Outer magnetospheric structure: Jupiter and Saturn compared

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A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 [46] For the purposes of this study a constant electron temperature of 150 eV and 120 eV has been assumed for Jupiter and Saturn, respectively, based on in situ plasma measurements by the Ulysses and Cassini spacecraft. The magnetic field profile was determined from a power law fit to the observed magnetodisk lobes with the assumption (based on Ulysses observations) that the field magnitude in the center of the magnetodisk is roughly 10% of its value in the lobes. The azimuthal velocity of the plasma was allowed to vary with radial distance according to the Achilleos et al. [2010a] model for Saturn and the Hill [1980] model for Jupiter. Finally, a mean ion charge of q i = 1 is assumed for both magnetospheres and the mean ion mass, m i , is set at 24 amu for Jupiter [Thomas et al., 2004] and 18 amu for Saturn [Khurana et al., 2007]. We further assume that pressure gradient forces are comparable to centrifugal forces at Saturn (k = 1) and an order of magnitude larger than centrifugal forces (k = 10) at Jupiter. This is the largest possible pressure gradient contribution allowed at each planet by Achilleos et al. [2010a, 2010b]. [47] Using these numbers the critical number density at both planets (Figure 6, red lines) is seen to decrease with radial distance, primarily due to the decreasing magnitude of the magnetic field. The electron number density measured by in situ spacecraft (Figure 6, blue lines) also decreases with radial distance and, for the Ulysses inbound pass at Jupiter, the two curves cross at roughly 50 R J , well inside the 88 R J distance to the magnetopause. Beyond this distance the number density at the center of the plasma sheet is almost an order of magnitude higher than the critical value suggesting that magnetodisk breakdown in this region is likely. Such breakdown is supported by the observation of tearing islands inside the dayside magnetodisk by Russell et al. [1999] as well as the magnetic “nulls” of Haynes [1995]. [48] Interpreting the Cassini data is more difficult. The low electron density in Saturn’s outer magnetosphere is near the ELS detection threshold and this, combined with the lack of clear current sheet crossings, makes it hard to tell how the in situ number density at the center of the plasma sheet compares with the critical value. Because of this, definite statements about the structure and dynamics of Saturn’s magnetodisk will have to await more extensive observations and higher‐sensitivity plasma measurements. Qualitatively, however, it appears likely that the measured number density is comparable to the critical value throughout much of Saturn’s magnetodisk but that it rarely exceeds this critical value to the extent observed at Jupiter. This implies that magnetodisk breakdown is less likely at Saturn (although the current observations cannot rule it out entirely) which is certainly consistent with the observational absence of a cushion region in the Saturnian magnetosphere. [49] In light of this discussion an important point must be raised with regards to the mean subsolar thickness of the cushion region seen at Jupiter. In estimating this value we have assumed that the mean subsolar standoff distance of the magnetopause is 75 R J . However, in section 4 it has been suggested that the Jovian cushion region is more likely to form when the magnetosphere is in an expanded configuration. It is thus possible that many of the Jovian cushion region observations used in this analysis were made when the magnetosphere was in just such an expanded configuration and that the mean magnetopause location for this particular subset of passes is actually greater than the 75 R J mean obtained by Joy et al. [2002]. Such biasing would result in the cushion region having a mean inertial subsolar thickness larger