Research Article

Saturn’s magnetic field revealed by the Cassini Grand Finale

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Science  05 Oct 2018:
Vol. 362, Issue 6410, eaat5434
DOI: 10.1126/science.aat5434
  • A meridional view of the results of the Cassini magnetometer observations during the Grand Finale orbits.

    Overlain on the spacecraft trajectory is the measured azimuthal field from the first Grand Finale orbit, revealing high-latitude auroral FACs and a low-latitude interhemispherical FAC system. Consistent small-scale axisymmetric internal magnetic field structures originating in the shallow interior are shown as field lines within the planet. A tentative deep stable layer and a deeper dynamo layer, overlying a central core, are shown as dashed semicircles. The A-, B-, C-, and D-rings are labeled, and the magnetic field lines are shown as solid lines. RS is Saturn’s radius, Z is the distance from the planetary equator, ρ is the perpendicular distance from the spin axis, and Bϕ is the azimuthal component of the magnetic field.

  • Fig. 1 Vector magnetic field measurements from nine Cassini Grand Finale orbits.

    (A to C) The IAU System III Saturn-centered spherical polar coordinates are adopted here. The peak measured magnetic field strength is ~18,000 nT, whereas the azimuthal component of the field (plotted on a different scale) is ~1/1000 of the total field outside the high-latitude auroral FACs region, which are labeled. Vertical dashed lines indicate the MAG range 3 (Embedded Image nT) time period. (C) The vertical dotted lines mark the inner edge of the D-ring mapped magnetically, and the solid vertical solid lines mark the outer edge of the D-ring mapped magnetically, as described in the text.

  • Fig. 2 Cassini’s Grand Finale trajectory shown in the meridional plane.

    The trajectory of Cassini Grand Finale orbit rev 275 (black/blue trace with arrows) in the ρ-Z cylindrical coordinate system—where ρ is the perpendicular distance from the spin axis, and Z is distance from Saturn’s planetary equator—is overlain on the average positions of Saturn’s auroral FACs region (gray region) and the newly discovered low-latitude FACs (red region). Thin gray traces with arrows are magnetic field lines. The interval along the trajectory during which the measured magnetic field was >10,000 nT is highlighted in blue. Small circles are at 3-hour intervals, and the beginning of day 142 of year 2017 is shown.

  • Fig. 3 Saturn’s magnetic equator position as directly measured along the Cassini Grand Finale orbits.

    (A) The measured cylindrical radial component of the magnetic field versus distance from Saturn’s planetary equator from nine Cassini Grand Finale orbits. (B) The distribution of the magnetic equator northward displacements (triangles) versus cylindrical radial distance compared with predictions from existing degree-3 internal field models (lines) (4, 7, 10, 11). The Z3 model is from (4), the Cassini 3 model is from (11), the SPV model is from (7), the Cassini (SOI, June 2007) model, and the Cassini, P11, V1, V2 model are both from (10). The Cassini (SOI, June 2007) model were derived from Cassini SOI data to July 2007 data, and the Cassini, P11, V1, V2 model is derived from Cassini SOI to July 2007 data combined with Pioneer 11 and Voyager 1 and 2 data (10).

  • Fig. 4 Saturn’s magnetic field beyond spherical harmonic degree 3 as directly measured during the Grand Finale orbits.

    (A and B) Residuals of the radial and meridional magnetic fields from the unregularized degree-3 internal field model (parameters are provided in table S1). It can be seen that the residuals are on the order of 100 nT, which is 10 times larger than the magnetodisk field.

  • Fig. 5 Saturn’s magnetic field beyond spherical harmonic degree 6 as directly measured during the Grand Finale.

    (A and B) Residuals of the radial and meridional magnetic fields from the unregularized degree-6 internal field model (parameters are provided in table S1). The field corresponding to the magnetodisk current has also been removed (parameters of the magnetodisk current model are provided in table S2). Latitudinally banded magnetic structures on the order of 25 nT are evident. Orbit-to-orbit deviations in Bθ between –60° and –40° latitudes are mostly due to the influences of southern hemisphere high-latitude FACs.

  • Fig. 6 Magnetic power spectrum of the Cassini 11 model.

    This new internal field model for Saturn is constructed from nine Cassini Grand Finale orbits with regularized inversion. Central values and five times the formal uncertainties derived from the regularized inversion are shown (Methods). It can be seen that the high-degree moments between degree 4 and degree 11 are on the order of 10 to 100 nT, whereas those above degree 11 are below the derived uncertainty.

  • Fig. 7 Small-scale axisymmetric magnetic field of Saturn at 0.75 RS.

    The values of ΔBr were computed by using the central values of the degree 4 to degree 11 Gauss coefficients (Table 1). These small-scale magnetic perturbations at 0.75 RS are on the order of 5000 nT, which amount to ~5 to 10% of the local background field (fig. S7). The plotted latitudinal range corresponds to the region of Cassini MAG range 3 measurements along the Grand Finale orbits.

  • Table 1 Gauss coefficients of a new model for Saturn’s internal magnetic field, which we refer to as the Cassini 11 model, constructed from nine orbits of Cassini Grand Finale MAG data with regularized inversion.

    The reported uncertainty is five times the formal uncertainties associated with the chosen regularization (Methods).

    Gauss coefficientValue (nT)Uncertainty (nT)
    g1021140.21.0
    g201581.11.2
    g302260.13.2
    g4091.14.2
    g5012.67.1
    g6017.28.2
    g70–59.68.1
    g80–10.58.7
    g90–12.96.3
    g10015.07.0
    g11018.27.1
    g1200.37.7
    RMS residual6.2
  • Saturn's magnetic field revealed by the Cassini Grand Finale

    Michele K. Dougherty, Hao Cao, Krishan K. Khurana, Gregory J. Hunt, Gabrielle Provan, Stephen Kellock, Marcia E. Burton, Thomas A. Burk, Emma J. Bunce, Stanley W. H. Cowley, Margaret G. Kivelson, Christopher T. Russell, David J. Southwood

    Materials/Methods, Supplementary Text, Tables, Figures, and/or References

    Download Supplement
    • Figs. S1 to S10
    • Tables S1 and S2 

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