Saturn's magnetosphere is an efficient emitter of Energetic Neutral Atoms (ENAs), given the presence of an extended neutral cloud around the planet that originates from the icy moon, Enceladus. The ENA emission is symptomatic of the global circulation of plasma in Saturn's magnetosphere. Energetic ions are injected from the outer magnetosphere following magnetotail dynamics and reconnection events. These ions then charge exchange with the neutral cloud, which is mostly confined to the spin plane, resulting in ENA production. The global ENA emission is dynamic, displaying sudden brightening on the nightside, and discrete rotating enhancements which circle the planet for many hours as energetic ions drift with the bulk plasma flow. These latter features have been linked with rotating signatures in the ultraviolet auroras, suggesting coupling via some transient system of field-aligned currents that forms following injection events. Indeed these injection events occur so often as to form Saturn’s dawn auroral arc.Our characterization of the ENA emission at Saturn is made possible using imagery from the Ion-Neutral Camera (INCA) that flew onboard Cassini. Observations were made over the entire mission lifetime. We present for the first time a statistical analysis of the complete INCA image set, using equatorial projections of the flux distribution to reveal the time-averaged morphology of Saturn's ENAs. We used a comprehensive data processing and equatorial projection algorithm to calibrate, clean and filter for all high inclination orbit days. In the final average pictures, many of the projected pixels consist of between tens to hundreds of days continuous exposure, all captured with a line-of-sight > 50° elevation (above the projection plane) and within 30 RS distance from the spacecraft.We find clear toroidal ENA distributions in O and H, and all INCA energy bands, with the emission dropping off sharply inside 5 RS radial distance in all cases. Average peak intensities occur at radial distances from ~7 RS (O, 170-230 keV) to ~10 RS (H, 24-55 keV). All toroids are offset towards the dayside by several RS, most clearly in the 24-55 keV H image, with a maximum intensity at ~13-14 RS from the planet centre on the dayside, compared to only ~10 RS on the nightside. The H ENA distribution is also enhanced around midnight local times, as previously observed in an early morphological study of 2007 data by Carbary et al. [2008], a net effect associated with reconnection return flows and transient ENA enhancements in this sector.We also explore possible organisation of the average global ENA intensity by Saturn’s rotating current systems associated with planetary period oscillations (PPOs, e.g., Provan et al. [2018]). We find that the ENA intensity is statistically modulated by periodic changes in expected plasma sheet thickness as controlled by field-aligned current interactions, a pattern evident in both north and south rotating system frames. With a thicker plasma sheet, more energetic ions are available to charge exchange within the background neutral cloud, and the LOS integral measure increases as a result (and vice versa). In this long-term picture, this effect may dominate over other possible PPO modulation effects on the appearance or evolution of transient ENA injection signatures.

We present a comparison of magnetic field data collected by the NASA Juno spacecraft, with the magnetosphere-ionosphere (MI) coupling model for the Jovian system developed by the University of Leicester. We study the magnetic field of Jupiter, in the Northern Hemisphere, for Perijoves 1-13. By virtue of the offset of the magnetic field to the rotation axis and the subsequent “wobble” of the Juno trajectory in magnetic coordinates, these northern hemisphere portions of PJs 1-13 see the spacecraft traversing the magnetic field lines connecting to the inner, middle, outer and tail regions of the magnetosphere. As such, even away from the close Perijove period, the observations contain evidence of the expected magnetic field perturbations associated with field-aligned currents associated with this fundamental MI coupling. In this study, therefore, we focus on investigating the nature of the field-aligned current signatures evident in the residual azimuthal field (having subtracted the Connerney et al 2018 JRM09 internal magnetic field model) along the magnetic field lines outside of the close periapsides. We map the residual azimuthal field signatures into the ionosphere, and calculate the corresponding ionospheric Pedersen current on an orbit by orbit basis. We compare the magnitude and distribution of these field-aligned current signatures to those expected from the Leicester model, and consider the observed orbit-by-orbit variation as a function of ionospheric colatitude and longitude. We deduce estimates for the field-aligned current densities on auroral field lines for each observation using the Pedersen currents and their distribution in co-latitude, and compare to the previous work of Kotsiaros et al [2019]. We discuss possible reasons for the variations we see, and present the next steps of our broader analysis.

