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GEOPHYSICAL RESEARCH LETTERS, VOL. 32, L14S06, doi:10.1029/2005GL022643, 2005

Dynamics of the Saturnian inner : First inferences from the Cassini magnetometers about small-scale 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 , the main ring system, the more during the first three 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 resides in 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 and confined to field signatures in the inner regions of the magnetosphere the equatorial plane by the , 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 , 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 to the multiphasic nature of its components, including the . 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 ) 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

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Figure 1. (left) Cassini trajectories (first 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, it spent most of its defined such that Saturn’s magnetic axis lies in the XZ time within 2 Rs of the equatorial plane during its second plane, and the Y axis lying in the Saturn’s magnetic and third inbound orbits since it flew by Titan very closely, equatorial plane.

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Figure 2. Magnetic field VHM observations during Cassini first and second orbits. (left) Magnetic field magnitude (in blue) and By (+20 nT) component (in nT) versus radial distance (in Rs) on DOY 182. High-frequency turbulence associated with plasma production is particularly noticeable in the transverse By component. (right) Magnetic field magnitude (in nT) versus time (in hours since 04:00 UT) on DOY 302. See color version of this figure in the HTML.

+ [11] The key observation we would like to address in cyclotron waves propagating at the H2O gyro-frequency the following sections is that the inner Saturnian magne- seemed to surround the magnetic depression, indicative tosphere appears to be in an extremely dynamic state as of the production of new plasma in the magnetospheric observed by MAG; and a set of illustrations supports this system. This depression was associated with a plasma observation. energy density of 900 eV cmÀ3 [Leisner et al., 2005]. [14] Particularly evident and abundant on the second 3.1. Short-Duration Depressions in Field Magnitude orbit (Figure 3 (left)), these events present average magnetic [12] Inside of 10 Rs, MAG reported for all orbits numer- field magnitude changes of 1–2 nT (1–2% of the back- ous intermittent diamagnetic depressions in which some of ground field of magnitude 50 and 90 nT) and average the magnetic pressure is replaced by increased plasma durations lasting between 2 and 6 minutes. Rough estimates pressure (Figure 2). for the longitudinal width of these structures give tens [13] These diamagnetic cavities were not reported or of thousands kilometers. They are mainly observed at not so abundant at the time of the Voyager and Pioneer distances inside of 8 Rs and for the majority lie close to flybys. Dougherty et al. [2005] described similar events the outer latitudinal boundary of the plasmadisc, also on the first Cassini orbit in a radial range 6–10 Rs. reported turbulent by Sittler et al. [1983]. The Cassini The largest depression was observed on DOY 182 at spacecraft later on the second orbit crossed the Saturnian 21:05 UT, near but inside the orbit of Dione and at a equatorial plane around 20:00 UT on day of year (DOY) radial distance of 5.93 Rs (Figure 2 (left)). In that case, 302 when MAG revealed an interval with a large increase well-marked transverse waves accompanied by left-hand in the noise level of the magnetic field observations, again

Figure 3. Magnetic field VHM observations during Cassini second orbit. (left) Zoom on the magnetic depressions. Residual magnetic field magnitude (in nT) versus time (in hours) for four different periods (3-h intervals, starting at 04:00 UT) on DOY 302. (right) Zoom on the magnetic enhancements. Magnetic field magnitude and components (in nT) versus time (in minutes) on DOY 302. Similar observations occurred on DOY 350 after 1640 UT and are reported on Figure 1. See color version of this figure in the HTML.