than the 20 R J figure obtained in section 2.3. A more accurate estimation of the cushion region’s thickness should become possible once future missions to Jupiter begin returning data. 4.3. Future Studies and Extensions [50] The ideas presented in this paper can be tested in two ways. A detailed study of the ion‐electron plasma in the outer regions of both planets magnetospheres should reveal whether plasma‐depleted flux tubes (of any magnetic configuration) exist in these systems. The above statement will hold true even when the magnetodisk is seen to persist all the way out to the magnetopause or, in the case of Saturn, when the magnetosphere is so compressed that no magnetodisk is observed at all. Such observations will, additionally, allow plasma in the outer magnetosphere to be better characterized in general and, as a result, allow the radial distance (and critical density) at which the magnetodisk breaks down to be better constrained. Extensive, high‐quality plasma measurements of this nature are currently unavailable (both at Jupiter and at Saturn) but this situation may change as new and improved spacecraft reach the outer planets. Meanwhile, in the more immediate future, qualitative statements may become possible as existing instrumental data sets are studied in more detail. We expect plasma‐depleted flux tubes to be common in the outer magnetospheres of both these planets, regardless of the associated magnetic field topology. [51] We have also suggested that the thickness of the cushion region is inversely (though not necessarily linearly) proportional to solar wind dynamic pressure and that, for extremely high dynamic pressures, it may not be seen at all. This is difficult to examine at present owing, primarily, to the difficulties involved in calculating the thickness of the cushion region (and the upstream solar wind dynamic pressure) for individual passes. The aforementioned lack of plasma data is also an important issue. The proposed EJSM and Juno missions (Juno is scheduled for launch in August 2011 while EJSM is currently under review) may improve the statistics in such a way that qualitative statements can be made on this possibility but the real answer must await a multispacecraft investigation of the cushion region. The EJSM mission, potentially consisting of two concurrent spacecraft, may yet provide such an opportunity. [52] The Juno mission will, for the first time, allow the full three dimensional structure of the Jovian magnetosphere to be explored and this, combined with high‐latitude Cassini orbits, will allow the effects of magnetodisk warping to be quantified at both planets. Finally, a better understanding of the microphysics of magnetotail reconnection (for example, do the Dungey and Vasyliũnas cycles share a common x line?) will allow many of our theoretical expectations, at both planets, to be better constrained and compared with observations. 5. Summary [53] This paper has characterized the Jovian outer magnetosphere (or cushion region) using, for the first time, observations made by multiple exploring spacecraft. We find that, 12 of 14
A04224 WENT ET AL.: OUTER MAGNETOSPHERIC STRUCTURE A04224 while the instantaneous location of both the inner and outer boundaries are highly variable, the cushion region has a mean subsolar thickness of order 20 R J . The cushion region, which is more evident in the morningside magnetosphere as opposed to afternoon, is interpreted by Kivelson and Southwood [2005] as a layer of plasma‐depleted flux tubes which have recently lost mass in the magnetotail as part of the Vasyliũnas and Dungey cycles. Using magnetometer and plasma data from Cassini and other spacecraft, we have shown that the Saturnian magnetosphere typically lacks this outer layer of quasi‐dipolar flux tubes with the Saturnian magnetodisk instead persisting right out to the magnetopause. [54] In spite of this observation, arguments are presented suggesting that Saturn’s