Juno Observations of Large-Scale Azimuthal Fields in Jupiter's Nightside Magnetosphere and Related Radial Currents

Abstract

We combine magnetic data from the first 46 data-taking periapsides of the polar orbiting Juno spacecraft spanning dawn to dusk via midnight to investigate azimuthal fields and related currents in Jupiter's nightside magnetosphere. Data are binned by perpendicular radial distance ρ from the magnetic axis over 4–32 R_J along empirical poloidal model field lines spanning from tail to middle magnetosphere regions (ionospheric colatitudes θ_i ∼ 5°–17°), and by local time (LT). The data are well organized by these parameters. On southern tail field lines () the azimuthal field is well represented as the sum of sweepback fields falling as 1/ρ that are near-independent of θ_i and LT, and a near-constant field consistent with ∼3.5 nT pointing sunward. The combination is swept back at dawn/midnight but swept forward at dusk outside ∼5 R_J. Outer magnetosphere (θ_i ∼ 12°–15.5°) azimuthal fields are instead swept back near-independent of LT, near-continuous with the tail field in the dawn sector, but with large shear at the tail interface and across outer magnetosphere field lines in the dusk-midnight sector. The tail region 1/ρ field is associated with a nightside inward polar axial current ∼5.8 MA located within θ_i ∼ 5° of the magnetic axis, while the dusk-midnight field shear measured near the ∼30 R_J study boundary provides an inward current ∼15.4 MA, related to the near-constant field. Azimuthal fields fall to small values across middle magnetosphere field lines (θ_i ∼ 15.5°–17°), associated with outward currents ∼21.2 MA per hemisphere near-independent of LT forming the nightside equatorial current sheet, balancing these inward currents.