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C. S. Arridge,
N. André,
H. J. McAndrews,
E. J. Bunce,
M. H. Burger,
K. C. Hansen,
H.-W. Hsu,
R. E. Johnson,
G. H. Jones,
S. Kempf, [......],
W. S. Kurth,
J. S. Leisner,
C. Paranicas,
E. Roussos,
C. T. Russell,
P. Schippers, E. C. Sittler,
H. T. Smith,
M. F. Thomsen,
M. K. Dougherty
[show abstract]
[hide abstract]
ABSTRACT: Saturn’s rich magnetospheric environment is unique in the solar system, with a large number of active magnetospheric processes
and phenomena. Observations of this environment from the Cassini spacecraft has enabled the study of a magnetospheric system
which strongly interacts with other components of the saturnian system: the planet, its rings, numerous satellites (icy moons
and Titan) and various dust, neutral and plasma populations. Understanding these regions, their dynamics and equilibria, and
how they interact with the rest of the system via the exchange of mass, momentum and energy is important in understanding
the system as a whole. Such an understanding represents a challenge to theorists, modellers and observers. Studies of Saturn’s
magnetosphere based on Cassini data have revealed a system which is highly variable which has made understanding the physics
of Saturn’s magnetosphere all the more difficult. Cassini’s combination of a comprehensive suite of magnetospheric fields
and particles instruments with excellent orbital coverage of the saturnian system offers a unique opportunity for an in-depth
study of the saturnian plasma and fields environment. In this paper knowledge of Saturn’s equatorial magnetosphere will be
presented and synthesised into a global picture. Data from the Cassini magnetometer, low-energy plasma spectrometers, energetic
particle detectors, radio and plasma wave instrumentation, cosmic dust detectors, and the results of theory and modelling
are combined to provide a multi-instrumental identification and characterisation of equatorial magnetospheric regions at Saturn.
This work emphasises the physical processes at work in each region and at their boundaries. The result of this study is a
map of Saturn’s near equatorial magnetosphere, which represents a synthesis of our current understanding at the end of the
Cassini Prime Mission of the global configuration of the equatorial magnetosphere.
KeywordsCassini–Saturn–Magnetospheric regions–Plasma processes
Space Science Reviews 05/2012; 164(1):1-83. · 3.61 Impact Factor
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A. Coustenis,
S. K. Atreya,
T. Balint,
R. H. Brown,
M. K. Dougherty,
F. Ferri,
M. Fulchignoni,
D. Gautier,
R. A. Gowen,
C. A. Griffith, [......],
C. Szopa,
R. Thissen,
M. G. Tomasko,
D. Toublanc,
H. Vali,
I. Vardavas,
V. Vuitton,
R. A. West,
R. Yelle,
E. F. Young
[show abstract]
[hide abstract]
ABSTRACT: TandEM was proposed as an L-class (large) mission in response to ESA’s Cosmic Vision 2015–2025 Call, and accepted for further
studies, with the goal of exploring Titan and Enceladus. The mission concept is to perform in situ investigations of two worlds
tied together by location and properties, whose remarkable natures have been partly revealed by the ongoing Cassini–Huygens
mission. These bodies still hold mysteries requiring a complete exploration using a variety of vehicles and instruments. TandEM
is an ambitious mission because its targets are two of the most exciting and challenging bodies in the Solar System. It is
designed to build on but exceed the scientific and technological accomplishments of the Cassini–Huygens mission, exploring
Titan and Enceladus in ways that are not currently possible (full close-up and in situ coverage over long periods of time).
In the current mission architecture, TandEM proposes to deliver two medium-sized spacecraft to the Saturnian system. One spacecraft
would be an orbiter with a large host of instruments which would perform several Enceladus flybys and deliver penetrators
to its surface before going into a dedicated orbit around Titan alone, while the other spacecraft would carry the Titan in
situ investigation components, i.e. a hot-air balloon (Montgolfière) and possibly several landing probes to be delivered through
the atmosphere.
