  The Cassini mission entered the Grand Finale phase in April 2017 and executed
22.5 highly inclined, close-in orbits around Saturn before diving into the
planet on September 15th 2017. Here we present our analysis of the Cassini
Grand Finale magnetometer (MAG) dataset, focusing on Saturn's internal magnetic
field. These measurements demonstrate that Saturn's internal magnetic field is
exceptionally axisymmetric, with a dipole tilt less than 0.007 degrees (25.2
arcsecs). Saturn's magnetic equator was directly measured to be shifted
northward by ~ 0.0468 +/- 0.00043 (1-sigma) $R_S$, 2820 +/- 26 km, at
cylindrical radial distances between 1.034 and 1.069 $R_S$ from the spin-axis.
Although almost perfectly axisymmetric, Saturn's internal magnetic field
exhibits features on many characteristic length scales in the latitudinal
direction. Examining Br at the a=0.75 $R_S$, c=0.6993 $R_S$ isobaric surface,
the degrees 4 to 11 contributions correspond to latitudinally banded magnetic
perturbations with characteristic width similar to that of the off-equatorial
zonal jets observed in the atmosphere of Saturn. Saturn's internal magnetic
field beyond 60 degrees latitude, in particular the small-scale features, are
less well constrained by the available measurements, mainly due to incomplete
spatial coverage in the polar region. A stably stratified layer thicker than
2500 km likely exists above Saturn's deep dynamo to filter out the
non-axisymmetric internal magnetic field. A heat transport mechanism other than
pure conduction, e.g. double diffusive convection, must be operating within
this layer to be compatible with Saturn's observed luminosity. The
latitudinally banded magnetic perturbations likely arise from a shallow
secondary dynamo action with latitudinally banded differential rotation in the
semi-conducting layer.