3of5 L14S06 ANDRE´ ET AL.: INNER MAGNETOSPHERE DYNAMICS L14S06 indicative of plasma production. This interval is particu- instruments suite on-board Cassini will be of great help to larly noticeable in Figure 2 (right) and will be described in reach this objective. the next section. [19] The similarities of our new reported observations [15] Most of the observed magnetic depressions are with observations of dynamic processes occurring in the Io found to correlate well with energy-time dispersion in the plasma torus is striking and let us envisage interesting low-energy plasma instrument observations [Hill et al., comparative studies to obtain a better understanding of the 2005; Burch et al., 2005]. These events are interpreted as interchange instability responsible (at least partly) for the signatures of centrifugally driven interchange motions radial transport of plasma in giant planet . injecting hot tenuous plasma toward Saturn and are associ- [20] The spacecraft was in orbit about ated with narrow but deep density cavities in the cooler from December of 1995 to September of 2003. Galileo background plasma. detected observational evidence for small-scale plasma transport by interchange motions, which were 3.2. Short-Duration Increases in Field Magnitude not identified at the time of the Voyager mission. Signatures [16] Close to the time when Cassini crossed the Saturnian of intermittent and short-lived, mass-loaded and empty flux equatorial plane (around 8 Rs) on its outbound second and tubes were identified in Io’s torus [Kivelson et al., 1997; third orbits (around the 21–24 LT sector of the magneto- Thorne et al., 1997; Bolton et al., 1997] and beyond sphere), MAG reported unusual magnetic field signatures of [Russell et al., 1999] through field and particle measure- flux tubes with greater magnetic pressure than their ments. A global picture of the large-scale outward motion of surroundings (Figure 3 (right)), embedded in regions where the Jovian plasma was given by Russell [2001]. As the the noise level of the magnetic field observations is greatly plasma moves outward through the magnetodisk, the enhanced. These unusual signatures have not been reported magnetodisk appears globally unstable and highly dynamic before in Saturn’s magnetosphere and are characterized by and small tearing islands can be observed within it [Russell very sharp boundaries, by abrupt and large changes in the et al., 1999], which act as sites of transient reconnection in magnetic field magnitude (3 nT in a background field of the nightside, with the subsequent release of and the magnitude 30 nT), and by average durations lasting between return of empty flux tubes into the inner magnetosphere 1 and 5 minutes. For some of the largest events presented on [Russell et al., 2000a, 2000b]. Figure 3, wavy processes are observed on their edges (see [21] The complete Cassini tour within the Saturnian for example around 19:00 UT on DOY 302) and could magnetosphere will also add new pieces and answers to suggest erosion of the structures and possible breakup into our puzzle and hopefully will enable us to reach at least the smaller flux tubes. same level of understanding of the global plasma circulation [17] We tentatively interpret these unusual magnetic field in the Jovian magnetosphere that we have learnt from observations as the signature of depleted flux tube missing Galileo, as summarized above. some plasma energy density compared to their surround- ings. Preliminar information about their mass content and [22] Acknowledgment. N. Andre´ would like to acknowledge the very simple pressure balance arguments assuming no tem- French Space Agency (CNES) for its financial support. perature differences (dn/n = À2/bdB/B with b  1) suggest density variations in these flux tubes of the order of 20%. If References confirmed, the role played by these flux tubes could be to Backes, H., et al. (2005), Titan’s magnetic field signature during the TA counterbalance outward plasma transport and to return encounter by Cassini, Science, 308, 992–995. inwards to the inner magnetosphere the magnetic flux Blanc, M., et al. (2002), Magnetospheric and Plasma Science with Cassini- Huygens, Space Sci. Rev., 104, 253. carried out by mass-loaded flux tubes. The exact mecha- Bolton, S. J., et al. (1997), Enhanced Whistler-mode emissions: Signatures nism producing these flux tubes has not been yet reported to of interchange motion in the Iorus, Geophys. Res. Lett., 24, 2123. our knowledge but we anticipate that reconnection in the Burch, J. L., J. Goldstein, T. W. Hill, D. T. Young, F. J. Crary, A. J. Coates, N. Andre´, W. S. Kurth, and E. C. Sittler Jr. (2005), Properties of local near tail on the nightside could be a candidate [Curtis et al., plasma injections in Saturn’s magnetosphere, Geophys. Res. Lett., 32, 1986]. Cassini will orbit these regions of the magnetosphere L14S02, doi:10.1029/2005GL022611. later during its 4-years tour of the Saturnian magnetosphere. Curtis, S. A., et al. (1986), The centrifugal flute instability and the genera- tion of Saturn kilometric radiation, J. Geophys. Res., 91, 10,989. Dougherty, M. K., et al. (2005), Cassini magnetometer observations during 4. Perspectives Saturn Orbit Insertion, Science, 307, 1266. Eviatar, A., et al. (1982), The plumes of Titan, J. Geophys. Res., 87, 8091. [18] The Cassini magnetometer observations reveal inner Goertz, C. K. (1983), Detached plasma in Saturn’s front side magneto- regions of the Saturnian magnetosphere in total ebullition sphere, Geophys. Res. Lett., 10, 455. Hill, T. W. (1976), Interchange instability of a rapidly rotating magneto- during the first three orbits of the spacecraft around the sphere, Planet. Space Sci., 24, 1151. planet. This observation is supported by various unusual Hill, T. W., A. M. Rymer, J. L. Burch, F. J. Crary, D. T. Young, M. F. signatures of magnetic field depressions and enhancements Thomsen, D. Delapp, N. Andre´, A. J. Coates, and G. R. Lewis (1997), Evidence for rotationally driven plasma transport in Saturn’s magneto- that we tentatively relate to plasma transport triggered by sphere, Geophys. Res. Lett., doi:10.1029/2005GL022620, in press. the interchange instability. We plan in the near future to Kivelson, M. G., et al. (1997), Intermittent short-duration magnetic field conduct very detailed case studies of these two types of anomalies in the Io torus: Evidence for plasma interchange?, Geophys. Res. Lett., 24, 2127. magnetic signatures in order to determine their exact prop- Leisner, J. S., et al. (2005), A case study of warm flux tubes in the E-ring erties and to firmly confirm our interpretation. Information plasma torus: Initial Cassini magnetometer observations, Geophys. Res. on the mass content, the composition and the radial velocity Lett., doi:10.1029/2005GL022652, in press. of the observed flux tubes is central to our analysis, since Lepping, R. P., E. C. Sittler Jr., W. H. Mish, S. A. Curtis, and B. T. Tsurutani (2005), Analysis of waves in Saturn’s dayside magnetosphere: the magnetometer can not have access to these parameters. Voyager 1 observations, J. Geophys. Res., 110, A05201, doi:10.1029/ The unique capabilities of the Magnetosphere and Plasma 2004JA010559.