Experimental Astronomy 04/2012; 23(3):893-946. · 1.82 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Plasma data from the Cassini Plasma Spectrometer experiment were used to
investigate the properties of the variable plasma environment of Titan's
orbit. Using Ion Mass Spectrometer data within +/- 3 hours of the Titan
flybys we could identify the different encounter types proposed earlier
(Rymer et al.) based on electron measurements. The ion data reveal
differences not only in characteristic energy and density but in the ion
composition as well. To be able to find (some of) the reasons of the
variability of the plasma environment, we examined how the variations
depend on the Saturnian local time (SLT) and on the position in the
Saturn Kilometric Radiation (SKR) based SLS3 longitude system. Both SKR
and SLT positions have significant influence on the plasma environment,
which shows up in the clustering of the encounters of different types as
well as in periodic variations of the ion moments.
AGU Fall Meeting Abstracts. 11/2009; -1:1642.
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[show abstract]
[hide abstract]
ABSTRACT: Yelle et al. [2008] have estimated from Cassini Ion Neutral Mass
Spectrometer (INMS) measurements that methane is escaping from
Titan’s upper atmosphere at the rate of
2.5-3.0×109 mol/cm2/s and in order to
explain this loss rate Strobel [2008] has proposed a hydrodynamic escape
model to explain such high loss rates. This translates to loss of
2.8×1027 methane mol/s. The consequence of this work is
the formation of a methane torus around Saturn which will dissociate to
CH3 and other fragments of methane. The CH3 will
then become ionized to form CH3+ with pickup
energies ≈ keV after which it can be detected by the Cassini Plasma
Spectrometer (CAPS) Ion Mass Spectrometer (IMS). Up till now the ion
composition within Saturn’s outer magnetosphere in the vicinity of
Titan’s orbit have yielded negative results with water group ions
W+ dominating. The water group ions probably result from the
emission of fast neutrals from the Enceladus torus via charge exchange
reactions but still gravitationally bound to Saturn [see Johnson et al.,
2005 and Sittler et al. 2006] and then become ionized in the outer
magnetosphere as ~≈keV pickup ions. The CAPS IMS produces two ion
composition data products, one called Straight Through (ST) and the
other Linear Electric Field (LEF). The first has a higher sensitivity,
while the latter has a greater discrimination in time-of-flight (TOF).
For ST data O+ and CH4+ have similar
TOF with the primary discriminator being the O- fragment which appears
at a different TOF than for mass 16 ions. One can also look for other
discriminators called ghost peaks. In case of LEF W+ ions produce TOF
peak close to that for atomic O+ and the methane will produce
TOF close to that for atomic C+ which has a significantly
different(shorter) TOF than O+. We will be reporting on our
continual search for methane ions within Saturn’s outer
magnetosphere. References: 1. Yelle, R. V., J. Cui and I.C.F.
Müller-Wodarg, JGR, 2008. 2. Strobel, D. F., Icarus, 193, 588,
[2008] 3. Johnson. R.E., et. Astrophys. J. Letts, 644, L137-L139, 2005
4. Sittler, E. C., Jr., et al., JGR, 111, A09223, 2006
AGU Fall Meeting Abstracts. 11/2009; -1:1232.
