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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. André, M. K. Dougherty, C. T. Russell, J. S. Leisner, And K. K. Khurana
To cite this version:
N. André, M. K. Dougherty, C. T. Russell, J. S. Leisner, And K. K. Khurana. Dynamics of the Saturnian inner magnetosphere: First inferences from the Cassini magnetometers about small-scale plasma transport in the magnetosphere. Geophysical Research Letters, American Geophysical Union, 2005, Cassini, special issue. �10.1029/2005GL022643�. �hal-00013046�
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
plasmadisc within the inner Saturnian magnetosphere during the first three orbits of the Cassini orbital tour. This corotation-dominated region is known to contain various neutral and plasma populations and Voyager spacecraft observations suggest important radial transport processes redistribute the locally created plasma out to the remote magnetospheric regions, by a mechanism yet to be identified. In this work we report on anomalous magnetic field signatures in the inner regions of the magnetosphere (inside of 8 Saturn radii) that may participate in the radial transport and that seem to be consistent with signatures of interchanging flux tubes. These unusual events are characterized by sharp boundaries and abrupt changes of the magnetic pressure, consisting of depressions or enhancements of the field magnitude. They present some similarities with previously identified signatures of interchanging flux tubes in the Io torus, allowing the interesting possibility of carrying out a comparative study of these two environments. Citation: Andre´, N., M. K. Dougherty, C. T. Russell, J. S. Leisner, and K. K. Khurana (2005), Dynamics of the Saturnian inner magnetosphere: First inferences from the Cassini magnetometers about small-scale plasma transport in the magnetosphere, Geophys. Res. Lett., 32, L14S06, doi:10.1029/2005GL022643.
1. Introduction
[2] The Saturnian magnetosphere is one of the most
complex physical environments in our Solar System due to the multiphasic nature of its components, including the planet itself, its large ring system, numerous satellites (more particularly the icy satellites and Titan) and various neutral, plasma and dust populations coupled in a very intricate way [Blanc et al., 2002].
[3] The diverse sources of the Saturnian
magneto-spheric plasma are divided into both external and inter-nal ones, whose contribution is by far dominant. The inner magnetospheric regions (out to R = 15 Saturn radii Rs) play host to the majority of the important plasma
and neutral sources [Richardson, 1998] and include the planetary ionosphere, the main ring system, the more diffuse E-ring, and numerous icy satellites (Mimas, Enceladus, Tethys, Dione, and Rhea). Titan which is the other dominant plasma source at Saturn resides in the outer magnetospheric regions at a radial distance of 20 Rs.
[4] The plasma created by the internal distributed sources
is trapped by the planetary magnetic field and confined to the equatorial plane by the centrifugal force, thereby giving rise to a thin disc of corotating plasma in the inner magnetospheric regions. This region is composed of a cool and dense plasma torus embedded into a hotter and more tenous plasma and extends basically out to 12 – 15 Rson the
dayside [Sittler et al., 1983]. Since the plasma which is added locally cannot build up indefinitely, a circulation system is set up, such that the plasma is either transported outward to the remote magnetospheric regions where it escapes into the interplanetary medium, or lost down the planetary field lines into the ionosphere. Plasma transport across and along the magnetic field lines plays a very important role in redistribution of the magnetospheric plasma which arises from the variety of different sources. The mechanisms responsible for this transport are yet not completely understood, with the transport probably operating through different modes and on different scales.
[5] Large-scale plasma circulation is often described as
the superposition of the planetary corotation flow and of the magnetospheric convection flow driven by the solar wind electric field. Small-scale motions and plasma instabilities may also significantly contribute to the redistribution of plasma throughout the magnetosphere. In the inner magnetospheric regions, which are corotation dominated, the outward transport is believed to be triggered by the centrifugal instability (a Rayleigh-Taylor type instability with the centrifugal force playing the role of gravity) and to proceed through the interchange of magnetic flux tubes [Hill, 1976]. In the outer magnetospheric regions, beyond the extended plasmadisc, irregular structures have been detected on the dayside which have been described as plumes of plasma from Titan [Eviatar et al. [1982] and/or blobs of plasma centrifugally detached from the outer Copyright 2005 by the American Geophysical Union.
boundary of the plasmadisc [Goertz, 1983; Curtis et al., 1986; Lepping et al., 2005].
[6] The investigation of the coupling between the
differ-ent compondiffer-ents of the Saturnian magnetospheric system and the identification of the energy sources and mechanisms for driving dynamical processes are important objectives of the dual technique magnetometer (MAG) experiment. We present here some of the latest Cassini MAG observations (utilizing all the available orbits obtained to date) which are suggestive of large dynamics of the innermost mag-netospheric regions, which we interpret as first evidence of plasma transport via the interchange instability. We plan to conduct more detailed multi-instrumental studies in the near future of the events presented here, both through specific case studies and through an overview analysis.
