Jupiter's Magnetic Field and Secular Variation During Juno's First 6 Years in Orbit

The Juno spacecraft, in polar orbit about Jupiter since July 2016, maps the gas giant's magnetic field with ever-increasing resolution in space and time. The JRM33 magnetic field model, based on vector observations of the field acquired during the Prime Mission's first 33 orbits, revealed an intense and asymmetrically non-dipolar magnetic field undergoing secular variation. The Lowes' radius fit to spherical harmonics through degree 18 yields a putative dynamo radius of 0.807 +/- 0.006 Rj, taken as the depth to which helium rain stabilizes the metallic hydrogen-helium mixture against convection. Direct comparison of spherical harmonic models (JRM33 and JRM09) representative of different epochs revealed secular variation of the field near the isolated and intense patch of negative flux near the equator known as the Great Blue Spot (GBS). The feature drifts eastward relative to the deep interior at a rate of a few cm/s; if carried at depth by zonal winds, they must penetrate to depths of ~3000 km where the electrical conductivity is sufficient to grip the magnetic field. A dedicated magnetic survey above the GBS was conducted during Extended Mission orbits 36-42 to better characterize the GBS and its evolution during the mission. Jupiter's planetary rotation period has been determined with greater accuracy than that provided by observations of its radio emissions (System III (1965): 9h 55m 29.711s +/-0.04s) via a subsequent spherical harmonic analysis allowing for time-dependent dipole coefficients. The drift of the dipole during Juno's prime mission (by 0.12°/yr) determined this way yields an improved planetary rotation period of 9h 55m 29.698s, if the migration of the dipole is attributed to the limited accuracy of the IAU adopted planetary rotation period. A similar result is obtained by direct comparison of the JRM33 model with models representing earlier epochs (Voyager in 1979 and Ulysses in 1992).