Stanley W. H. Cowley

The Cassini spacecraft spent 13 years orbiting Saturn, quasi‐continuously measuring Saturn's radio emissions. One of the key elements of the radio spectrum at Saturn is the Saturn Kilometric Radiation (SKR), and several studies have been written on events called Low Frequency Extensions (LFEs) which represent strong, continuous extensions of the SKR down to lower frequencies. A previous machine learning approach, trained on LFEs observed by Cassini, returned several thousand LFEs across the entire mission. We joined events with very short ( minutes) inter‐event separation to produce a data set of 4553 individual LFEs. We find the events have a median duration of 5.65 hr, with a long‐tail distribution up to several days, and thus we separate them into “short” ( hours or planetary rotation) and “long” events for subsequent analysis. We show several formats of spatial maps of LFE location, and these indicate a broad clustering of LFEs in the post‐midnight region of hr local time (LT). Averaged spectrograms illustrate the median morphological shape of LFEs as a function of LT and latitude. Key features such as the broadening of the emission near dawn, and the extinction of the signal at highest latitudes can be clearly seen in these visualizations. Comparison of the start times of LFEs with the Planetary Period Oscillation (PPO) northern and southern phases reveals an organization by phase consistent with the physical picture of preferential viewing when the plasma sheet was thinned, if appropriate phase propagation delays are accounted for.

Using the methodology of Cowley et al. ( https://doi.org/10.1029/2025JA033993 ), we employ ∼8 years of Juno azimuthal magnetic field data to derive colatitude profiles of Jupiter's nightside (17‐05 hr via midnight) ionospheric equatorward Pedersen current from near the magnetic pole to inner magnetosphere field lines at 18° colatitude, from which we quantify large‐scale field‐aligned currents on spatial scales ∼1°. Polar‐tail Pedersen currents are ∼1–2 MA per radian of azimuth, with regions of downward and upward currents of order tens nA m ⁻² . Stronger downward currents ∼50–200 nA m ⁻² flow in an outer magnetosphere layer that is wider at dusk (∼7°) than at dawn (∼2°), with peak Pedersen currents near the outer/middle magnetosphere boundary reaching ∼12 MA rad ⁻¹ at dusk falling to ∼2.5 MA rad ⁻¹ at dawn. Upward currents of order hundreds of nA m ⁻² flow on middle magnetosphere field lines across which the Pedersen current falls to small values. The Pedersen currents are also combined with Juno/JADE‐I plasma angular velocity measurements to determine Jupiter's effective ionospheric height‐integrated Pedersen conductivity, together with the power input to thermospheric heating. Outer magnetosphere effective conductivities are ∼0.4 mho at dusk, falling to ∼0.25 mho at midnight, and ∼0.1 mho pre‐dawn. No significant increase in conductivity is found in the middle magnetosphere region of strong upward current, suggesting that thermospheric winds closely track the plasma velocity in this region. Latitude‐integrated powers show a similar dusk‐dawn asymmetry, with a total nightside power per hemisphere of ∼370 TW, of which ∼55% is input to thermospheric heating and 45% to the magnetospheric plasma.

Using the methodology of Cowley et al. (2025, https://doi.org/10.1029/2025JA033993), we employ ~8 years of Juno azimuthal magnetic field data to derive colatitude profiles of Jupiter’s nightside (17-05 hr via midnight) ionospheric equatorward Pedersen current from near the magnetic pole to inner magnetosphere field lines at 18° colatitude, from which we quantify large-scale field-aligned currents on spatial scales ~1°. Polar-tail Pedersen currents are ~1-2 MA per radian of azimuth, with regions of downward and upward currents of order tens nA m-2. Stronger downward currents ~50-200 nA m-2 then flow in an outer magnetosphere layer that is wider at dusk (~7°) than at dawn (~2°), with peak Pedersen currents near the outer/middle magnetosphere boundary reaching ~12 MA rad-1 at dusk falling to ~2.5 MA rad-1 at dawn. Upward currents of order hundreds nA m-2 flow on middle magnetosphere field lines across which the Pedersen current falls to small values. The Pedersen currents are also combined with Juno/JADE-I plasma angular velocity measurements to determine Jupiter’s effective ionospheric height-integrated Pedersen conductivity, together with the power input to thermospheric heating. Outer magnetosphere effective conductivities are ~0.4 mho at dusk, falling to ~0.25 mho at midnight, and ~0.1 mho pre-dawn. No significant enhancement is found in the middle magnetosphere region of strong upward current, suggesting that thermospheric winds near-track the plasma velocity in this region. Latitude-integrated powers show a similar dusk-dawn asymmetry, with a total nightside power per hemisphere of ~370 TW, of which ~55% is input to thermospheric heating and 45% to the magnetospheric plasma.

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.

