Cassini Observations of Plasma Mass Loading Near Ence-ladus

R.L.Tokar1, R.E.Johnson2, T.W.Hill3, D.H.Pontius3, F. J.Crary4, D.B.Reisenfeld5, A.J. Coates6, W.S. Kurth7, M.F. Thomsen1, L.Gilbert6, D.T. Young4, E.C. Sittler8, M.H. Burger2 and

D.A.Gurnett7

1Space Science and Applications, Los Alamos National Laboratory, Los Alamos, NM 87545, rlt@lanl.gov

2The University of Virginia, Charlottesville,VA3Rice University, Houston, TX

4Southwest Research Institute, San Antonio, TX5The University of Montana, Missoula, MT

6University College London, UK7The University of Iowa, Iowa City, IA

8Goddard Space Flight Center, Greenbelt, MD

On July 14, 2005, the Cassini spacecraft executed a close flyby of the Saturnian

satellite Enceladus, passing within about 175 km of its surface. During this encounter, the

Cassini plasma spectrometer (1), CAPS, observed a significant slowing and deflection of

the flow velocity of ions (H2O+, H3O+,O+,OH+), extending at least 30 Enceladus radii away

from the satellite (1 RE = 249 km). This effect is caused by ions that are formed in the neu-

tral atmosphere that is escaping from Enceladus and then picked up by Saturn’s rotating

magnetic field. The ese data are used here to show the interaction differs from that at Io

and to infer the mass loading rate close to Enceladus which we and to connect that rate to

the neutral escape rate seen by the Cassini ion and neutral mass spectrometer escaping

neutral atmosphere of predominantly H2O seen by the Cassini ion and neutral mass spec-

trometer.

The pick-up procees may also account e strong source of H2O molecules is for athe

‘missing’ source ofn important contributor both to the neutral OH torus observed re-

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motely by the Hubble Space Telescope (HST) (2) and to the heavy ion plasma in Saturn’s

inner magnetosphere observed in situ by CAPS (3).

In 1993, the Faint Object Spectrograph on the HST observed large concentrations of OH

near Saturn’s satellite Tethys (32). Subsequent observations by HST and theoretical modeling

of both HST and Voyager plasma data established the presence of a dense toroidal neutral OH

cloud in Saturn’s inner magnetosphere (2, 4-5). This cloud extends from about 3 to 8 Rs (1 Rs =

Saturn’s radius = 60330 km) with maximum concentration (~103 cm-3) inferred near the orbit of

Enceladus (3.95 Rs). The OH cloud is produced by dissociation of H2O, and although the peak

concentration near Enceladus indicates suggested that the largest source of water molecules wa-

sis from the region near Enceladus, the nature of this source was unkownin that region. Models

indicate that the source region near 4 Rs must provide at least ~ 80% of the total OH source,

which is estimated to be ~ 0.4-11028 H2O/s (2, 5-6).

Fig. 1 shows the Cassini trajectory on July 14, 2005 in a frame of reference centered at

Enceladus, with distance in units of Enceladus radii. A cylindrical coordinate system is shown in

the top panel with Z parallel to Saturn’s spin axis and the perpendicular distance of Cassini

from the Z axis. In the bottom panel the Cassini trajectory is projected on the equatorial plane,

where the ordinate is the radial distance toward Saturn and the abscissa is the azimuthal coordi-

nate, positive in the direction of corotation. The orbital speed of Enceladus is about 12.6 km/s

while the thermal plasma corotates with Saturn at about 38 km/s near Enceladus. Thus the coro-

tating plasma overtakes Enceladus, forming a “corotational wake” in the positive azimuthal di-

rection. Note that Cassini passeds upstream of this wake. Enhanced ion flux is observed by

CAPS when the instrument viewing is favorable, which occurs in the two time intervals colored

red on the trajectory.

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An overview of the Enceladus encounter from the perspective of the Cassini radio and

plasma wave science (RPWS) investigation (7) is shown in Fig. 2, showing electric field power

spectral density as a function of frequency (vertical axis) and time (horizontal axis). RPWS pro-

vides an estimate of the total electron density at Cassini from the measured frequency of the up-

per hybrid resonance band, and this estimate is used in this study to constrain the CAPS data

analysis procedure. The upper hybrid resonance is observed throughout the Saturnian magneto-

sphere for radial distances less than ~8 RS, and the measured frequency is a known function of

the total electron density. The upper hybrid resonance emission (fUH in Fig. 2) is somewhat com-

plex, with a smoothly varying narrowband emission at low frequencies and a more sporadic,

broadband extension to higher frequencies. Nevertheless, the upper hybrid line is reliably identi-

fied and indicates that the total electron density, shown approximately by the right-hand scale,

smoothly increases from about 45 cm-3 at 19:30 UT to about 70 cm-3 at time t3. It should be

noted that there is a short period very close to Enceladus when the narrowband emission is not

identifiable, likely because of the effect of few-micron sized dust particles impacting the space-

craft. During this time, it is not possible for RPWS to precisely determine the electron density,

but there does not appear to be evidence for a significant increase in the density (>~20%) at clos-

est approach.

