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Dynamics of the Saturnian Inner Magnetosphere: First Inferences from the Cassini Magnetometers About Small-Scale Plasma Transport in the Magnetosphere N

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Dynamics of the Saturnian Inner Magnetosphere: First Inferences from the Cassini Magnetometers About Small-Scale Plasma Transport in the Magnetosphere N GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L14S06, doi:10.1029/2005GL022643, 2005 Dynamics of the Saturnian inner magnetosphere: First inferences from the Cassini magnetometers about small-scale plasma transport in the magnetosphere N. Andre´ Centre d’Etudes Spatiales des Rayonnements, Toulouse, France M. K. Dougherty Blackett Laboratory, Space and Atmospheric Physics, Imperial College, London, UK C. T. Russell, J. S. Leisner, and K. K. Khurana Institute of Geophysics and Planetary Physics, University of California, Los Angeles, USA Received 7 February 2005; revised 18 April 2005; accepted 16 May 2005; published 14 June 2005. [1] The Cassini magnetometers reveal a very dynamic and neutral sources [Richardson, 1998] and include the plasmadisc within the inner Saturnian magnetosphere planetary ionosphere, the main ring system, the more during the first three orbits of the Cassini orbital tour. diffuse E-ring, and numerous icy satellites (Mimas, This corotation-dominated region is known to contain Enceladus, Tethys, Dione, and Rhea). Titan which is various neutral and plasma populations and Voyager the other dominant plasma source at Saturn resides in spacecraft observations suggest important radial transport the outer magnetospheric regions at a radial distance of processes redistribute the locally created plasma out to the 20 Rs. remote magnetospheric regions, by a mechanism yet to be [4] The plasma created by the internal distributed sources identified. In this work we report on anomalous magnetic is trapped by the planetary magnetic field and confined to field signatures in the inner regions of the magnetosphere the equatorial plane by the centrifugal force, thereby giving (inside of 8 Saturn radii) that may participate in the radial rise to a thin disc of corotating plasma in the inner transport and that seem to be consistent with signatures of magnetospheric regions. This region is composed of a cool interchanging flux tubes. These unusual events are and dense plasma torus embedded into a hotter and more characterized by sharp boundaries and abrupt changes of tenous plasma and extends basically out to 12–15 Rs on the the magnetic pressure, consisting of depressions or dayside [Sittler et al., 1983]. Since the plasma which is enhancements of the field magnitude. They present some added locally cannot build up indefinitely, a circulation similarities with previously identified signatures of system is set up, such that the plasma is either transported interchanging flux tubes in the Io torus, allowing the outward to the remote magnetospheric regions where it interesting possibility of carrying out a comparative study of escapes into the interplanetary medium, or lost down the these two environments. Citation: Andre´, N., M. K. planetary field lines into the ionosphere. Plasma transport Dougherty, C. T. Russell, J. S. Leisner, and K. K. Khurana across and along the magnetic field lines plays a very (2005), Dynamics of the Saturnian inner magnetosphere: First important role in redistribution of the magnetospheric inferences from the Cassini magnetometers about small-scale plasma which arises from the variety of different plasma transport in the magnetosphere, Geophys. Res. Lett., 32, sources. The mechanisms responsible for this transport L14S06, doi:10.1029/2005GL022643. are yet not completely understood, with the transport probably operating through different modes and on different scales. 1. Introduction [5] Large-scale plasma circulation is often described as [2] The Saturnian magnetosphere is one of the most the superposition of the planetary corotation flow and of the complex physical environments in our Solar System due magnetospheric convection flow driven by the solar wind to the multiphasic nature of its components, including the electric field. Small-scale motions and plasma instabilities planet itself, its large ring system, numerous satellites (more may also significantly contribute to the redistribution of particularly the icy satellites and Titan) and various neutral, plasma throughout the magnetosphere. In the inner plasma and dust populations coupled in a very intricate way magnetospheric regions, which are corotation dominated, [Blanc et al., 2002]. the outward transport is believed to be triggered by the [3] The diverse sources of the Saturnian magneto- centrifugal instability (a Rayleigh-Taylor type instability spheric plasma are divided into both external and inter- with the centrifugal force playing the role of gravity) and nal ones, whose contribution is by far dominant. The to proceed through the interchange of