In the Earth's magnetosphere wave-particle interaction is a major ion energization process, playing an important role for the atmospheric escape. A common type of ion heating is associated with low-frequency broadband electric wave fields. For such waves the energy is not concentrated to a certain narrow frequency range and exhibits no peaks or dips in a power spectrum. If there are enough fluctuations close to the ion gyrofrequency the electric field may still come in resonance with gyrating ions and heat them perpendicular to the background magnetic field. We perform a proof-of-concept study to investigate if this heating mechanism may contibute significantly to the energization of planetary ions also in the induced magnetosphere of Venus. We assume Alfv & eacute;nic fluctuations and estimate the electric field spectral density based on magnetic field observations. We find typical estimated electric spectral densities of a few (mV/m)2 ${(\text{mV/m})}<^>{2}$/Hz close to Venus. This corresponds to a heating rate of a few eV/s. We consider an available interaction time of similar to ${\sim} $ 300 s and conclude that this mechanism could increase the energy of an oxygen ion by about a keV. Observed thermal energies are in the range 100-1,000 eV and thus, resonant wave heating may also be important at Venus.

Observations of energetic neutral atoms (ENAs) are a useful tool for analyzing ion and neutral abundances in planetary magnetospheres. Saturn's magnetosphere is dominated by high densities of water group neutrals which originate from the icy moon Enceladus and are confined close to the equatorial plane due to the planet's rapid rotation rate. Hot plasma populations are mainly created by magnetotail reconnection events and driven inward with the subsequent magnetic field dipolarization to form a so-called "injection". As this hot plasma interacts with the ambient neutral population, charge exchange creates ENAs whose motion is not governed by the magnetic field anymore, such that they can be observed remotely allowing us to image Saturn's ring current on a global scale.

Magnetic Dipolarization, a hallmark indicator of the formation of field‐aligned currents, plays a crucial role in the energy dissipative processes that occur within planetary magnetospheres. Saturn's magnetic dipolarization was found to recur after one planetary rotation, suggesting a potential correlation with the corotating current systems. The observation of enhanced Energetic Neutral Atoms (ENA) within Saturn's inner magnetosphere serves as a diagnostic tool, revealing the existence of a rotating, dynamic Partial Ring Current (PRC) system. By utilizing multiple datasets from Cassini, this study advances the understanding of the interrelationship between ENA emission and magnetic dipolarization. Our results suggest the possibility of a coupling between the PRC system revealed by the ENA emission and magnetic dipolarization. Furthermore, a temporal correlation was found between the ENA emissions and two distinct radio emissions. This study discusses potential causal relationships among these phenomena and first time proposes a global unified physical picture.

The proton population in Venus' plasma environment is characterized during periods of solar minimum and maximum using data from a particle mass‐energy spectrometer. Such characterizations at different levels of solar activity provides physical insight into solar‐cycle‐dependent plasma phenomena around the planet, for example mirror modes in the magnetosheath. Statistical distributions of proton bulk speeds and temperatures are generated using a previously developed method which applies Maxwellian fits to measurements of the protons' velocity distribution function. Spatial maps and probability‐density histograms comparing the proton parameters between the two time periods are presented. The temperatures perpendicular ( T ⊥ ) and parallel ( T ∥ ) to the background magnetic field are found to be 20%–35% lower during solar maximum. Though the overall distributions of the temperature ratio T ⊥ / T ∥ do not change, the regions with higher anisotropy ( T ⊥ / T ∥ > 1) are found farther downstream from the bow shock during solar maximum than minimum. This is consistent with the previously observed growth of mirror modes during solar maximum and their decay during minimum.