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A M Rymer,
D G Mitchell,
T W Hill,
B H Mauk, E C Sittler,
J Leisner,
C T Russell,
K Khurana,
Y Jia,
F J Crary,
D T Young,
A J Coates,
J Saur,
S Jacobsen,
M K Dougherty
[show abstract]
[hide abstract]
ABSTRACT: It is increasingly well accepted that, despite its diminutive size, the tiny icy moon Enceladus is the dominant source of water group neutrals and charged particles throughout Saturn's magnetosphere through the copious gas and dust emanations from its South pole. During two recent Cassini flybys the spacecraft plasma instruments were oriented such that they looked along a magnetic flux tube nominally connecting the Enceladus plume to Saturn's ionosphere. Two of the remarkable discoveries from these observational campaigns were, 1) high energy (10s-100s of keV) field aligned ion beams propagating from Saturn toward the plume and 2) lower energy field aligned electron beams which were observed to 'flicker' in energy from 10s of eV to several 100 eV. Initial speculation was that this is evidence of an Alfven wing type interaction, such as exists at Io due to significant mass loading in the wake of the moon. It was subsequently realised that the magnetic field signature is not consistent with this simple picture, leading us to speculate that there exists a more filamentary Birkeland current system with the observed variability linked to the highly dynamic and variable nature of the Enceladus outgassing. Ions could be accelerated by wave activity or field-aligned potential drops just above the ionosphere, but we have yet to ascertain if either is sufficient to explain the observed very high energy ion beams. Additionally we will show that similar phenomena exist near the L-value of Enceladus, but away from the moon - implying the existence of a significant extended Enceladus plasma torus.
AGU Spring Meeting Abstracts. 01/2009; 32:06.
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[show abstract]
[hide abstract]
ABSTRACT: The remarkable observation that Enceladus, a small icy satellite of Saturn, is actively venting has led to the suggestion that ejected water molecules are the source of the toroidal atmosphere observed at Saturn for over a decade using the Hubble Space Telescope (HST). Here we show that the venting leads directly to a new feature, a narrow Enceladus neutral torus. The larger torus, observed using HST, is populated by charge exchange, the process that limits the lifetime of the neutrals in the Enceladus torus.
The Astrophysical Journal 12/2008; 644(2):L137. · 6.02 Impact Factor
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[show abstract]
[hide abstract]
ABSTRACT: Upward flow of ionospheric plasma into the induced magnetic tail of
Venus was inferred some time ago from Pioneer Venus Orbiter (PVO)
measurements, which were used to derive upward flow and acceleration of
H+, D+ and O+ within the nightside ionosphere [1]. The measurements
revealed that the polarization electric field in the nightside
ionosphere produced the principal upward force on these light ions.
Other electrodynamic forces were unimportant because the plasma beta in
the nightside ionosphere is much greater than one. The resulting
vertical flow of H+ and D+ was found to be the dominant escape mechanism
of hydrogen and deuterium, corresponding to loss rates consistent with
large oceans in early Venus [2]. Recently, plasma measurements made from
Venus Express have clearly identified H+, D+ and O+ flowing away from
Venus, down its magnetic tail [3]. The primary source of tail-flowing O+
is from the high altitude day-to-night flow system. Similarly, at
unmagnetized Titan, ions have been observed to flow away from the moon
along its induced magnetic tail by the Plasma Science Instrument (PLS)
on Voyager 1 and the Casini Plasma Spectrometer (CAPS) on Cassini. In
both cases, the ions have been inferred to be of ionospheric origin.
Although the plasma beta is also greater than one in much of Titan's
ionosphere, ion acceleration is expected to be more complex, especially
because the subsolar point and the subflow points can be 180 degrees
apart. Following what we learned at Venus, upward acceleration of light
ions by the polarization electric field opposing gravity in the
wake-side ionosphere of Titan is described. Additional electrodynamic
forces resulting from the interaction of Saturn's magnetosphere with
Titan's ionosphere will be examined using a recent hybrid model [4].
Comparisons between the wake-side flows on Venus and Titan will be made.
[1] Hartle, R. E. and J. M. Grebowsky, Adv. Space Res., 15, (4)117,
1995. [2] Donahue, T. M. and R. E. Hartle, Geophys. Res. Lett., 19,
2449, 1992. [3] Barabash, S., et al., Nature, 450, 650, 2007. [4]
Lipatov, A. S., E. C. Sittler, Jr. and R. E. Hartle, 2007 Fall AGU mtg.,
EOS, P23B-1366, 2007.
AGU Fall Meeting Abstracts. 11/2008; -1:1324.