magnetic flux tubes inner magnetospheric regions (out to R = 15 Saturn radii [Hill, 1976]. In the outer magnetospheric regions, beyond Rs) play host to the majority of the important plasma the extended plasmadisc, irregular structures have been detected on the dayside which have been described as Copyright 2005 by the American Geophysical Union. plumes of plasma from Titan [Eviatar et al. [1982] and/or 0094-8276/05/2005GL022643$05.00 blobs of plasma centrifugally detached from the outer L14S06 1of5 L14S06 ANDRE´ ET AL.: INNER MAGNETOSPHERE DYNAMICS L14S06 Figure 1. (left) Cassini trajectories (first orbit in black from DOY 181 to 185, second orbit in blue from DOY 300 to 304, and third orbit in red from DOY 348 to 352) in the X-Y KSM plane and modelled average magnetopause boundaries from Slavin et al. [1983] (in magenta). (right) Distance (in Rs) to the Saturnian equatorial plane versus radial distance (in Rs) during the first three orbits (same colour system used). Superposed on these two figures are the intervals where Cassini is in immersion within the inner magnetospheric regions (in green) and the intervals on which we focus in more details in the present paper (green crosses). Empty circles indicate the beginning of each new day and black crosses the last observed inbound magnetopause crossing. See color version of this figure in the HTML. boundary of the plasmadisc [Goertz, 1983; Curtis et al., whereas its outbound orbits progressively show a tendency 1986; Lepping et al., 2005]. to get closer to the equatorial plane. Closest approaches on [6] The investigation of the coupling between the differ- these first three orbits occur respectively at 1.35, 6.18, and ent components of the Saturnian magnetospheric system 4.78 Rs, enabling us therefore to obtain a reasonable and the identification of the energy sources and mechanisms coverage of the inner Saturnian magnetosphere. for driving dynamical processes are important objectives of [9] We identify plasmadisc regions in the magnetometer the dual technique magnetometer (MAG) experiment. We data by examining large-scale changes in the field magni- present here some of the latest Cassini MAG observations tude, orientation, and activity. Once within the plasmadisc, (utilizing all the available orbits obtained to date) which owing to the presence of dense plasma, the level of are suggestive of large dynamics of the innermost mag- magnetic fluctuations increases, and the magnetic field netospheric regions, which we interpret as first evidence appears stretched and begins to turn slightly quasi-dipolar of plasma transport via the interchange instability. We as we get closer to the planet. We content ourselves in this plan to conduct more detailed multi-instrumental studies work to focus on observations out to R = 10 Rs. However it in the near future of the events presented here, both seems that Cassini may have been totally immersed within through specific case studies and through an overview an extended plasmadisc on its inbound second and third analysis. orbits (with a clear distinction observed between outer and inner magnetospheric regions [Backes et al., 2005] 2. Inner Magnetosphere Observations and that in addition the spacecraft may have re-entered into plasmadisc-like regions regularly on all outbound [7] The Cassini spacecraft has orbited the Saturnian trajectories, even when the spacecraft was at large distances system three times during 2004, beginning with the Saturn from the planet and more importantly from the Saturnian Orbit Insertion in early July, and this data has resulted in equatorial plane. These observations are beyond the scope unique in-situ observations of the Saturnian magneto- of the present paper. sphere. Figure 1 shows these three orbits in term of Local Time(LT)coverageandintermofdistancetothe Saturnian equatorial plane, i.e. the plane that contains the main internal plasma sources. During its first three 3. Inner Magnetosphere Dynamics orbits, the spacecraft encountered the Saturnian magneto- [10] The MAG instrument on-board Cassini consists of sphere through its dawn-noon sector. The last magneto- two separate magnetometers, a Helium magnetometer on pause crossings observed inbound suggested that the the end of the 11-meter boom which operates either in a magnetosphere was relatively extended at the time of the Vector (VHM) or a scalar mode, and a fluxgate magne- first orbit compared to its average dimensions as modelled tometer, half way down the boom. In the present paper we by Slavin et al. [1983], whereas it was closer to the use VHM high-resolution data. To represent the magnetic predicted model values during the time of the second field observations, we use the planetocentric KSM coor- and third orbits. dinate system, with the positive X axis directed towards [8] While the spacecraft stayed far from the equatorial the Sun, the positive Z axis pointing northward and plane during the majority of its first orbit,
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