Venus’ lack of an intrinsic magnetic field allows the solar wind to closely interact with its atmosphere [1], making it a prime target for investigating how unmagnetized atmospheric bodies in our Solar System [2] or elsewhere [3] interact with magnetized plasma flows. This close interaction means that solar-activity correlations exhibited by the solar wind and other heliospheric parameters [4, 5] cause solar-cycle variations in Venus’ plasma environment and plasma phenomena. We investigate these variations by characterizing the proton population around Venus during periods of solar minimum (2006–2009) and maximum (2010–2014). We use data from the Ion Mass Analyser (IMA) instrument, a particle mass-energy spectrometer which was onboard the Venus Express (VEX) mission. We apply a previously developed methodology which fits Maxwellian models to measurements of the protons’ velocity distribution functions [6] to produce statistical distributions of bulk speeds and temperatures in various regions of Venus’ plasma environment. We also present spatial maps and probability-density histograms comparing the proton parameters between the two time periods. We find that the temperatures perpendicular (T⊥) and parallel (T) to the background magnetic field are 20–35% lower in the magnetosheath during solar maximum. This suggests that the heating of particles as they cross the bow shock varies between the two time periods. We also find that the regions in the magnetosheath with highest temperature ratio T⊥/T are farther downstream from the bow shock during solar maximum than minimum. This is consistent with previous observations of how mirror-mode structures presumably generated at the bow shock strictly decay as they are convected into the magnetosheath during solar minimum, whereas during solar maximum they first grow and then decay [7]. We also present ongoing work to further characterize the plasma environment as a function of upstream solar-wind parameters (such as Mach number or cone angle) and bow shock geometry. We discuss preliminary results concerning energy conversion processes at Venus’ bow shock. REFERENCES [1] Y. Futaana, G. Stenberg Wieser et al., “Solar Wind Interaction and Impact on the Venus Atmosphere,” Space Science Reviews, vol. 212, no. 3-4, 2017. [2] C. Bertucci, F. Duru et al., The induced magnetospheres of mars, venus, and titan, 2011, vol. 162, no. 1-4. [3] C. Dong, M. Jin et al., “Atmospheric escape from the TRAPPIST-1 planets and implications for habitability,” Proceedings of the National Academy of Sciences of the United States of America, vol. 115, no. 2, 2017. [4] C. T. Russell, E. Chou et al., “Solar and interplanetary control of the location of the Venus bow shock,” Journal of Geophysical Research, vol. 93, no. A6, 1988. [5] P. R. Gazis, “Solar cycle variation in the heliosphere,” Reviews of Geophysics, vol. 34, no. 3, 1996. [6] A. Bader, G. Stenberg Wieser et al., “Proton Temperature Anisotropies in the Plasma Environment of Venus,” Journal of Geophysical Research: Space Physics, vol. 124, no. 5, 2019. [7] M. Volwerk, D. Schmid et al., “Mirror mode waves in Venus’s magnetosheath: Solar minimum vs. solar maximum,” Annales Geophysicae, vol. 34, no. 11, 2016.