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[show abstract]
[hide abstract]
ABSTRACT: At the time of the Voyager 1 close encounter with Titan, enhancements of
density and sharp decreases of electron temperature were observed by the
PLS instrument. A few smaller plumes were identified inside and outside
Titan orbit and their displacements correlated with variations in solar
wind dynamic pressure observed by Voyager PLS one and two corotation
periods earlier. At the time, two conflicting interpretations of this
phenomenon were proposed. In one interpretation (Eviatar et al., JGR,
87, 8091, 1982, the enhancements were regarded as plumes drawn out of
the ionosphere of Titan by the corotation electric field. The secondary
enhancements were taken to be old plumes that had been wrapped around
Saturn, had begun to decay and to merge into the magnetosphere
environment. An alternative interpretation, proposed by Goertz (GRL,
1983, 10, 455), viewed them as blobs of plasmas detached by flute or
Kelvin-Helmholtz instability from the central body of Saturn plasma in
the inner magnetosphere. The Voyager PLS instrument was unable to make
a firm composition determination which would have resolved the question.
In this study, we use Cassini/CAPS data to identify plumes and blobs of
plasma and classify them by source by means of composition and
temperature. We find that all three types of plasma bodies, primary
plumes, secondary wrapped around plumes and sloughed off blobs exist in
the Titan-dominated region of the magnetosphere.
AGU Fall Meeting Abstracts. 11/2008; -1:09.
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[show abstract]
[hide abstract]
ABSTRACT: Crossings of Saturn's magnetopause made by the Cassini spacecraft
between 12 and 17 March 2006 are analysed. Magnetic field and plasma
data are used to identify excursions into the magnetosheath bounded by
crossings of the magnetopause current layer. During most of this period
Cassini's trajectory was approximately parallel to the magnetopause
boundary given by a model of the surface. Minimum variance analysis
(MVA) of the magnetic field vector measurements is used to determine the
normal to the boundary for each crossing of the current layer. The
normals corresponding to the crossings made on 12, 13 and 17 March
oscillate about the normal predicted by the surface model. This suggests
the presence of regular boundary waves with a direction of propagation
found to be close to parallel to Saturn's rotational equator, and not
coincident with the expected solar wind flow direction in the local
magnetosheath. Based on this propagation direction and the
magnetospheric and magnetosheath magnetic fields we propose that these
waves were generated by the Kelvin-Helmholtz instability. In addition we
discuss the possibility that on 15 and 16 March nonlinear
Kelvin-Helmholtz waves produced a strongly perturbed magnetopause
boundary that may have led to local magnetic reconnection.
AGU Fall Meeting Abstracts. 11/2008; -1:08.
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[show abstract]
[hide abstract]
ABSTRACT: We present evidence in the CAPS data for two `states` encountered in the
magnetospheric tail. One state is represented by mass-loaded flux tubes,
subcorotating with a stretched and lagging field configuration. These
flux tubes have yet to pinch-off and release plasma downtail in the form
of plasmoids or a planetary wind. The second state consists of the
depleted, less-stretched flux tubes which indicate plasma release in the
nightside tail. We discuss the plasma characteristics of both states and
the implications for tail dynamics and global mass- loss.
AGU Spring Meeting Abstracts. 04/2008; -1:01.
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[show abstract]
[hide abstract]
ABSTRACT: We discuss the results of the hybrid simulation of Titan's environment
in case of T9 encounter. The simulations are based on recent analysis
of the Cassini Plasma Spectrometer (CAPS) ion measurements during the
T9 flyby through the induced magnetic tail of Titan [Sittler et al,
2008]. This new result changes our previous model of the interaction of
Saturn's rotating magnetosphere with Titan from one that was discussed
in the recent publications. The current simulation shows that mass
loading by pickup ions H+, H_2+, CH4+ and N2+ is stronger than in the
previous simulations. In our hybrid simulations we use Chamberlain
profiles for the exosphere's components. We also include a simple
ionosphere model. Special attention will be paid to comparing our
numerical results with Cassini T-9 observations. We shall estimate the
mass loading rate and the energy input to the upper atmosphere from
ambient and pickup ions for the T9 encounter. E C Sittler et al., Spring
AGU Meeting, 2008.
