One-dimensional hybrid simulations of planetary ion
pickup: Techniques and verification
M. M. Cowee,1R. J. Strangeway,1C. T. Russell,1and D. Winske2
Received 30 July 2006; revised 20 September 2006; accepted 25 October 2006; published 20 December 2006.
 Previously, hybrid simulation techniques using massless fluid electrons and kinetic
ions have been successfully applied to study the electromagnetic plasma waves
generated by ion pickup in the solar wind, where instability is driven by the large drift
velocities of newborn ion populations. For ion pickup at Jupiter and Saturn’s
magnetospheres where instability is driven by heavy ions with a ring velocity
distribution, we show that the one-dimensional hybrid simulation technique can
successfully reproduce the behavior of this instability as predicted by linear dispersion
theory as well as the important nonlinear wave-particle interactions. The simulated ion
cyclotron waves have frequencies near the ion gyrofrequency and are generated as the
anisotropic newborn ion ring distribution scatters to a more isotropic configuration.
Simulated maximum wave amplitudes and instability growth rates increase with newborn
ion density and pickup velocity. For appropriate heavy pickup ion densities and velocities
the simulated wave amplitudes are within the range observed by spacecraft.
Citation: Cowee, M. M., R. J. Strangeway, C. T. Russell, and D. Winske (2006), One-dimensional hybrid simulations of planetary
ion pickup: Techniques and verification, J. Geophys. Res., 111, A12213, doi:10.1029/2006JA011996.
 Ion pickup processes occur in many planetary environ-
ments in our solar system. When neutral particles from
ionization, and charge exchange they become subject to
electric and magnetic forces. If pickup ions are significantly
accelerated they form an anisotropic population with suffi-
cient free energy for wave generation. Electromagnetic
plasma waves which appear to be generated by unstable
populations of newborn pickup ions have been identified in
the planetary environments of Earth [Le et al., 2001], Venus
[Russell et al., 2006], Mars [Barabash et al., 1991; Russell et
al., 1990], Jupiter [Warnecke et al., 1997; Russell and
Kivelson, 2001; Russell et al., 2003], and Saturn [Smith
and Tsurutani, 1983; Leisner et al., 2006]. At Venus, Mars,
and comets, newborn ions picked up solar wind can have
supra-Alfvenic speeds in the directions both parallel (drift)
and perpendicular (gyration) to the ambient magnetic field,
depending on the angle between the solar wind velocity and
the magnetic field. Inside Jupiter and Saturn’s magneto-
spheres, neutral particles from the satellites and rings are
ionized,picked upbythecorotating magnetodiskplasmaand
accelerated to sub-Alfvenic speeds in the direction perpen-
dicular to the planet’s ambient magnetic field. Because
Jupiter and Saturn’s magnetic fields are, to first order,
perpendicular to the plane of the magnetodisk where ion
pickup takes place, the newborn ion populations there will
have very little parallel drift velocity.
 Wavegenerationbypickupionpopulations inthesolar
wind has been well studied in cometary environments.
Observations of comets Giacobini-Zinner and Halley [Le et
al., 1989; Thorne and Tsurutani, 1987] detected right-hand
polarized waves near the water group ion gyrofrequencies.
Because newly ionized cometary ions have a large velocity
component parallel to the ambient magnetic field, the wave
frequency is Doppler shifted between the frame of reference
of the ions (or the spacecraft) and the solar wind; the waves
are left-hand polarized in the frame of the ions but right-hand
polarized in the solar wind. Hybrid simulations of this
instability have reproduced the waves and shown that as
the newborn cometary ions exchange energy with the waves
they pitch angle scatter forming a more isotropic velocity
distribution [Gary et al., 1989, and references therein].
 In the Jovian and Saturnian magnetospheres, left-hand
polarized waves were detected at frequencies near the local
pickup ion gyrofrequencies because in the perpendicular
pickup geometry of these environments Doppler shifts are
small [Warnecke et al., 1997; Leisner et al., 2006]. The
perpendicular pickup geometry favors the generation of
highly anisotropic ‘‘ring’’-type ion velocity distributions
with T?> Tk, where ? and k denote directions relative to
the ambient field, B0(Figure 1). Instability driven by a zero
drift ring velocity distribution was previously simulated
using hybrid techniques for conditions conditions in the
Earth’s magnetotail by Convery and Gary ; however,
they used ring densities, velocities, and temperatures much
higher than occurs at Jupiter and Saturn and did not consider
heavy ions. Machida et al.  used the hybrid simulation
to reproduce proton cyclotron waves near the Jovian moon,
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 111, A12213, doi:10.1029/2006JA011996, 2006
1Institute of Geophysics and Planetary Physics, University of
California, Los Angeles, California, USA.