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Richardson, J. D. (1998), Thermal plasma and neutral gas in Saturn’s Slavin, J. A., et al. (1983), A Pioneer-Voyager study of the solar wind magnetosphere, Rev. Geophys., 36, 501. interaction with Saturn, Geophys. Res. Lett., 10,9. Russell, C. T., et al. (1999), The fluctuating magnetic field in the middle Thorne, R. M., et al. (1997), Galileo evidence for rapid interchange trans- Jovian magnetosphere: Initial Galileo observations, Planet. Space Sci., port in the Io torus, Geophys. Res. Lett., 24, 2131. 47, 133–142. Russell, C. T., et al. (2000a), Substorms at Jupiter: Galileo observations of thansient reconnection in the near tail, Adv. Space Res., 26, 1499. ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ Russell, C. T., et al. (2000b), Implications of depleted flux tubes in the N. Andre´, Centre d’Etudes Spatiales des Rayonnements, 9 avenue du Jovian magnetosphere, Geophys. Res. Lett., 27, 3133. colonel Roche, F-31028 Toulouse, France. ([email protected]) Russell, C. T. (2001), The dynamics of planetary magnetospheres, Planet. M. K. Dougherty, Blackett Laboratory, Space and Atmospheric Physics, Space Sci., 49, 1005. Imperial College, London SW7 2BZ, UK. Sittler, E. C., K. W. Ogilvie, and J. D. Scudder (1983), Survey of K. K. Khurana, J. S. Leisner, and C. T. Russell, Institute of Geophysics low-energy plasma in Saturn’s magnetosphere: Voyagers 1 and Planetary Physics, University of California, Los Angeles, Los Angeles, and 2, J. Geophys. Res., 88, 8847. CA 90095–1567, USA.

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