AGU Spring Meeting Abstracts. 04/2008; -1:06.
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[show abstract]
[hide abstract]
ABSTRACT: The plasma wake formed by the interaction of Saturn's rotating
magnetosphere with Titan was observed by the Cassini Plasma Spectrometer
(CAPS) Electron Spectrometer (ELS) and Ion Mass Spectrometer (IMS)
during the T9 flyby (Coates et al., 2007; Szego et al, 2007). Recent
analysis of the CAPS ion mass spectrometer measurements by Sittler et
al. [2008] has revealed that protons dominate the ion composition of
Saturn's magnetosphere plasma flowing upstream of Titan during the T9
flyby. They show that light magnetospheric ions predominantly extend as
far as T9's trajectory because Titan is sufficiently below Saturn's
neutral sheet (Bertucci, et al., 2007), the region expected to confine
the bulk of the heavy ions. Previous interaction studies for other
flybys included significant fractions of heavy ions in the magnetosphere
such as O+ along with protons. In contrast, the momentum of the
magnetospheric plasma interacting with Titan during T9 is significantly
lower than considered in previous flyby studies. In this case, the
lighter proton magnetosphere plasma interacts noticeably with the
lighter pickup ions, H2+ and H+, playing a more important role in mass
loading and deflecting magnetosphere plasma around Titan. The influence
of these processes extends to great altitudes (and possibly out to the
Hill sphere), well beyond the inner region where mass loading by N2+ and
CH4+ pickup ions is dominant. This outer region begins beyond an
altitude of about a Titan radius, where H2 and H are the dominant
exosphere constituents as are their pickup ion progeny. The extent of
this much larger sphere of influence on the interaction is analyzed
using previously developed pickup ion and mass loading models. Also
described are the effects of the heavier pickup ions, N2+ and CH4+, on
the interaction closer to Titan. Results from a hybrid model of Lipatov
et al. [2008] will also be applied to this study. Coates, A. J., et al.,
GRL, Vol. 34, L24S05, 2007. Szego, K., et al., GRL, Vol. 34, L24S03,
2007. Sittler, E, C. et al., Spring AGU meeting, 2008. Bertucci, C., et
al., GRL, Vol. 34, L24S02, 2007. Lipatov, A., et al. Spring AGU meeting,
2008.
AGU Spring Meeting Abstracts. 04/2008; -1:06.
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A. J. Coates,
H. J. McAndrews,
C. S. Arridge,
G. H. Jones,
F. J. Crary,
D. T. Young,
K. Szego, E. C. Sittler,
M. F. Thomsen,
R. L. Tokar,
C. Bertucci,
M. K. Dougherty
[show abstract]
[hide abstract]
ABSTRACT: Titan spends most of its time inside Saturn's magnetosphere, thus most
of Cassini's encounters to date have been with Titan surrounded by
magnetospheric plasma. During the encounter on 13 June 2007, however,
the magnetosheath was close enough that the encounter itself happened
very close to the magnetopause. In this talk we present the upstream
plasma conditions, and show the measured electron and ion spectra during
this so far unique event. Magnetosphere and magnetosheath plasma are
both clearly seen, and at times there are mixed populations. Titan's
ionosphere is clearly distinguished by cold plasma, and very heavy
negative ions are seen by CAPS ELS near closest approach. We discuss
the plasma results and their implications.
AGU Fall Meeting Abstracts. 11/2007; -1:1021.
