A spherical harmonic model of the magnetic field of Jupiter is obtained
from vector magnetic field observations acquired by the Juno spacecraft
during 32 of its first 33 polar orbits. These Prime Mission orbits
sample Jupiter's magnetic field nearly uniformly in longitude (∼11°
separation) as measured at equator crossing. The planetary magnetic
field is represented with a degree 30 spherical harmonic and the
external field is approximated near the origin with a simple external
spherical harmonic of degree 1. Partial solution of the underdetermined
inverse problem using generalized inverse techniques yields a model
(“JRM33”) of the planetary magnetic field with spherical harmonic
coefficients reasonably well determined through degree and order 13.
Useful information regarding the field extends through degree 18, well
fit by a Lowes' spectrum with a dynamo core radius of 0.81 Rj,
presumably the outer radius of the convective metallic hydrogen region.
This new model provides a most detailed view of a planetary dynamo and
evidence of advection of the magnetic field by deep zonal winds in the
vicinity of the Great Blue Spot (GBS), an isolated and intense patch of
flux near Jupiter's equator. Comparison of the JRM33 and JRM09 models
suggests secular variation of the field in the vicinity of the GBS
during Juno's nearly 5 years of operation in orbit about Jupiter. The
observed secular variation is consistent with the penetration of zonal
winds to a depth of ∼3,500 km where a flow velocity of ∼0.04 ms−1 is required to match the observations.