2. Inner Magnetosphere Observations
[7] The Cassini spacecraft has orbited the Saturnian
system three times during 2004, beginning with the Saturn Orbit Insertion in early July, and this data has resulted in unique in-situ observations of the Saturnian magneto-sphere. Figure 1 shows these three orbits in term of Local Time (LT) coverage and in term of distance to the Saturnian equatorial plane, i.e. the plane that contains the main internal plasma sources. During its first three orbits, the spacecraft encountered the Saturnian sphere through its dawn-noon sector. The last magneto-pause crossings observed inbound suggested that the magnetosphere was relatively extended at the time of the first orbit compared to its average dimensions as modelled by Slavin et al. [1983], whereas it was closer to the predicted model values during the time of the second and third orbits.
[8] While the spacecraft stayed far from the equatorial
plane during the majority of its first orbit, it spent most of its time within 2 Rs of the equatorial plane during its second
and third inbound orbits since it flew by Titan very closely,
whereas its outbound orbits progressively show a tendency to get closer to the equatorial plane. Closest approaches on these first three orbits occur respectively at 1.35, 6.18, and 4.78 Rs, enabling us therefore to obtain a reasonable
coverage of the inner Saturnian magnetosphere.
[9] We identify plasmadisc regions in the magnetometer
data by examining large-scale changes in the field magni-tude, orientation, and activity. Once within the plasmadisc, owing to the presence of dense plasma, the level of magnetic fluctuations increases, and the magnetic field appears stretched and begins to turn slightly quasi-dipolar as we get closer to the planet. We content ourselves in this work to focus on observations out to R = 10 Rs. However it
seems that Cassini may have been totally immersed within an extended plasmadisc on its inbound second and third orbits (with a clear distinction observed between outer and inner magnetospheric regions [Backes et al., 2005] and that in addition the spacecraft may have re-entered into plasmadisc-like regions regularly on all outbound trajectories, even when the spacecraft was at large distances from the planet and more importantly from the Saturnian equatorial plane. These observations are beyond the scope of the present paper.
3. Inner Magnetosphere Dynamics
[10] The MAG instrument on-board Cassini consists of
two separate magnetometers, a Helium magnetometer on the end of the 11-meter boom which operates either in a Vector (VHM) or a scalar mode, and a fluxgate magne-tometer, half way down the boom. In the present paper we use VHM high-resolution data. To represent the magnetic field observations, we use the planetocentric KSM coor-dinate system, with the positive X axis directed towards the Sun, the positive Z axis pointing northward and defined such that Saturn’s magnetic axis lies in the XZ plane, and the Y axis lying in the Saturn’s magnetic equatorial plane.
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.
L14S06 ANDRE´ ET AL.: INNER MAGNETOSPHERE DYNAMICS L14S06
[11] The key observation we would like to address in
the following sections is that the inner Saturnian magne-tosphere appears to be in an extremely dynamic state as observed by MAG; and a set of illustrations supports this observation.
3.1. Short-Duration Depressions in Field Magnitude [12] Inside of 10 Rs, MAG reported for all orbits
numer-ous intermittent diamagnetic depressions in which some of the magnetic pressure is replaced by increased plasma pressure (Figure 2).
[13] These diamagnetic cavities were not reported or
not so abundant at the time of the Voyager and Pioneer flybys. Dougherty et al. [2005] described similar events on the first Cassini orbit in a radial range 6 – 10 Rs.
The largest depression was observed on DOY 182 at 21:05 UT, near but inside the orbit of Dione and at a radial distance of 5.93 Rs (Figure 2 (left)). In that case,
well-marked transverse waves accompanied by left-hand
cyclotron waves propagating at the H2O+ gyro-frequency
seemed to surround the magnetic depression, indicative of the production of new plasma in the magnetospheric system. This depression was associated with a plasma energy density of 900 eV cm 3 [Leisner et al., 2005].
[14] Particularly evident and abundant on the second
orbit (Figure 3 (left)), these events present average magnetic field magnitude changes of 1 – 2 nT (1 – 2% of the back-ground field of magnitude 50 and 90 nT) and average durations lasting between 2 and 6 minutes. Rough estimates for the longitudinal width of these structures give tens of thousands kilometers. They are mainly observed at distances inside of 8 Rs and for the majority lie close to
the outer latitudinal boundary of the plasmadisc, also reported turbulent by Sittler et al. [1983]. The Cassini spacecraft later on the second orbit crossed the Saturnian equatorial plane around 20:00 UT on day of year (DOY) 302 when MAG revealed an interval with a large increase in the noise level of the magnetic field observations, again 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.