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[show abstract]
[hide abstract]
ABSTRACT: One of the major discoveries1 of Cassini to date is the south polar icy
plume at Enceladus (R ~ 4 RS). Models2 predict that this plume may be a
source of both the extended (2-8 RS) OH neutral cloud observed by the
Hubble space telescope3, and a new feature, a narrow (~0.5 to 1.0 RS)
neutral water group torus centered on the Enceladus orbit. As the
corotating and magnetically confined thermal plasma (mostly water group
ions) streams through the gravitationally bound water group neutrals,
charge exchange between the ions and neutrals is expected4 to occur
yielding slower ions subsequently "picked up" by Saturn's magnetic
field. The phase space density of these ions should show characteristics
of a ring velocity distribution within the source, combined with
subsequent scattering into a shell and possible adiabatic cooling at
larger radial distances. Therefore, via analysis of Cassini in situ ion
counting data, it may be possible to indirectly detect the neutral
Enceladus torus, confirming the predictions in (2). In this study,
Cassini plasma spectrometer (CAPS) data for equatorial orbits with
favorable viewing5 are analyzed. The radial distance range of about 3.5
to 6.5 RS is considered covering data across the Enceladus orbit.
Assuming flow speeds near co-rotation as reported in (6) yields a
modeled water group ion core that is subtracted from the measured data.
The resulting residual ion counting data has velocity space signatures
resembling pick up ions. The strongest source region is identified about
the Enceladus orbit with radial extent at least 1 RS, in qualitative
agreement with predictions. Peak phase space density of these ions is
perpendicular to the magnetic field, resembling a ring, as expected
within the source region. At larger radial distances (e.g. R = 6 RS),
the ring signature has evolved to a shell and the expected adiabatic
cooling due to transport from the source outward is observed. Similarly
strong pick up ion sources are not observed near the orbits of either
Tethys or Dione. 1.) Science, "Cassini at Enceladus", special section,
10 March 2006. 2.) Johnson, R.E. et al., The Astrophysical Journal, pg
L137, 20 June 2006. 3.) Shemansky et al., Nature, 27 May 1993.
4.) Johnson, R.E., M. Liu, and E.C. Sittler, Jr., Geophys. Res. Letts.,
32, 17 Dec 2005. 5.) Wilson, R.J. et al., this meeting. 6.) Sittler,
E.C. et al., Geophys. Res. Letts., 32, 15 June 2005.
AGU Fall Meeting Abstracts. 11/2007; -1:06.
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[show abstract]
[hide abstract]
ABSTRACT: Cassini observations in Saturn's magnetotail during 2006 show evidence
of periodic encounters with the plasma and current sheet. In this talk
we discuss these encounters as observed by the Cassini magnetometer
(MAG) and electron spectrometer (ELS). We emphasise that the tail
observations can simply be interpreted in terms of periodic vertical
oscillations of the sheet, obviating the need for a sub-corotating plume
structure as advocated by some authors. Furthermore, many encounters
with the sheet are "double-peaked" which can also be simply explained by
the vertical oscillation paradigm. We explore these vertical
oscillations, consider simple structural models to fit the observations,
and compare our results with data from other instruments (e.g., Carbary
et al. 2007).
AGU Fall Meeting Abstracts. 11/2007; -1:0194.
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[show abstract]
[hide abstract]
ABSTRACT: Within Saturn's magnetosphere, plasma produced by the system of rings
and moons continually loads the field lines. To avoid unlimited mass
build-up, the release of plasma is predicted downtail either as a
planetary wind or during substorm-like processes. The newly emptied flux
tubes will then return to the dayside, depleted of plasma, to complete
the circulation. The goal of the present study is to determine the
equatorial flows in the night-side region to explore this scenario. Does
the plasma appear to corotate? Are radial flows observed both outwards
suggestive of tailward plasma release and inwards indicating the return
of newly closed field-lines? If so, at what radial distances and local
times are these observed? What plasma characteristics do these flows
exhibit? Approximately 30 entries into the plasma sheet have been
identified between October 2005 and April 2007 when Cassini was between
5 RS and 45 RS from Saturn, at local times between 5 to 20 LT, and close
to the equatorial plane. This data set includes the preliminary events
previously presented by Sittler et al., [2007a, b, c], where both
outward and inward flows super-imposed on rotational motion were
identified. For each plasmasheet encounter we employ data from the
Cassini Plasma Spectrometer (CAPS) to determine preliminary ion flow
directions and magnitude, as well as the ion and electron energy spectra