This study investigates Jupiter's dawnside magnetodisc, using plasma and magnetic field measurements from Juno orbits 5 to 12 to refine a radial force‐balance magnetodisc model. This iterative vector potential model examines variations in the azimuthal magnetodisc current, coupled with a magnetosphere‐ionosphere coupling model from which the radial current is simultaneously obtained. Three key force‐balance parameters are used: the hot plasma parameter (pV, Pa m T⁻¹), the mass outflow rate of cold iogenic plasma, and the height‐integrated ionospheric Pedersen conductivity. Axisymmetric equilibrium outputs are compared to Juno's residual magnetic field and heavy ion density data between 15 and 60 R_J. Optimal parameter values for each orbit and overall current distributions are determined. Averaged modeled values are (1.63 ± 0.17) × 10⁷ Pa m T⁻¹ for the hot plasma parameter, 1,340 ± 350 kg s⁻¹ for the mass outflow rate, and 0.26 ± 0.08 mho for the Pedersen conductivity. The overall modeled magnetodisc azimuthal current to 60 R_J is 266 ± 23 MA, varying similarly to the currents determined by Connerney et al. (2020, https://doi.org/10.1029/2020JA028138) but typically ∼50 MA larger. Of this total, the hot plasma current 158 ± 13 MA is larger than the cold plasma current 109 ± 23 MA, and dominates in the inner region. The cold plasma current typically becomes the larger component beyond ∼35 R_J and exhibits greater orbit‐to‐orbit variability. The mass outflow rate from Io is the primary driver of magnetodisc current variability. The north‐south summed radial magnetosphere‐ionosphere coupling current 104 ± 31 MA is typically ∼40% of the total azimuthal current, with variations that are only weakly correlated.

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.

We compare Hubble Space Telescope observations of Jupiter's FUV auroras with contemporaneous conjugate Juno in situ observations in the equatorial middle magnetosphere of Jupiter. We show that bright patches on and equatorward of the main emission are associated with hot plasma injections driven by ongoing active magnetospheric convection. During the interval that Juno crossed the magnetic field lines threading the complex of auroral patches, a series of energetic particle injection signatures were observed, and immediately prior, the plasma data exhibited flux tube interchange events indicating ongoing convection. This presents the first direct evidence that auroral morphology previously termed “strong injections” is indeed a manifestation of magnetospheric injections, and that this morphology indicates that Jupiter's magnetosphere is undergoing an interval of active iogenic plasma outflow.

We analyze magnetic data obtained during northern high‐altitude traversals of Jovian middle magnetosphere (MM) field lines during the first 10 inbound data‐taking passes of the Juno spacecraft for azimuthal field perturbations associated with magnetosphere‐ionosphere coupling currents. Full traversals across the MM region occur on all 10 passes at radial distances ∼7 to 16 R_J (T1 traversals), followed in four cases by reversed traversals (T2) at ∼4 to 7 R_J. Signatures of upward field‐aligned currents were observed in all cases, closely collocated with the statistical main auroral oval when mapped along field lines to the ionosphere. Two T1 sheets carry currents ∼10 MA per radian of azimuth in ionospheric colatitudinal layers ∼1.2°, closely comparable with the ∼8.5 MA rad⁻¹ theoretical model value of Cowley et al. (2008, https://doi.org/10.5194/angeo-26-4051-2008). The other T1 sheets typically carry half this current ∼5.1 MA rad⁻¹ in layers half the width ∼0.56°, thus with comparable current densities ∼425 nA m⁻². The T2 currents are smaller ∼3.4 MA rad⁻¹ with current densities ∼120 nA m⁻², a difference that may relate to locations on opposite sides of the main oval as reflected in near‐contemporaneous ultraviolet observations, though ionospheric field strengths are not greatly different. Comparison with northern near‐periapsis currents observed inside ∼2 R_J on the same passes by Kotsiaros et al. (2019, https://doi.org/10.1038/s41550-019-0819-7) yields a cross‐correlation coefficient ∼0.7 with our T1 currents, though Kotsiaros et al.’s mean current value of ∼3.8 MA rad⁻¹ is somewhat less than our overall mean T1 value ∼6.2 MA rad⁻¹. The differences may have spatial, temporal, or methodological origins.

Cassini's 2017 proximal orbits provided the opportunity to examine the auroral field‐aligned currents in the northern hemisphere dawn sector in relation to wider magnetospheric conditions. We combine three recent studies to examine the response of the dawn region auroral field‐aligned currents and the azimuthal ring currents to compressions and expansions of the Saturnian magnetosphere. For compressions of Saturn's magnetosphere resulting in tail reconnection, the currents within the downward current sheet, located equatorward of the main auroral oval, increases in strength with increasing total ring current and location of the peak downwards current moves inwards toward Saturn. While the inverse relation occurs during intervals of quiet or expanded magnetospheric conditions. During compression events there is an increase in the energetic particle intensities, in particular in the protons (35–506 keV), within the downward current region. This current system is akin to an Earth‐like “region 2” field aligned current within Saturn's magnetosphere, with tail reconnection occurring when the magnetosphere is compressed resulting in a partial nightside ring current closed by a downward current near to dawn. Within the upward current sheet, mapping to Saturn's main auroral oval, both non‐rotating subcorotating current and the rotating Planetary Period Oscillations (PPOs) currents flow. The upward current is strongly modulated by the PPOs but also increases in strength, with enhanced high‐energy protons, during intervals of magnetospheric compressions and tail reconnection. We conclude that the enhanced plasma injected into the midnight‐dawn sector during tail reconnection events results in an enhanced subcorotation current system.