Abstract We study Jupiter's nightside magnetospheric azimuthal magnetic field using measurements from the first 58 data‐taking periapsides of the Juno spacecraft, spanning post‐dawn to pre‐dusk local times (LTs). The methodology follows Provan et al. (2024, https://doi.org/10.1029/2024JA032677 ) but extends the analysis to 40 Jupiter radii distances, allowing better discrimination of field variations at dusk. The field has two components, (a) a field varying along field lines inversely with perpendicular distance from the magnetic axis, taken to be associated with magnetosphere‐ionosphere coupling, and (b) a quasi‐uniform field consistent with a field ∼4 nT pointing sunward/antisunward in the southern/northern hemisphere. These components are separated by subtracting the quasi‐uniform field, yielding colatitude profiles of the ionospheric current versus LT from which the atmospheric torque on the magnetospheric plasma is calculated. Currents peak on outer magnetosphere field lines at ∼12.5 MA per radian of azimuth at dusk, falling to ∼6.5 MA rad −1 at midnight but remain at ∼2 MA rad −1 polar values at dawn. Corresponding outer magnetosphere torques are ∼1.1 × 10 18 N m in the dusk‐midnight quadrant, falling to ∼0.2 × 10 18 N m in the midnight‐dawn quadrant. The quasi‐uniform field also provides a torque on the equatorial plasma, corotational in the midnight‐dawn sector ∼0.6–0.8 × 10 18 N m augmenting the atmospheric torque, but anti‐corotational in the dusk sector opposing the atmospheric torque. We propose that the quasi‐uniform fields and related ionospheric LT asymmetries are associated with the confinement of iogenic plasma outflow in the down‐tail direction, possibly augmented by more direct interaction with the solar wind at the magnetopause.
Cowley et al. (Fri,) studied this question.