and the ion composition. Our preliminary results find that the ion flows
are generally in the corotation direction, but significantly lagging the
corotation speed. There is also generally an outward (tailward)
component to the derived velocities. Of the events surveyed, the IMS
instrument was looking tailward (and therefore able to see inward flows)
approximately 60 % of the time. However, inward flows (i.e. VR < 0)
were only observed approximately 10 % of the time, demonstrating the
rarity of such signatures in this region. Moreover, the ion composition
persistently has a strong water-group component, rather than being
relatively depleted of heavy ions as expected for recently emptied flux
tubes returning to the inner magnetosphere. E. C. Sittler et al.,
Cassini Observations of Saturn's Dawn-Magnetotail Region: Preliminary
Results, Magnetospheres of the Outer Planets (MOP), San Antonio, TX,
June 25-29, 2007a. E. C. Sittler et al., Cassini Observations of
Saturn's Dawn-Magnetotail Region: Preliminary Results, Europlanet,
EPSC2007-A-00428, Potsdam, Germany, August 19-24, 2007b. E. C. Sittler
et al., Cassini Observations of Saturn's Magnetotail Region:
Preliminary results, Fall AGU, San Francisco, CA, December 10-14, 2007c.
AGU Fall Meeting Abstracts. 11/2007; -1:1085.
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[show abstract]
[hide abstract]
ABSTRACT: The composition and structure of neutral exospheres imbedded in moving
plasmas can be determined by measurements of the velocity distributions
of their pickup ion progeny. In turn, the velocity distributions are
dependent on the spatial structure of the neutral source gases. Since
Titan's neutral exosphere extends into the Saturn's magnetosphere (or
solar wind) and well above its ionopause, it serves as a good place to
analyze such characteristics. They are analyzed using pickup ion
measurements made by the Cassini Plasma Spectrometer (CAPS) at Titan
[e.g., Hartle et al., 2006] and an ion kinetic model. The model [Hartle
and Sittler, 2007] is an expression describing the phase space density
of pickup ions, which is derived from the Vlasov equation with an ion
source that explicitly accounts for the velocity and spatial variation
of the exosphere source gases. A fundamental parameter in the phase
space density expression is the ratio of the gyroradius to the neutral
scale height., α = rg/H. Titan's exosphere includes H, H2, CH4 and
N2, with scale heights near the exobase of ~ 2400, 1200, 149 and 85 km,
and α of ~ 0.1, 0.4, 27, and 82, respectively. This structured
exosphere yields pickup ions whose phase space distributions are
beam-like when α >> 1 and fluid- like when α <<
1. Downstream from the source peak, the light pickup ions, with α
<< 1, are easily observed because the phase space density is
almost uniform over the orbit phases. On the other hand, the phase
space distribution of the heavier ions, with α >> 1, peaks
over a narrow velocity and spatial range. This beam-like nature makes it
more difficult to observe the heavy ions because the downstream
positions and viewing directions are constrained. Examples of these
extremes will be discussed. Hartle et al., Planet. Space Sci. vol. 54,
1211, 2006 Hartle and Sittler, J. Geophys. Res., in review, 2007
AGU Spring Meeting Abstracts. 04/2007; -1:08.
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[show abstract]
[hide abstract]
ABSTRACT: 1] On 14 July 2005, Cassini passed within 175 km of Enceladus' surface enabling a direct in situ measurement of water escaping from the surface by the Ion and Neutral Mass Spectrometer (INMS) and the observation of a stellar occultation by the Ultraviolet Spectrometer (UVIS). We have developed a three-dimensional, Monte Carlo neutral model to simultaneously model the INMS and UVIS measurements of water gas density and column density, respectively. The data are consistent with a two-component atmosphere; the first with a weak, distributed source on the surface which, if global, has a source rate of $8 Â 10 25 H 2 O/s, and the second with a much larger source localized at the south pole with a source rate $10 28 H 2 O/s. This latter source is possibly coincident with the ''tiger stripe'' series of fractures revealed by the Imaging Science Subsystem instrument where the ice was measured to be warmer than the surrounding regions by the Composite Infrared Spectrometer instrument. We estimate the plasma mass loading rate due to interaction between the plume and magnetospheric plasma is 2–3 kg/s for a plume source of 10 28 H 2 O/s. Pickup of water group ions in the plume slows down the plasma to $10 km/s relative to Enceladus in the region of, and downstream of, the south polar plume. This is consistent with the mass loading rate inferred from magnetic field perturbations detected during the Cassini flyby and suggests an additional source may be needed to explain the plasma flow deflections detected by the Cassini Plasma Spectrometer.