2Los Alamos National Laboratory, Los Alamos, New Mexico, USA.
Copyright 2006 by the American Geophysical Union.
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Io, but analytic study has shown that protons do not play a
role in the generation of the heavy ion cyclotron waves
observed there [Huddleston et al., 1997]. Machida et al.
from inward radial diffusion rather than ion pickup and did
not consider heavy ion cyclotron waves as they had not been
observed at Io by Voyager [Thorne and Scarf, 1984]. They
concluded that extending the study to include more realistic
conditions in the Jovian magnetosphere, which would
include heavy ions, was not possible at that time because of
the increased computational runtime and cumulative numer-
 This paper presents the first self-consistent simulation
of the heavy ion, zero-drift, ring anisotropy instability result-
ing from ion pickup into the sub-Alfvenic plasma environ-
ments at Jupiter and Saturn. We use ion pickup waves in the
Io plasma torus as our primary example to show that the
instability behavior is well reproduced in simulations. Using
the hybrid simulation is important because although linear
dispersion theory has explained the existence of pickup ion
waves at Io and predicted their dispersion properties, it
cannot predict wave amplitudes which are key to relating
the observed waves to specific plasma or pickup ion con-
ditions. Additionally, linear dispersion theory cannot predict
during ion pickup. Simulations with ion injection to model
the continuous creation of new pickup ions is not included
this study but will be the subject of future work.
 The paper is organized as follows: section 2 briefly
summarizes the observations of ion cyclotron waves and
plasma conditions in the Io torus; section 3 describes the
linear dispersion analysis; section 4 describes the hybrid
simulation technique; section 5 shows that the hybrid
simulation reproduces the behavior predicted by linear
theory at Io; section 6 discusses future application of the
one-dimensional (1-D) hybrid simulation to understanding
the ion pickup processes in planetary environments. In
addition, section 5 briefly shows simulation results appli-
cable to Saturn’s E-ring.
2.Observations at Io
 The Galileo spacecraft collected magnetic field data
during six of its seven passes by Io and observed ion
cyclotron waves each time. To first order, the waves were
left-hand circularly polarized and propagated along the
ambient magnetic field with frequencies near the local ion
gyrofrequency. Wave amplitude varied along each pass,
conditions and the mass loading process. On every pass
waves near the gyrofrequencies of SO2
observed and S+waves were also observed on two of the
passes. During some periods of time only one of the two
a distance of 7 RIoinward and 20 RIooutward of Io. For a
more detailed discussion of the wave properties and example
wave spectra, refer to Russell and Kivelson  and
Russell et al. .
 During the Voyager flyby, plasma composition mea-
surements showed the surrounding plasma torus was mainly
composed of thermalized atomic O and S ions with only less
than 1% SO2
Io measured similar composition at closest approach with an
amplitudes of the waves are directly proportional to the
number density,mass,and pickup velocity ofions generating
them, Huddleston et al.  estimated the SO2
loading rate at Io as ?8 ? 1026ions/s. This value is sensitive
+ions [Bagenal, 1994]. The first Galileo pass by
+density of 5% [Kivelson et al., 1996]. Assuming that the
Figure 1. SchematicillustrationofthepickupgeometryofIogenicions,showingthepickupioncycloidal
plasma. From Huddleston et al. .
COWEE ET AL.: HYBRID SIMULATION OF ION PICKUP WAVES
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we can better constrain the properties of the background
plasma and the newborn ions and the dynamics and associ-
ated timescales of ion pickup.
Yin, and Peter Gary at Los Alamos National Laboratory for their expertise
for Cassini data. The work was supported by the National Aeronautics and
Space Administration under research grant NAG 5-12022, by a grant from
the Jet Propulsion Laboratory for the analysis of data from the Cassini
Mission, and by summer student funding from the Institute of Geophysics
and Planetary Physics at Los Alamos National Laboratory.
 Amitava Bhattacharjee thanks the reviewers for their assistance in
evaluating this paper.
The authors wish to thank Dan Winske, Lin
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? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?
M. M. Cowee, C. T. Russell, and R. J. Strangeway, Institute of
Geophysics and Planetary Physics, University of California, Los Angeles,
Los Angeles, CA 90095, USA. (email@example.com)
D. Winske, Los Alamos National Laboratory, Los Alamos, New Mexico,
Figure 11. Simulated maximum fluctuations magnetic
field energy densities for ring densities between 0.3 and
0.8 and ring velocities of 15 km/s (circles) and 20 km/s
(squares). Run parameters given in Table 4.
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