Jupiter’s Magnetic Field and Rotation Period in the Extended Missions

The Juno spacecraft continues to map the gas giant’s complex magnetic field with ever-increasing resolution in space and time, taking advantage of the natural evolution of Juno’s polar orbit and time on target. At the beginning of the prime mission in 2016, Juno’s cloud-topping periapsis occurred just northward of the equator. With each subsequent orbit, Juno’s perijove marches northward by ∼1°, owing to the apsidal precession of the orbit caused by Jupiter’s tidal bulge. Our recent spherical harmonic models derived from Juno measurements through orbit 66 of Extended Mission 1 (EM1, periJove at 56 degrees north latitude) routinely introduce a correction to the planet’s rotation period along with resolution of spherical harmonic coefficients corresponding to smaller spatial scales. Jupiter’s planetary rotation period (per IAU) has been determined with greater accuracy than that provided by observations of its radio emissions (System III (1965): 9h 55m 29.711s +/-0.04s). The secular variation of the magnetic field during Juno’s mission through orbit 66 (by ~0.122°/yr) yields an improved planetary rotation period of 9h 55m 29.697s, if the variation is attributed to the limited accuracy of the IAU adopted planetary rotation period. Much of the apparent motion of the Great Blue Spot (GBS), the localized patch of intense magnetic field near the equator, can be accounted for by inaccuracy of System III (1965). As Juno’s periJove migrates further northward in EM1 (through orbit 76) and EM2, the polar regions will be mapped at lower altitudes affording comparison with fluid motions such as those probed by Juno’s Microwave Radiometer (MWR). The latter half of EM1 orbits will complete mapping of the mid latitude high flux band, and EM2 will map the field with periJoves to 81 degrees north latitude where the circumpolar cyclones encircle the pole.