J. Geophys. Res. 01/2007; 112.
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R L Tokar,
R E Johnson,
T W Hill,
D H Pontius,
W S Kurth,
F J Crary,
D T Young,
M F Thomsen,
D B Reisenfeld,
A J Coates,
G R Lewis, E C Sittler,
D A Gurnett
[show abstract]
[hide abstract]
ABSTRACT: During the 14 July 2005 encounter of Cassini with Enceladus, the Cassini Plasma Spectrometer measured strong deflections in the corotating ion flow, commencing at least 27 Enceladus radii (27 x 252.1 kilometers) from Enceladus. The Cassini Radio and Plasma Wave Science instrument inferred little plasma density increase near Enceladus. These data are consistent with ion formation via charge exchange and pickup by Saturn's magnetic field. The charge exchange occurs between neutrals in the Enceladus atmosphere and corotating ions in Saturn's inner magnetosphere. Pickup ions are observed near Enceladus, and a total mass loading rate of about 100 kilograms per second (3 x 10(27) H(2)O molecules per second) is inferred.
Science 04/2006; 311(5766):1409-12. · 31.20 Impact Factor
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R E Hartle, E C Sittler,
F M Neubauer,
R E Johnson,
H T Smith,
F Crary,
D J Mccomas,
D T Young,
A J Coates,
D Simpson,
S Bolton,
D Reisenfeld,
K Szego,
J J Berthelier,
A Rymer,
J Vilppola,
J T Steinberg,
N Andre
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ABSTRACT: The Cassini plasma spectrometer (CAPS) instrument made measurements of Titan's plasma environment when the Cassini Orbiter flew through the moon's plasma wake October 26, 2004 (flyby TA). Initial CAPS ion and electron measurements from this encounter will be compared with measurements made by the Voyager 1 plasma science instrument (PLS). The comparisons will be used to evaluate previous interpretations and predictions of the Titan plasma environment that have been made using PLS measurements. The plasma wake trajectories of flyby TA and Voyager 1 are similar because they occurred when Titan was near Saturn's local noon. These similarities make possible direct, meaningful comparisons between the various plasma wake measurements. They lead to the following: (A) The light and heavy ions, H + and N + /O + , were observed by PLS in Saturn's magnetosphere in the vicinity of Titan while the higher mass resolution of CAPS yielded H + and H 2 + as the light constituents and O + /CH 4 + as the heavy ions. (B) Finite gyroradius effects were apparent in PLS and CAPS measurements of ambient O + ions as a result of their absorption by Titan's extended atmosphere. (C) The principal pickup ions inferred from both PLS and CAPS measurements are H + , H 2 + , N + , CH 4 + and N 2 + . (D) The inference that heavy pickup ions, observed by PLS, were in narrow beam distributions was empirically established by the CAPS measurements. (E) Slowing down of the ambient plasma due to pickup ion mass loading was observed by both instruments on the anti-Saturn side of Titan. (F) Strong mass loading just outside the ionotail by a heavy ion such as N 2 + is apparent in PLS and CAPS measurements. (G) Except for the expected differences due to the differing trajectories, the magnitudes and structures of the electron densities and temperatures observed by both instruments are similar. The high-energy electron bite-out observed by PLS in the magnetotail is consistent with that observed by CAPS.
Planetary and Space Science 01/2006; 54:1211-1224. · 2.22 Impact Factor