outer magnetosphere must contain a large number of plasma‐depleted flux tubes. The nondipolar geometry of these flux tubes is discussed from a number of perspectives, emphasizing the complicating factors of magnetodisk warping and variations in the size of the magnetospheric cavity. We show that the Jovian magnetodisk typically breaks down well inside the planetary magnetopause while, at Saturn, evidence for this breakdown is weaker although it cannot, at present, be ruled out entirely. While conclusive statements must await higher‐quality plasma data and statistics, we tentatively propose that Saturn’s magnetodisk typically persists right out to the magnetopause, robbing any plasma‐depleted flux tubes that lay beyond it of the essential space that is required for them to relax into a more dipolar configuration reminiscent of the cushion region seen at Jupiter. Observational tests of this theory have been proposed and potential developments and extensions of this work are discussed. [55] Acknowledgments. The authors would like to acknowledge useful discussions with Krishan Khurana with regards to the content of this paper. D. R. Went was funded by an STFC postgraduate studentship at Imperial College London. [56] Masaki Fujimoto thanks the reviewers for their assistance in evaluating this paper. References Achilleos, N., C. S. Arridge, C. Bertucci, C. M. Jackman, M. K. Dougherty, K. K. Khurana, and C. T. Russell (2008), Large‐scale dynamics of Saturn’s magnetopause: Observations by Cassini, J. Geophys. Res., 113, A11209, doi:10.1029/2008JA013265. Achilleos, N., P. Guio, and C. S. Arridge (2010a), A model of force balance in Saturn’s magnetodisc, Mon. Not. R. Astron. Soc., 401, 2349–2371, doi:10.1111/j.1365-2966.2009.15865.x. Achilleos,N.,P.Guio,C.S.Arridge,N.Sergis,R.J.Wilson,M.F. Thomsen, and A. J. Coates (2010b), Influence of hot plasma pressure on the global structure of Saturn’s magnetodisk, Geophys. Res. Lett., 37, L20201, doi:10.1029/2010GL045159. André, N., et al. (2007), Magnetic signatures of plasma‐depleted flux tubes in the Saturnian inner magnetosphere, Geophys. Res. Lett., 34, L14108, doi:10.1029/2007GL030374. Arridge, C. S., N. Achilleos, M. K. Dougherty, K. K. Khurana, and C. T. Russell (2006), Modeling the size and shape of Saturn’s magnetopause with variable dynamic pressure, J. Geophys. Res., 111, A11227, doi:10.1029/2005JA011574. Arridge, C. S., C. T. Russell, K. K. Khurana, N. Achilleos, S. W. H. Cowley, M. K. Dougherty, D. J. Southwood, and E. J. Bunce (2008a), Saturn’s magnetodisc current sheet, J. Geophys. Res., 113, A04214, doi:10.1029/2007JA012540. Arridge, C. S., K. K. Khurana, C. T. Russell, D. J. Southwood, N. Achilleos, M. K. Dougherty, A. J. Coates, and H. K. Leinweber (2008b), Warping of Saturn’s magnetospheric and magnetotail current sheets, J. Geophys. Res., 113, A08217, doi:10.1029/2007JA012963. Arridge, C. S., L. K. Gilbert, G. R. Lewis, E. C. Sittler, G. H. Jones, D. O. Kataria, A. J. Coates, and D. T. Young (2009), The effect of spacecraft radiation sources on electron moments from the Cassini CAPS electron spectrometer, Planet. Space Sci., 57, 854–869, doi:10.1016/j.pss.2009. 02.011. Badman, S. V., and S. W. H. Cowley (2007), Significance of Dungey‐cycle flows in Jupiter’s and Saturn’s magnetospheres, and their identification on closed equatorial field lines, Ann. Geophys., 25, 941–951. Bagenal, F., and J. D. Sullivan (1981), Direct plasma measurements in the Io torus and inner magnetosphere of Jupiter, J. Geophys. Res., 86, 8447–8466, doi:10.1029/JA086iA10p08447. Bagenal, F., P. Delamere, A. Steffl, and M. Horanyi (2004), Time variability of plasma production in the Io torus, Eos Trans. AGU, 85(17), Jt. Assem. Suppl., Abstract SM51A‐03. Balogh, A., M. K. Dougherty, R. J. Forsyth, D. J. Southwood, E. J. Smith, B. T. Tsurutani, N. Murphy, and M. E. Burton (1992), Magnetic field observations during the Ulysses flyby of Jupiter, Science, 257, 