Thermal plasma composition in Saturn’s magnetosphere

doi:10.1016/S0032-0633(00)00036-2

Planetary and Space Science

Volume 48, Issues 7–8, 1 June 2000, Pages 775-783

https://doi.org/10.1016/S0032-0633(00)00036-2 Get rights and content

Abstract

A brief description is given to the composition of the thermal ions in the Saturnian magnetosphere with the neutral gas cloud as the main source. It is predicted that, if the neutral cloud density peaks near Enceladus’ orbit with a total number density of the water-group neutrals reaching 10⁴ cm⁻³, the molecular ions like OH⁺ and H₂O⁺ could be as abundant as O⁺ throughout the Saturnian magnetosphere. At the same time, a significant amount of H₃O⁺ and O⁺₂ ions can be found in the region with L<5. The composition of the Saturnian plasma could therefore be far more complex than previously thought.

Introduction

Even though the last closeup observations of the Saturnian system were by the Voyager spacecraft in 1980 and 1981, there is a steady stream of exciting new observations of the physical phenomena of this ringed planet from ground-based and space-born observatories over the last few years. In some areas, the new measurements have completely supplanted our old ideas. One example is the distribution of neutral gas cloud which has now been found to be extremely dense by the Hubble Space Telescope observations (Shemansky and Hall, 1992, Shemansky, 1998). Its impact on the composition and dynamics of the Saturnian magnetosphere is still to be carefully assessed. In this work, we will examine several major ingredients in the neutral cloud origin of the magnetospheric plasma. These include source (Section 2), composition (Section 3), and configuration (Section 4).

Section snippets

Source

It is useful to compare the Saturnian magnetosphere with the Jovian magnetosphere. Both planets have similar rotational periods (P=9 h 55.5 min for Jupiter and 10 h 39 min for Saturn) so that for both of them the corotational electric field tends to dominate the solar wind convective electric field in shaping the magnetospheric plasma motions (Brice and Ioannidis, 1970, Mendis and Axford, 1974, Vasyliunas, 1983). Jupiter’s large magnetic dipole moment (equatorial magnetic field B_eq=4.2 Gauss)

Composition

According to the Voyager observations of the thermal plasma environment of the Saturnian system (Bridge et al., 1981, Bridge et al., 1982, Richardson and Sittler, 1990), the ion composition within 5R_S is dominated by heavy ions. Because of the limitation in mass resolution of the Voyager plasma instrument the exact composition of the corotating ions was not identified except that they are likely to be ion species related to the water group ions (i.e., O⁺). With the neutral cloud composition

Configuration

The formulation of the one-dimensional radial diffusion equation is applicable to an azimuthally symmetric system. However, the measurements by Voyager 1 and 2 spacecraft showed that significant longitudinal asymmetry existed in the structure of the plasma disc. Firstly, both inbound passes near noon detected a plasma density discontinuity at L≈15 (Sittler et al., 1983). The plasma inside this boundary was characterized by a cold electron temperature (T_e<40 eV) and high number density. On the

Discussion

It is possible that the electron temperature at L<4 is much smaller than 1 eV as a result of collisional cooling by the dense neutral gas cloud (Shemansky and Hall, 1992). Consequently, the ion number density could be kept low in this region even though the neutral gas density is relatively high (≈10⁴ cm⁻³). The situation is somewhat similar to the structure of the cometary ionosphere of Comet Halley in which the inner region of cold electron temperature (T_e≈500 K) is surrounded by a plasma of

Acknowledgements

I thank D. E. Shemansky, R. E. Johnson, A. Kopp and J. D. Richardson for informative discussions, and Margit Steinmetz, F. Both and Apple Wang for assistance in preparing the manuscript. This work was supported in part by the German Aerospace Agency (DLR) and in part by the National Science Council Grant No. 88Afa0700296.

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Thermal plasma composition in Saturn’s magnetosphere

Abstract

Introduction

Section snippets

Source

Composition

Configuration

Discussion

Acknowledgements

Icarus

Icarus

Icarus

Icarus

Icarus

Planet. Space Sci.

Icarus

Icarus

Jupiter’s magnetic field and magnetosphere

Giant planet magnetospheres

Ann. Rev. Earth Planet. Sci.

Radial diffusion of low-energy plasma ions in Saturn’s magnetosphere

J. Geophys. Res.

Dawn-dusk electric field asymmetry of the Io plasma torus

Geophys. Res. Lett.

Plasma observations near Saturn: Initial results from Voyager 1

Science

Plasma observations near Saturn: Initial results from Voyager 2

Science

Extreme ultraviolet observations from Voyager 1 encounter with Saturn

Science

The electron temperature in the inner Coma of comet P/Halley

Astron. Astrophys.

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Astrophys. J.