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.
indicative of plasma production. This interval is particu-larly noticeable in Figure 2 (right) and will be described in the next section.
[15] Most of the observed magnetic depressions are
found to correlate well with energy-time dispersion in the low-energy plasma instrument observations [Hill et al., 2005; Burch et al., 2005]. These events are interpreted as signatures of centrifugally driven interchange motions injecting hot tenuous plasma toward Saturn and are associ-ated with narrow but deep density cavities in the cooler background plasma.
3.2. Short-Duration Increases in Field Magnitude [16] Close to the time when Cassini crossed the Saturnian
equatorial plane (around 8 Rs) on its outbound second and
third orbits (around the 21 – 24 LT sector of the magneto-sphere), MAG reported unusual magnetic field signatures of flux tubes with greater magnetic pressure than their surroundings (Figure 3 (right)), embedded in regions where the noise level of the magnetic field observations is greatly enhanced. These unusual signatures have not been reported before in Saturn’s magnetosphere and are characterized by very sharp boundaries, by abrupt and large changes in the magnetic field magnitude (3 nT in a background field of magnitude 30 nT), and by average durations lasting between 1 and 5 minutes. For some of the largest events presented on Figure 3, wavy processes are observed on their edges (see for example around 19:00 UT on DOY 302) and could suggest erosion of the structures and possible breakup into smaller flux tubes.
[17] We tentatively interpret these unusual magnetic field
observations as the signature of depleted flux tube missing some plasma energy density compared to their surround-ings. Preliminar information about their mass content and very simple pressure balance arguments assuming no tem-perature differences (dn/n = 2/b dB/B with b 1) suggest density variations in these flux tubes of the order of 20%. If confirmed, the role played by these flux tubes could be to counterbalance outward plasma transport and to return inwards to the inner magnetosphere the magnetic flux carried out by mass-loaded flux tubes. The exact mecha-nism producing these flux tubes has not been yet reported to our knowledge but we anticipate that reconnection in the near tail on the nightside could be a candidate [Curtis et al., 1986]. Cassini will orbit these regions of the magnetosphere later during its 4-years tour of the Saturnian magnetosphere.
4. Perspectives
[18] The Cassini magnetometer observations reveal inner
regions of the Saturnian magnetosphere in total ebullition during the first three orbits of the spacecraft around the planet. This observation is supported by various unusual signatures of magnetic field depressions and enhancements that we tentatively relate to plasma transport triggered by the interchange instability. We plan in the near future to conduct very detailed case studies of these two types of magnetic signatures in order to determine their exact prop-erties and to firmly confirm our interpretation. Information on the mass content, the composition and the radial velocity of the observed flux tubes is central to our analysis, since the magnetometer can not have access to these parameters. The unique capabilities of the Magnetosphere and Plasma
instruments suite on-board Cassini will be of great help to reach this objective.
[19] The similarities of our new reported observations
with observations of dynamic processes occurring in the Io plasma torus is striking and let us envisage interesting comparative studies to obtain a better understanding of the interchange instability responsible (at least partly) for the radial transport of plasma in giant planet magnetospheres.
[20] The Galileo spacecraft was in orbit about Jupiter
from December of 1995 to September of 2003. Galileo detected observational evidence for small-scale plasma transport by flux tube interchange motions, which were not identified at the time of the Voyager mission. Signatures of intermittent and short-lived, mass-loaded and empty flux tubes were identified in Io’s torus [Kivelson et al., 1997; Thorne et al., 1997; Bolton et al., 1997] and beyond [Russell et al., 1999] through field and particle measure-ments. A global picture of the large-scale outward motion of the Jovian plasma was given by Russell [2001]. As the plasma moves outward through the magnetodisk, the magnetodisk appears globally unstable and highly dynamic and small tearing islands can be observed within it [Russell et al., 1999], which act as sites of transient reconnection in the nightside, with the subsequent release of ions and the return of empty flux tubes into the inner magnetosphere [Russell et al., 2000a, 2000b].
[21] The complete Cassini tour within the Saturnian
magnetosphere will also add new pieces and answers to our puzzle and hopefully will enable us to reach at least the same level of understanding of the global plasma circulation in the Jovian magnetosphere that we have learnt from Galileo, as summarized above.
[22] Acknowledgment. N. Andre´ would like to acknowledge the French Space Agency (CNES) for its financial support.
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M. K. Dougherty, Blackett Laboratory, Space and Atmospheric Physics, Imperial College, London SW7 2BZ, UK.
K. K. Khurana, J. S. Leisner, and C. T. Russell, Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Los Angeles, CA 90095 – 1567, USA.