We examine and propose to fundamentally modify the classical theory of solar wind formation. To form a supersonic solar wind, the classical theory requires that a subsonic flow speed must start at a specific initial speed from the coronal base, called eigenspeed, go along a continuous eigenfunction, and reach the sonic point, which is where the flow speed equals the sonic speed, while the critical condition, which is where the effective driving force is zero, is satisfied. Any mismatch between the sonic point and critical condition distances results in either subsonic winds when the initial speed is below the eigenspeed, or no solar wind when the initial speed is above the eigenspeed. Because the critical condition is determined by the solar wind temperature profile, which depends on ionization process at the top of the chromosphere and the heating process around the coronal base but not by the processes at the sonic point, the required match between the two is generally not met and hence the momentum equation in the conventional theory encounters difficulty when the initial speed is above the eigenspeed. To resolve the difficulty, we propose a discontinuity between the sonic point and the critical condition to reach supersonic solar wind solutions. As a result, supersonic solar winds can be produced when the initial speed in the coronal base is greater than the eigenspeed. The critical solution or eigen function provided by the conventional solar wind model describes the condition that separates the supersonic solar winds from subsonic ones.

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 rho from the magnetic axis over 4-32 R-J along empirical poloidal model field lines spanning from tail to middle magnetosphere regions (ionospheric colatitudes theta(i) similar to 5 degrees-17 degrees), and by local time (LT). The data are well organized by these parameters. On southern tail field lines (similar to 5 degrees< theta(i )<= 11 degrees) the azimuthal field is well represented as the sum of sweepback fields falling as 1/rho that are near-independent of theta(i) and LT, and a near-constant field consistent with similar to 3.5 nT pointing sunward. The combination is swept back at dawn/midnight but swept forward at dusk outside similar to 5 R-J. Outer magnetosphere (theta(i) similar to 12 degrees-15.5 degrees) 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/rho field is associated with a nightside inward polar axial current similar to 5.8 MA located within theta(i) similar to 5 degrees of the magnetic axis, while the dusk-midnight field shear measured near the similar to 30 R-J study boundary provides an inward current similar to 15.4 MA, related to the near-constant field. Azimuthal fields fall to small values across middle magnetosphere field lines (theta(i) similar to 15.5 degrees-17 degrees), associated with outward currents similar to 21.2 MA per hemisphere near-independent of LT forming the nightside equatorial current sheet, balancing these inward currents.

&lt;p&gt;We study Jupiter&amp;#8217;s magnetic field and plasma parameters during Juno&amp;#8217;s prime mission, using data from Juno&amp;#8217;s FGM magnetometer and ion measurements from the&amp;#160; Jovian Auroral Distributions Experiment Ion (JADE-I) sensor on Juno.&amp;#160; We compare the observed poloidal magnetic field and&amp;#160; plasma density, angular velocity and temperature profiles, with predictions from the Nichols et al. (2015) &amp;#160;axisymmetric magnetic vector potential model.&amp;#160; This &amp;#160;magnetodisc model balances the j x B force of the azimuthal magnetodisc currents with the outwards forces of the plasma pressure gradient, plasma pressure anisotropy and the centrifugal force associated with the rotating plasma.&amp;#160; By varying the model parameters for each orbit we model how Jupiter&amp;#8217;s mass outflow rate, plasma angular velocity and &amp;#8216;hot&amp;#8217; and &amp;#8216;cold&amp;#8217; plasma temperatures and densities vary throughout Juno&amp;#8217;s prime mission.&amp;#160; We further examine how changes in magnetospheric conditions are related to variations in the magnetosphere&amp;#8211;ionosphere coupling parameters, in particular by studying the azimuthal and radial currents and the ionospheric field-aligned current density.&amp;#160;&lt;/p&gt;

