Slobodan Jurac

Sputtering of water ice surfaces and the production of extended neutral atmospheres

Plasma and UV photon bombardment of an icy object in the outer solar system can lead to ejection of atoms and molecules from the surface which can, in turn, produce an extended neutral atmosphere. We present new laboratory studies of the sputtering of water ice by keV ions (H+ through Ne+) made using a sensitive microbalance technique that allows measurements at very low ion fluences. These results for the sputtering yield of ice by keV O+ ions, the dominant sputtering agents in the Saturnian magnetosphere, are much larger than those used previously to model the neutral cloud associated with the icy satellites. The data presented are used to recalculate previously published sputtering rates for the icy satellites of Jupiter and Saturn, and for the E-ring grains at Saturn. The new results can account, in part, for the discrepancy between the predicted and observed OH cloud near Tethys in Saturns inner magnetosphere. We compare the yields induced by the incident ions to the recently measured UV photosputtering yield, and discuss possible synergism between UV photon and plasma ion induced erosion.

Saturn: Search for a missing water source

[1] The origin of the large hydroxyl radical (OH) cloud near the inner moons of Saturn, indicative of a surprisingly large water-vapor source, has represented a puzzle since its discovery in 1992. A new set of Hubble Space Telescope measurements is used to constrain the OH spatial densities and to pinpoint the source region. Our model indicates that the vast majority of the water vapor (>80%) originates from Enceladus’s orbital distance. This may indicate the presence of a dense population of small, as of yet unseen, bodies concentrated near Enceladus; collisions between these fragments are the suggested mechanism for producing the necessary amounts of water vapor. We show that collisions between plasma ions and neutral molecules substantially inflate the OH cloud, and increase the OH loss rate, requiring a water source three times larger than previous estimates. INDEX TERMS: 6275 Planetology: Solar System Objects: Saturn; 2756 Magnetospheric Physics: Planetary magnetospheres (5443, 5737, 6030); 6280 Planetology: Solar System Objects: Saturnian satellites; 6213 Planetology: Solar System Objects: Dust. Citation: Jurac, S., M. A. McGrath, R. E. Johnson, J. D. Richardson, V. M. Vasyliunas, and A. Eviatar, Saturn: Search for a missing water source, Geophys. Res. Lett. , 29(24), 2172, doi:10.1029/2002G L015855, 2002.

Charging of ice grains by low‐energy plasmas: Application to Saturn's E ring

The charging of ice grains in planetary plasmas is studied, including the effects of secondary electron emission and backscattering of the incident electrons. It is shown that existing charging models can not be simply extrapolated to the low-energy electron regime (below 30 eV) common in planetary magnetospheric plasmas. We derive expressions for the electrical potential of a grain immersed in a low-energy plasma which more carefully account for electron reflection and the threshold for secondary electron emission. Using plasma parameters from Voyager PLS experiment, we calculate the potential of Saturns E ring grains to vary from −5.5 V at 4 Rs to 5 V at 10 Rs.

Energetic nitrogen ions within the inner magnetosphere of Saturn

[1] We investigate the importance of nitrogen ions within Saturn’s magnetosphere and their contribution to the energetic charged particle population within Saturn’s inner magnetosphere. This study is based on the Voyager observations of Saturn’s magnetosphere and Cassini observations. The latter have shown that water group ions dominate both the plasma and energetic particle populations but that nitrogen ions over a broad range of energies were observed at � 5% abundance level. In the outer magnetosphere, methane ions were predicted to be an important pickup ion at Titan and were detected at significant levels in the outer magnetosphere and at Titan. O + ions were found to be the dominant heavy ion in the outer magnetosphere, � 60%, with methane ions being � 30% of the heavy ions and N + being a few percent. The two major sources of nitrogen ions within Saturn’s magnetosphere are Titan’s atmosphere and primordial nitrogen trapped in the icy crust of Saturn’s moons and its ring particles deep within the magnetosphere. It is important to understand the source, transport, and sinks of nitrogen in ordertodeterminewhethertheyhaveaprimordialoriginorarefromTitan’satmosphere.The energetic component is important, since it can come from Titan, be implanted into the surfaces of the icy moons, and reappear at plasma energies via sputtering obfuscating the ultimatesource.Aswewillshow,suchimplantationofnitrogenionscanproduceinteresting chemistry within the ice of Saturn’s moons. The emphasis will be on the nitrogen, but the oxygenandotherwatergroupionsarealsoconsidered.Wearguethatneutralcloudsofheavy atoms and molecules within Saturn’s outer magnetosphere may be the dominant source of energetic heavy ions observed within the inner magnetosphere. Pickup heavy ions in the outer magnetosphere have energies � 1–4 keV when born. If they diffuse radially inward, whileconservingthefirstandsecondadiabaticinvariants,theycanhaveenergiesgreaterthan several hundred keVinside of Dione’s L shell. We will show how observations relate to the various sources and acceleration processes such as ionization, collisions, wave-particle interactions, and radial diffusion.

The dependence of plasma and magnetic field correlations in the solar wind on geomagnetic activity

The solar wind structures measured by four spacecraft (ISEE 1, ISEE 3, IMP 8, and Wind) are correlated and examined in the light of their geoeffectiveness. The average plasma and magnetic field correlations range from about 0.6 to 0.7 during quiet periods and increase substantially during the geomagnetic storms. Using the Ap and Dst indices as a measure for the level of geomagnetic disturbance, we find that correlations of plasma density, |B|, Bz and By are about 0.85 for the most geoeffective events. Earths foreshock significantly degrades the correlation when one or both spacecraft are within ∼30 RE of Earth during quiet periods, and this degradation extends beyond 50 RE during geoeffective periods. Excluding the foreshock effect, the correlations show little dependence on the spacecraft separation along the Sun-Earth line, but depend strongly on the spacecraft separation in the direction perpendicular to the Earth-Sun line, consistent with previous findings. The decrease in correlation in the perpendicular direction is less severe during geoeffective periods, suggesting that geoeffective features have larger scale sizes. The front normals of geoeffective features are predominantly in the propagation direction, consistent with radially outward propagating ejecta.