Fig. 3 shows CAPS data during the Enceladus encounter. The CAPS instrument com-

prises three plasma sensors, an electron spectrometer, ELS, an ion mass spectrometer, IMS, and

an ion beam spectrometer, IBS (1). This study employs data from ELS and IMS, depicted in Fig.

3, with the penetrating radiation background removed. The top panel shows ELS electron count-

ing rate as a function of energy and time, summed over the ELS field of view and uncorrected

for spacecraft potential. Two electron populations are visible in the figure, a ‘cold’ (~2-3 eV)

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peak and a ‘hot’ (~20eV) peak. During this time interval, the spacecraft potential is less than the

ELS minimum energy, making electron density and temperature extraction difficult. However,

constraining the ELS data with the RPWS total electron density depicted in Fig. 2, the space-

craft potential is estimated to be ~ -2V and the density and temperature of the ‘cold’ and ‘hot’

electron components are estimated to be Nc=70.3 cm-3, Tc=1.35eV, and Nh=0.2 cm-3 and Th=12.5

eV. During this time interval (19:30 to 20:30 UT) the cold density (Nc) and temperature (Tc) pa-

rameters determined from ELS data vary by ~40% and ~20% respectively.

The middle panel of Fig. 3 depicts IMS ion counting rate as a function of energy and

time. IMS detection of the thermal ion plasma strongly depends on instrument viewing, con-

trolled in part by Cassini orientation. IMS observes the slowing and deflection of the ion flow

velocity upstream of Enceladus between the times (t1,t2). Enhanced flux is also observed follow-

ing closest approach between the times (t3,t4). Near closest approach IMS viewing was not opti-

mum; nevertheless signatures of ion “pick-up” were observed as discussed below. The bottom

panel of Fig. 3 shows the ion composition obtained by IMS. The ions are from the “water

group”, with O+, OH+, H2O+ and H3O+ detected.

To extract ion bulk parameters from the data shown in the middle panel of Fig. 3, a for-

ward modeling procedure is employed similar to that discussed in (8). The ions are assumed to

be O+; including the IMS measured composition shown in Fig. 3 would have a small effect on

the calculated plasma moments because all the ions have mass per charge near 16. The ion

phase space density is assumed to be a convecting isotropic maxwellian distribution far from

closest approach. With this assumption, the free parameters in the model are the ion temperature

and two components of the ion flow velocity (radial and azimuthal). The ion flow velocity is

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constrained to lie in Saturn’s equatorial plane, and the total ion density is constrained to equal

the RPWS total electron density discussed above.

The measured flow is slowed and deflected from the corotation direction as far as 30 Enceladus

radii from the moon, as shown in Fig. 4. Similar deflection was observed near Jupiter's moon Io

(9), which is the dominant source of plasma for Jupiter’s magnetosphere. In Fig. 4 the observed

plasma flow velocity vectors are superimposed on streamlines calculated from a modification of

a model developed for Io (10). Mass loading drives a pickup current that diverges at gradients in

the mass loading rate per unit magnetic flux. Those currents are diverted along magnetic field

lines into the planet's ionosphere, and the electric fields required there to complete the circuit are

mapped back into the magnetosphere, thus modifying the corotation electric field. Deflection at

Enceladus occurs much farther from the surface than at Io, indicating an extended neutral gas

cloud with mass loading distributed over a volume much larger than the satellite. In this model,

mass-loading is taken to be proportional to neutral density, which is assumed to varies vary as

the inverse square of distance from Enceladus (11). Beyond ~ 3.4RE, the Hill sphere, this ap-

proximation breaks down as Saturn’s gravity and the centgrifugal confinement dominate, but

such changes will not significantly affect our estimate Using this model atmosphere t The

streamlines in Fig. 4 correspond to a total mass-loading rate of 90 kg/s (31027 H2O/s) and a

Pedersen conductance of 0.1 S in Saturn’s ionosphere.

(Moved this discussion until later and expanded—this is a general audience so connec-

tions need to be very clear)The total mass loading rate required by the model to roughly simulate

(crudely) the magnitude of the observed velocity perturbations is remarkably close to that re-

quired to explain the OH cloud observed remotely by HST (5,6). This coincidence may or may

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not be fortuitous. The mass loading assumed in the electrodynamics model is presumably domi-

nated by charge exchange collisions, which do not change the local plasma density (consistent

with the fact that no dramatic density increase is observed near closest approach – see Fig. 2).

They do, however, convert slow neutrals escaping Enceladus (11) into fast neutrals that are slung

downstream of the satellite in the corotation direction. These fast neutrals are potential sources

of the OH cloud. More detailed modeling, including a proper treatment of ion chemistry, will be

required to test this hypothesis.