1515–1518, doi:10.1126/science.257.5076.1515. Bame, S. J., D. J. McComas, B. L. Barraclough, J. L. Phillips, K. J. Sofaly, J. C. Chavez, B. E. Goldstein, and R. K. Sakurai (1992), The Ulysses solar wind plasma experiment, Astrophys. J. Suppl. Ser., 92, 237–265. Bunce,E.J.,S.W.H.Cowley,D.M.Wright,A.J.Coates,M.K. Dougherty,N.Krupp,W.S.Kurth,andA.M.Rymer(2005),Insitu observations of a solar wind compression‐induced hot plasma injection in Saturn’s tail,Geophys. Res. Lett., 32, L20S04, doi:10.1029/ 2005GL022888. Bunce, E. J., et al. (2008), Origin of Saturn’s aurora: Simultaneous observations by Cassini and the Hubble Space Telescope, J. Geophys. Res., 113, A09209, doi:10.1029/2008JA013257. Burton, M. E., M. K. Dougherty, and C. T. Russell (2009), Model of Saturn’s internal planetary magnetic field based on Cassini observations, Planet. Space Sci., 57, 1706–1713, doi:10.1016/j.pss.2009.04.008. Cowley, S. W. H., and E. J. Bunce (2003), Modulation of Jupiter’s main auroral oval emissions by solar wind induced expansions and compressions of the magnetosphere, Planet. Space Sci., 51, 57–79. Cowley, S. W. H., E. J. Bunce, T. S. Stallard, and S. Miller (2003), Jupiter’s polar ionospheric flows: Theoretical interpretation, Geophys. Res. Lett., 30(5), 1220, doi:10.1029/2002GL016030. Cox, A. N. (Ed.) (2001), Allen’s Astrophysical Quantities, 4thed., Springer, New York. Delamere, P. A., F. Bagenal, V. Dols, and L. C. Ray (2007), Saturn’s neutral torus versus Jupiter’s plasma torus, Geophys. Res. Lett., 34, L09105, doi:10.1029/2007GL029437. Dougherty, M. K., K. K. Khurana, F. M. Neubauer, C. T. Russell, J. Saur, J. S. Leisner, and M. E. Burton (2006), Identification of a dynamic atmosphere at Enceladus with the Cassini magnetometer, Science, 311, 1406– 1409, doi:10.1126/science.1120985. Dungey, J. W. (1961), Interplanetary magnetic field and the auroral zones, Phys. Rev. Lett., 6, 47–48, doi:10.1103/PhysRevLett.6.47. Espinosa, S. A., and M. K. Dougherty (2000), Periodic perturbations in Saturn’s magnetic field, Geophys. Res. Lett., 27, 2785–2788, doi:10.1029/2000GL000048. Goertz, C. K. (1983), Detached plasma in Saturn’s front side magnetosphere, Geophys. Res. Lett., 10, 455–458, doi:10.1029/GL010i006p00455. Gombosi, T. I., T. P. Armstrong, C. S. Arridge, K. K. Khurana, S. M. Krimigis, N. Krupp, A. M. Persoon, and M. F. Thomsen (2009), Saturn’s magnetospheric configuration, in Saturn From Cassini‐Huygens, edited by M. Dougherty, L. W. Esposito, and S. M. Krimigis, pp. 203–255, Springer, New York. Haynes, P. L. (1995), Dynamic phenomena in the Jovian magnetosphere based on observations during the Ulysses flyby, Ph.D. thesis, Imp. Coll. London, London. Haynes, P. L., A. Balogh, M. K. Dougherty, D. J. Southwood, and A. Fazakerley (1994), Null fields in the outer Jovian magnetosphere: Ulysses observations, Geophys. Res. Lett., 21, 405–408, doi:10.1029/93GL01986. Hill, T. W. (1980), Corotation lag in Jupiter’s magnetosphere—Comparison of observation and theory, Science, 207, 301–302, doi:10.1126/science. 207.4428.301. Joy, S. P., M. G. Kivelson, R. J. Walker, K. K. Khurana, C. T. Russell, and T. Ogino (2002), Probabilistic models of the Jovian magnetopause and bow shock locations, J. Geophys. Res., 107(A10), 1309, doi:10.1029/ 2001JA009146. Khurana, K. K., M. K. Dougherty, C. T. Russell, and J. S. Leisner (2007), Mass loading of Saturn’s magnetosphere near Enceladus, J. Geophys. Res., 112, A08203, doi:10.1029/2006JA012110. Kivelson, M. G. (1976), Jupiter’s distant environment, in Physics of Solar Planetary Environments, edited by D. J. Williams, pp. 836–853, AGU, Washington, D. C. Kivelson, M. G. (2005), Transport and acceleration of plasma in the magnetospheres of Earth and Jupiter and expectations for Saturn, Adv. Space Res., 36, 2077–2089, doi:10.1016/j.asr.2005.05.104. 13 of 14
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Outer magnetospheric structure: Jupiter and Saturn compared

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