We report on a new international community coding project to provide shared scientific computer code that performs common calculations to aid in planning scientific observations, modeling, and data analysis. We have developed code which calculates Jupiter’s internal and external magnetic fields. All magnetic field model code is provided in four programming languages (C++, IDL, MATLAB and Python). The code is freely available on GitHub. For Jupiter’s internal magnetic field, we present a number of spherical harmonic internal magnetic field models. These include JRM33, the latest Jupiter internal magnetic field model (Connerney et al. in J. Geophys. Res., Planets 127(2):e07055, 2022), as well as older jovian models (e.g. JRM09 (Connerney et al. in Geophys. Res. Lett. 45(6):2590–2596, 2018), O6 (Connerney in Planetary Radio Emissions III, pp. 13–33, 1992), VIP4 (Connerney et al. in J. Geophys. Res. 103(A6):11,929–11,940, 1998) and VIPAL (Hess et al. in J. Geophys. Res. Space Phys. 116(A5):A05217, 2011)). The internal magnetic field code can be easily modified for other planets by simply inputting another spherical harmonic magnetic field model. We have also developed code to calculate the magnetic field perturbations due to the azimuthal and radial currents flowing externally around Jupiter in the jovian magnetodisc according to the model of Connerney et al. (J. Geophys. Res. 86(A10):8370–8384, 1981; J. Geophys. Res. Space Phys. 125(10):e28138, 2020). The internal and external magnetic field codes can be combined to model the magnetic field in Jupiter’s magnetosphere. Finally, we provide field-line tracing software (C++ and a Python wrapper for C++) that utilizes the internal and external magnetic field models. The software can be used to trace along field lines from any position in the jovian magnetosphere to, for example, the ionosphere or an equator, and can also be utilized at different planets.

&lt;p&gt;We present simultaneous Juno and Hubble Space Telescope of Jupiter&#039;s far-ultraviolet auroras obtained as part of a programme of observations covering 3 years of Juno&#039;s Extended Mission.&amp;#160; We show that bright, expanded dusk-side southern main emission is associated with large-scale convection dynamics, dusk-side main emission arcs are associated with field-aligned currents, and equatorward diffuse emission and patches are associated with plasma injections in the middle magnetosphere occurring within intervals of enhanced plasma density, ongoing interchange motion and magnetospheric convection.&amp;#160; These results shed light on the relation between the main auroral emission and magnetosphere-ionosphere coupling currents, and radial force balance in the magnetosphere. We also report on unusually bright and expanded southern auroral emissions observed during PJ 43.&lt;/p&gt;

Abstract 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 compare Jupiter's dawn‐side main auroral emission intensity observed by the Hubble Space Telescope with the simultaneous magnitude of the dawn‐side magnetospheric equatorial radial current as observed by Juno during Orbits 3–7. We show that the peak auroral intensity and the square of the radial current per radian of azimuth are strongly correlated with R ≈ 0.9, and that the chance that the two phenomena are unrelated is negligible. We also fit empirical profiles of the total radial current flowing per radian of azimuth to the observed values and estimate the precipitating electron energy flux during each observation, and show this exhibits a similar correlation to the current. We find 1 mW m −2 of precipitating energy flux is associated with ∼5–11 kR of auroral emission, consistent with modeling studies. This is the first demonstration of a statistical relationship between the intensity of Jupiter's auroras and the strength of the magnetospheric currents, specifically the radial current, the j × B force of which accelerates plasma in the sense of planetary rotation. This result provides compelling evidence that the magnetosphere‐ionosphere coupling current system at Jupiter plays a key role in powering the planet's dawn side main auroral emission. We further show that there is no association between the auroral intensity and the rate of change of the magnetic energy density outside the current sheet.

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.

We analyze a database of 8920 measurements of thermal ion vector velocities obtained by the Cassini CAPS/IMS instrument in the near‐equatorial (±10° latitude) region of Saturn's magnetosphere in the radial range of 5.5–21.5 Saturn radii ( R S ) for the presence of modulations due to planetary period oscillations (PPOs). The data span 2004–2012, from Saturn late southern summer to early northern spring, and are analyzed using PPO phases derived previously from Cassini magnetic field data. For the near‐equinox and northern spring data 2008–2012 we show that both northern and southern PPO modulations are present in the azimuthal velocity component with similar amplitudes, typically ∼3 ± 1.5 km s −1 in the inner part of the system inside ∼10–12 R S , rising to typically ∼9 ± 3.5 km s −1 in the outer part of the system to ∼20 R S . Oscillation phases are overall consistent with expectations for PPO modulations driven from the corresponding polar ionospheres in both cases. For the late southern summer data 2004–2007, southern PPO modulations are also present with similar properties, but no clear northern modulations were detected, indicating weaker amplitudes in this case. These findings mirror similar properties in the amplitudes of magnetic field PPO oscillations observed in previous studies. Our results constitute the first direct detection of PPO‐related velocity modulations in Saturn's magnetospheric plasma. However, no clear PPO effects were found in either the radial or colatitudinal velocities, indicating weaker modulations, if any, in these components.