In addition to these ion flow perturbations, the IMS also provided a direct observation of

water group ion pick-up near Enceladus. This is depicted in Fig. 5, showing IMS ion data near

closest approach at 19:55 UT. Near closest approach, CAPS viewing was poor, resulting in the

low count rates seen in Fig. 3 near Enceladus. Nevertheless, eight look directions were measured

in a plane roughly perpendicular to an unperturbed flow and covering pitch angles (angles rela-

tive to the magnetic field) of 95° and 140°. In these data, the flux of water group ions was

strongly anisotropic. The greatest flux was observed from two nearly opposite directions, both of

which are between 95° and 110° from the magnetic field. In these directions, the peak flux oc-

curred at energies corresponding to 11 and 26 km/s [Frank… what mass are you assuming?],

consistent with a ring distribution perpendicular to the magnetic field. A ring distribution results

from recently ionized particles, and tends to evolve into a thermal distribution on a time scale of

many ion cyclotron periods (2/Wci ~ 3 s for water group ions.) In the example shown, the ob-

served peak velocities and directions correspond to ions produced in a 20 km/s flow, which sub-

sequently slowed to 15 km/s. This is typical of the other ion spectra obtained in the ±5 minute

period about closest approach. [ Frank … need final figure and this clearer if possible].

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Based on the deflections observed by CAPS in the ion flow and the ion thermal distributions,

considerable pick-up occurs in the neutral gas escaping from Enceladus. Our estimated ioniza-

tion/ pick-up rate (~3 x10^27H2O/s) is surprsingly close to the rate that has been estimated for

gas loss from Enceladus and also close to the estimate of the missing source of neutrals for the

OH cloud (2,6). This is consistent with the picture that the neutrals escape from Encladus with

speeds that are more than an order of magnitude lower than the orbit speed, so that a significant

fraction of the escaping neutrals are ionized within the observed interaction region. Therefore,

the ion mass loading rate above gives an independent estimate of the neutral source rate.

Using our measured electron densities and temperatures and our ion densities and flow speeds,

the electron lifetime of a water molecule near Enceladus is seen to be an order of magnitude

smaller than the charge exchange ionization rate. (Do we want the numbers in?) Since the pho-

toionization rate is two orders of magnitude smaller than charge excahnge, CAPS data indicate

that the increase in the plasma density in the Enceladus encounter region is small and not very

different from that observed by CAPS on other much more distant passes through the Enceladus

orbit region (ref?). This result is also consistent with the RWPS data discussed above. There-

fore, the observed mass loading is very different from that for the gas escaping from Io (ref) as it

is totally dominated by charge exchange. This is a process by which a ‘fast’ ion captures an elec-

tron from a slow neutral escaping from Enceladus, producing, on average, a ‘fast’ neutral and a

slow ion (Johnson et al 2005). Following such an interaction the local ion density does not

change but the rotating fields pick-up and accelerate the ion producing the observed mass load-

ing. Since the slow neutrals escaping from Enceladus are tightly constrained to a region about

the orbit of Enceladus, charge exchange produces an enormous expansion of this cloud by creat-

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ing fast neutrals that are, likely, a principal source of the large scale OH cloud seen by HST.

These fast neutrals also populate the outer magnetopshere with water products affecting the local

ion composition (Johnson et al 2005. 12) and, possibly, the composition of Titan’s upper atmos-

phere (Hartle et al. 2005, 13). More detailed modeling of this interaction region, including a

treatment of the ion chemistry in the Enceladus neutral cloud, are in progress.

[Needs a concluding paragraph. REJ? ]

References and Notes

1. Young caps instrument paper

42. Shemansky 1993 HST nature

3. Young et al. Science paper

54. Richardson 1998 JGR

25. Jurac 2002 GRL missing water source

6. R.D. Richardson and S. Jurac GEOPHYSICAL RESEARCH LETTERS, VOL. 31, L24803, doi:10.1029/2004GL020959, 2004; Jurac, S., and J. D. Richardson (2005), Geophys. Res. Lett,doi:10.1029/2004JA010635, in press.

Richardson/Jurac 2004 (5?) ion tori model GRL

7. Gurnett RPWS instrument paper

8. Tokar GRL 2005

9. Frank Galilieo science 1996

10. Hill and Pontius, 1998 JGR

11. Observed by Cassini INMS, J. H. Waite, private communication.

12 Plasma-Induced Clearing and Redistribution of Material Embedded in Planetary Mag-

netospher R.E. Johnson, M. Liu, E.C Sittler, Jr. Geophys Research Lett submitted –the revision

has actually been submitted

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13. recent Hartle submission or Crary Titan paper

12. We wish to thank the large number of scientists and engineers on the CAPS and RPWS

teams who made the results reported here possible. The work at Los Alamos was performed un-

der the auspices of the US Department of Energy. The work of U.S. co-authors was supported

by JPL contracts 1243218 with Southwest Research Institute and _______ with the University of

Iowa. Work in the UK was supported by the Particle Physics and Astronomy Research Council.

Cassini is managed by the Jet Propulsion Laboratory for NASA.

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Fig. 1: Trajectory-should be final. [Needs caption]

Fig. 2: RPWS spectrogram-revision expected. [Needs caption.]

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Fig. 3: CAPS data-should be final. [Needs caption.]

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Fig. 4: Duane modeling-need final. [Needs caption.]

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Fig. 5: Frank Pick-Up Need Final. [Needs caption.]

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