstream solar wind pickup O+ ions born upstream of Mars are insensitive to the
field values) and (2) fields taken from a three-dimensional finer details of the field pattern due to their very large
global MHD model of the solar wind interaction with Mars gyroradii. The field strength in the subsolar magnetic barrierSMP 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 non-
gyrotropic (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 down-
stream 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 Figure 6. (top) Same as Figure 4 (vx – vz scatterplot) but
if it crossed one of the four bins/planes described in the for the wake bin 15. The lines indicate projections of the
model section of the paper. Scatterplots are shown in directions opposite to the modeled instrument pointing
Figures 4 – 6. Ions are accepted from a full 4p solid angle direction for the spin number indicated. The mark on the
for these scatterplots. The upstream distribution is a line indicates the speed corresponding to a 55-keV oxygen
slightly modified nongyrotropic ring beam distribution ion. (bottom) Same as top panel but scatterplot in vx – vy
and looks very similar to the distribution that would be 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 spin modulated, and we also chose the peak fluxes for these
direction. comparisons; however, SLED had lower time/angle reso-
lution than our model did, which introduces a factor of 3
4.2. Calculated Ion Fluxes difference in the peak flux. The model ion fluxes are about
[21] We simulated the ion fluxes that SLED should see at a factor of 10– 20 greater than the measured fluxes and a
16 angles around its spin cycle at our four different bin scaling factor is included with the data shown in Figure 8
locations. Only ions whose velocity vectors put them within in order to facilitate the comparison. The channel 2 (72 –
the 40 instrument (full-width) acceptance cone were 223 keV) SLED fluxes were also placed entirely into a
recorded for each of the 16 look directions. The ions were single 5-keV interval for comparison purposes, which
further sorted according to their energy in 5-keV bins. introduces another scaling factor of 29.6 for this channel.
Figure 7 shows results for the inbound bow shock crossing [23] We also integrated the model ion fluxes over energy
for all instrument look directions for which any ions were for the SLED channels 1 and 2 energy ranges (55 –72 and
recorded (in this case, 8 of the directions recorded fluxes 72– 223 keV, respectively) in order to obtain a ‘‘SLED’’
while the other 8 directions did not record any ions). The count rate for each of the 16 look directions (0 through 15)
largest fluxes of O+ ions with energies near 55 keV (the SLED and for each of the four locations around Mars. However,
lower energy limit) were recorded for directions 10 and 11, we still used the better angle resolution of the model in
whereas smaller fluxes were recorded for direction 5 and order to see how sharp the peaks are for the angular
very little or no flux for the other directions. These directions dependence (i.e., time dependence). The results of this
(i.e., 5, 10, and 11) are indicated by the straight lines in procedure are shown in Figure 9. The spin variations look
Figures 4 – 6. For the two shock crossings or for the upstream very similar for the two shock locations and for the
case, directions 10 and 11 intersect the ring distribution at upstream location; the maximum flux appears near spin
positive vz where the ion flux is large, and direction 5 numbers 10 and 11. On the other hand, for the wake the
intersects the ring at negative vz where the ion flux is much maximum flux appears at spin look direction number 5. The
smaller. This asymmetry is due to the nongyrotropy of the ion model ion fluxes are confined to a rather narrow angular
distribution which, in turn, is due to increasing production range in Figure 9 (2 spin numbers or 45). The SLED
rate with decreasing distance from the planet. integration period is about a third of a spacecraft spin period,
[22] Figure 8 shows flux versus energy for the other three which is 5 to 6 of our spin angular intervals. Conse-
spatial locations but only for the look direction (or spin quently, the peak fluxes in a spin cycle from Figure 9 should
angle number) at which the flux was a maximum. Some be divided by a factor of 3 before being compared with the
SLED data from Afonin et al. [1991] are also indicated in SLED maximum flux (in a spin cycle) (from Figure 2).
Figure 8. The Afonin fluxes are multiplied by the solid Table 1 shows these comparisons for the two shock loca-
angle of the instrument (0.39 sr). The SLED fluxes were tions and for the wake. To reiterate, both the instrument andSMP 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 Figure 9. Model flux for SLED channels 1 and 2 energy
simulated SLED ‘‘data.’’ 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
5. Discussion and Summary same way as the SLED fluxes except that higher time
[24] Our combined test particle and MHD model (with resolution (i.e., angular resolution, 16 spin positions) is
nexo = 4000 cm3 in equation (1)) predicts ion fluxes in the retained for the model.CRAVENS ET AL.: PICKUP IONS NEAR MARS SMP 7-9
Table 1. Model-SLED Comparisonsa
Shock 1 Shock 2 Wake
Data (maximum in spin cycle) 1200 3000 300
Model (maximum in spin cycle) 33,000 39,000 3300
Model times 1/3 (modified maximum) 11,000 13,000 1100
Model times 1/3 1/6 (modified maximum and for lower O densities) 1830 2170 180
a
Oxygen flux 55 – 72 keV (channel 1) (cm2 s1 sr1).
rately, this is true only for those time periods when SLED of a radial distance from Mars of 10 Rm, and, hence,
actually did observe any ion flux. An examination of the proportional to the direct escape rate of oxygen produced
Orbit 42 data in Figure 2 indicates that for many time by the dissociative recombination of molecular oxygen ions
periods SLED did not record any oxygen flux. First, let us in the ionosphere. Oxygen can also be lost from the planet
consider this issue and then later we will discuss the factor due to ionization of bound oxygen atoms located closer to
of 6 model/data ratio. the planet than 10 Rm yet still in the solar wind or due to
[25] The model predicts the existence of significant direct outflow of ionospheric ions (mainly on the nightside).
pickup ion fluxes outside the bow shock, whereas SLED [27] In summary, our quantitative model supports the
only occasionally measured significant ion fluxes outside identification of the ions observed in channel 1 of the SLED
the shock. As suggested by Afonin et al. [1991], it is instrument as pickup oxygen ions that are created by the
difficult for pickup ions to enter the narrow SLED entrance ionization of oxygen atoms in the distant part of the exo-
cone at the energies measurable by SLED. Pickup ion sphere, as suggested by Kirsch et al. [1991]. Our model also
distributions, especially upstream of the shock, depend very explains the spin modulation exhibited by the SLED data.
sensitively on the solar wind speed and on the IMF Furthermore, the flux of energetic oxygen ions observed
direction. For the interplanetary conditions pertaining dur- near the orbit of the Phobos 2 provides new information on
ing Circular Orbit 42 (and adopted in the model) the ring the distant part of the hot oxygen corona.
beam distribution was intersected by look directions 10 and
11 (within the half-angle of 20), but if the IMF angle with [28] Acknowledgments. Support from NASA Planetary Atmospheres
respect to the solar wind direction had been 15 less than grant NAG5-4358 and NSF grant ATM-9815574 as well as financial
the value used (VB = 67) no intersection would have been support for the SLED instrument from the Irish National Board for Science
present for energies in excess of the SLED channel 1 lower and Technology is gratefully acknowledged. Computational support from
the Kansas Center for Advanced Scientific Computing and NSF MRI grant
limit of 55 keV. The magnetic field values we adopted for is also acknowledged.
Orbit 42 were ‘‘average’’ values estimated from upstream [29] Janet G. Luhmann thanks Stas Barabash and Hannu E. J. Koskinen
conditions, but an examination of Figure 2 indicates that for their assistance in evaluating this paper.
even during this one orbital pass the IMF direction varied
considerably. These variations were probably sufficient to References
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