&lt;p&gt;We study magnetosphere-ionosphere coupling at Jupiter during the Juno prime mission, considering magnetic field observations from Juno&amp;#8217;s Perijoves 1-32.&amp;#160; We compare the azimuthal magnetic field and the associated determination of Jupiter&amp;#8217;s ionospheric meridional Pedersen current, with predictions from a model of magnetosphere-ionosphere coupling developed at the University of Leicester.&amp;#160; We find that the Leicester model closely predicts the magnitude of the residual azimuthal field component of the field across the middle and outer magnetosphere regions, and across the tail.&amp;#160; However, we highlight two areas of discrepancies between the model and the data. On field lines mapping to the outer magnetosphere region, the model predicts an increase in the magnitude of the Bphi component of the magnetic field with ionospheric colatitude, whilst we observe a decrease.&amp;#160; This could suggest that the community needs an updated ionospheric angular velocity flow model for the Juno era. Furthermore, we do not observe the predicted upward-directed current at the boundary between the outer magnetosphere and field lines mapping to the tail.&amp;#160; Currently the model includes a constant ionospheric conductivity.&amp;#160; We suggest that the model might be improved by considering a variable ionospheric conductivity.&amp;#160; Finally, we produce maps of meridional ionospheric currents and discuss the variation of ionospheric currents with local time.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

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.

The response times of the coupled magnetosphere-ionosphere-thermosphere system are, on average, greater than the autocorrelation timescales of solar wind forcing. This means that the system is rarely, if ever, in equilibrium. Departures from equilibrium are a key component of the Expanding-Contracting Polar Cap (ECPC) model of convection excitation in both the magnetosphere and ionosphere, driven by the Dungey reconnection cycle of opening and re-closing magnetospheric field lines. Averaging over sufficiently long timescales reduces data to the equivalent of steady-state conditions, which hides the physical mechanisms involved and allows us to map electric fields from interplanetary space to the ionosphere–but this is not valid, either physically or generally, because of magnetic induction effects. Only for transient phenomena on sufficiently short timescales do the mechanisms associated with non-equilibrium fully manifest themselves. Nevertheless, because of both ever-changing solar wind conditions and Earth’s dipole tilt, eccentricity and rotation, the magnetosphere is always tending towards a perpetually-evolving equilibrium configuration and there are important implications of transient events for understanding the general behavior of the coupled magnetosphere-ionosphere-thermosphere system and its response to solar wind forcing. We here discuss one example: as a consequence of the importance of departures from equilibrium inherent in the ECPC model, the solar wind dynamic pressure PSW influences the magnetosphere-ionosphere convection response to the generation of open field lines by reconnection in the dayside subsolar magnetopause. We here demonstrate this effect in a statistical survey of observations and show that it is as predicted by the ECPC model and that, through it, PSW has an influence on flux transport in the magnetosphere-ionosphere system.

Global magnetospheric effects resulting from the passage at Earth of large-scale structures have been well studied. The effects of common and short-term features, such as discontinuities and current sheets (CSs), have not been studied in the same depth. Herein we show how a seemingly unremarkable interplanetary feature can cause widespread effects in the magnetosheath-magnetosphere system. The feature was observed by Advanced Composition Explorer inside an interplanetary coronal mass ejection on 10 January 2004. It contained 1) a magnetic field dip bounded by directional discontinuities in field and flows, occurring together with 2) a density peak in what we identify as a bifurcated, non-reconnecting current sheet. Data from an array of spacecraft in key regions of the magnetosheath/magnetosphere (Geotail, Cluster, Polar, and Defense Meteorological Satellite Program) provide context for Wind’s observations of flapping of the distant (R ∼ −226 RE) magnetotail. In particular, just before the flapping began, Wind observed a hot and tenuous plasma in a magnetic field structure with enhanced field strength, with the By and Bz components rotating in a fast tailward flow burst. Closer inspection reveals a large flux rope (plasmoid) containing lobe plasma in a tail strongly deflected and twisted by interplanetary non-radial flows and magnetic field By. We try to identify the origin of this ‘precursor to flapping’ by looking at data from the various spacecraft. Working back towards the dayside, we discover a chain of effects which we argue were set in motion by the interplanetary CS and its interaction with the bow shock. These effects include 1) a compression and dilation of the magnetosphere, 2) a local deformation of the postnoon magnetopause, and, 3) at the poleward edge of the oval in an otherwise quiet polar cap flow, a strong (3 km/s) sunward flow burst in a double vortex-like structure flanked by two sets of field-aligned currents. Clearly, an intertwined set of phenomena was occurring at the same time. We learn that multi-spacecraft analysis can give us great insight into the magnetospheric response to transient changes in the solar wind.

Saturn’s magnetosphere is an efficient emitter of energetic neutral atoms (ENAs), created through charge exchange of energetic ions with the extended neutral cloud originating from the icy moon Enceladus. We present an analysis using the complete image set captured by Cassini’s Ion Neutral Camera to characterize Saturn’s average ENA morphology. Concentric tori are formed around the planet by oxygen and hydrogen ENAs, with intensity peaks between 7 and 10 RS radial distance, with a ∼1–2 RS dayside offset. Nightside intensity is brighter than the dayside, likely the result of enhancements following large‐scale plasma injections from the magnetotail, and influence of the noon‐midnight electric field. Global intensity is clearly modulated with the near‐planetary rotation period. This Cassini‐era profile of Saturn’s ENA emission advances our understanding of how volcanic moons can influence plasma dynamics in giant magnetospheres and is timely ahead of the planned JUICE mission, which carries the first dedicated ENA detector to Jupiter.

We consider the physical origin of reconnection‐related events in Saturn’s equatorial current sheet revealed by Cassini, specifically the dipolarization events discussed by Yao et al. ( https://doi.org/10.1029/2018JA025837 ) that recur close to the ∼10.7 h planetary period oscillation (PPO) period. We argue that Yao et al.’s preferred recurrence explanation in terms of complete rotations of dipolarized field/current structures around the planet at close to the PPO period is highly implausible. All‐mission observations of ion flows at relevant radial ranges ∼20–30 Saturn radii are essentially invariably sub‐corotational, including reconnection‐related hot ion injections, thus requiring implausible structure propagation at significant speeds through the plasma. We further show their assertion that the nightside events occur at PPO phases unfavorable for reconnection is incorrect. These events instead occur under PPO conditions associated with outward radial plasma displacement and/or a thinning plasma sheet with falling colatitudinal field, shown previously to be optimal for modulated reconnection bursts leading to dipolarizations and plasmoids. We instead suggest these events are related to well‐documented Vasyliunas cycle disturbances, when two reconnection episodes happen to be triggered at similar favorable PPO phases on successive PPO cycles. Yao et al.’s dayside events do occur at phases unfavorable for recurrent reconnection, but may then simply be an effect of periodic field/plasma modulations during the regular PPO cycle combined with subcorotating small‐scale structures often present in the plasma sheet. Both dayside and nightside events can thus be understood within existing knowledge of PPO modulations of the structure and dynamics of Saturn’s equatorial current sheet.

We present an axially asymmetric steady state model of Jupiter's magnetosphere‐ionosphere coupling with variable ionospheric conductivity dependent on the field‐aligned current density. We use Juno and Galileo data to construct a simple model of the equatorial magnetic field, and develop a method for solving the system of partial differential equations describing magnetosphere‐ionosphere coupling. Using this model, we study the behavior of the system with different radial mass transport rates of magnetospheric plasma and the effect of additional field‐aligned currents associated with Jupiter's nightside partial ring current. We compare the model magnetodisc current intensities with those determined directly from magnetic field measurements in various local time sectors, and find that the value of mass transport rate of 2,000 kg s−1 , larger than usually estimated, better accounts for the observed radial currents. We also find that the inclusion of field‐aligned currents associated with Jupiter's partial ring current helps to explain the local time variation of the radial currents, reducing the discrepancy between the model and the observations.