Saturn’s kilometric radio (SKR) and energetic neutral atom (ENA) emissions are important remote diagnostics of the planet’s magnetospheric dynamics, intensifying during periods of global-scale plasma injection, and displaying characteristic planetary periodicity (e.g. Ye et al., 2011, Kinrade, Bader et al., 2021). Here we focus on the narrowband emissions between 5-40 kHz, thought to originate near density gradients at the edges of the plasma torus (e.g. Gurnett et al., 1981, Ye et al., 2009), and test the hypothesis that narrowband SKR production might be enhanced by inward-moving plasma following global injection events. Global scale ENA signatures have been associated with both 5 and 20 kHz nSKR emissions, particularly at dusk-evening local times (e.g. Wing et al., 2020, Wu et al., 2021) where plasma injections are expected to have moved inwards through the magnetosphere, possibly triggering interchange instabilities (e.g. Mitchell et al., 2015, Azari et al., 2018, Kinrade et al., 2020). Figure A: A calibrated and re-binned Cassini INCA image of Saturn’s equatorial ENA emission (24-55 keV Hydrogen, X-Y plane). This dataset and the open-source repository location are detailed in Bader & Kinrade, et al. (2020). We use a new set of calibrated equatorial ENA projections - captured by the Cassini INCA - to test the relationship between Saturn’s ENA and narrowband SKR emissions. The narrowband SKR emission intensity peak often coincides with the rotation of ENA enhancement through the dusk local time sector, complementing the findings of Wing et al. (2020). We test for radial distance dependence by constraining ENA keograms over a set of distances and local time sectors covering the edges of the plasma torus, and quantify the relative timing of nSKR enhancements through correlation of the ENA intensity with flux density in the 5 and 20-40 kHz emission bands. We also observe periods of strong 5 kHz SKR emission when the ENA emission is absent, even during times of favourable viewing, indicating that this relationship is complex (e.g. Wu et al., 2022). These results contribute towards our developing picture of how global plasma injection events can influence Saturn’s inner magnetosphere, linking together two valuable sources of remotely-sensed global emissions, the ENAs and SKR. Figure B: A re-working of the Wang et al. (2010) narrowband SKR example from 2007 (left-hand SKR polarisation shown in top panel), plus a keogram of the median ENA intensity between 1-20 RS (bottom panel). Some viewing artefacts remain here in this pre-published version of the ENA keogram (DOY 073 and 078), but the persistence of the rotating ENA enhancement over several days is clear. INCA projection / SKR viewing geometry are best on DOY 076 when Cassini was high above the north hemisphere ( > 50º latitude) at a range of ~ 30 RS. The bursts of narrowband SKR coincide with the ENA enhancement rotating through the dusk local time sector. This work is timely given the expected arrival of the JUICE mission at Jupiter in 2031, which carries an advanced ENA camera. ENA emissions have already been detected from Jupiter and the Io and Europa plasma torii by instruments onboard Cassini and JUNO (e.g. Mauk et al., 2003; 2020), and the arrival of JUICE will provide an opportunity to replicate this analysis, comparing the much-different Jovian ENA and associated radio emissions with those of Saturn’s neutral-dominated magnetosphere. References * Azari et al. (2018), Interchange Injections at Saturn: Statistical Survey of Energetic H+Sudden Flux Intensifications, JGR Space Physics, https://doi.org/10.1029/ 2018JA025391. * Bader, Kinrade et al. (2020), A complete dataset of equatorial projections of Saturn's energetic neutral atom emissions observed by Cassini-INCA, JGR Space Physics, https://doi.org/10.1029/2020JA028908. * Gurnett et al. (1981), Narrowband electromagnetic emissions from Saturn's magnetosphere, Nature, https://www.nature.com/articles/292733a0. * Kinrade et al. (2020), Tracking Counterpart Signatures in Saturn's Auroras and ENA Imagery During Large‐Scale Plasma Injection Events, JGR Space Physics, https://doi.org/10.1029/2019JA027542. * Kinrade, Bader et al. (2021), The Statistical Morphology of Saturn’s Equatorial Energetic Neutral Atom Emission, Geophysical Research Letters, https://doi.org/10.1029/2020GL091595. * Mauk et al. (2003), Energetic neutral atoms from a trans-Europa gas torus at Jupiter, Nature, https://www.nature.com/articles/nature01431. * Mauk et al. (2020), Juno Energetic Neutral Atom (ENA) Remote Measurements of Magnetospheric Injection Dynamics in Jupiter's Io Torus Regions, JGR Space Physics, https://doi.org/10.1029/2020JA027964. * Mitchell et al. (2015), ‘Injection, Interchange, and Reconnection: Energetic Particle Observations in Saturn’s Magnetosphere’ in Magnetotails in the Solar System, https://doi.org/10.1002/9781118842324.ch19. * Wang et al. (2010), Cassini observations of narrowband radio emissions in Saturn's magnetosphere, JGR Space Physics, https://doi.org/10.1029/2009JA014847. * Wing et al. (2020), Periodic Narrowband Radio Wave Emissions and Inward Plasma Transport at Saturn's Magnetosphere, The Astronomical Journal, https://doi.org/10.3847/1538-3881/ab818d. * Wu et al. (2021), Statistical Study on Spatial Distribution and Polarization of Saturn Narrowband Emissions, The Astrophysical Journal, https://doi.org/10.3847/1538-4357/ac0af1. * Wu et al., (2022), Reflection and Refraction of the L-O Mode 5 kHz Saturn Narrowband Emission by the Magnetosheath, Geophysical Research Letters, https://doi.org/10.1029/2021GL096990. * Ye et al., (2009), Source locations of narrowband radio emissions detected at Saturn, JGR Space Physics, https://doi.org/10.1029/2008JA013855.

We examined Hubble Space Telescope images of Saturn’s northern UV aurora in 2013–2017, identified 29 short-lived flashes, and examined simultaneous magnetometer data collected by the Cassini orbiter. When observation cadence permitted, a flash lifetime of 4–17 min (subject to exposure time-related uncertainties), and a 40–70 min recurrence period were found. An occurrence map shows a strong preference in both local time (14–19 LT) and latitude (75–85°). These transient flashes are identified in both the presence and absence of Saturn’s main auroral oval, indicating the lack of dependence on the main emission power. The concurrent magnetic field pulsations generally form a sawtooth shape, and the local field strength experiences a change of 0.2 to 2.0 nT (depending on the distance of Cassini). The quasiperiodic pulsation events were all detected when the spacecraft was in the southern hemisphere with conjugate flashes in northern aurora, suggesting these events occur on closed field lines, and typically showing a sudden transition to a less lagging, more southward magnetic field configuration. We also found the ionospheric footprint of the spacecraft must be close to the region of flashes for magnetic field pulsations to be detected, implying a localised rather than global driving process.

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.

Observations of energetic neutral atoms (ENAs) are a useful tool for analyzing ion and neutral abundances in planetary magnetospheres. They are created when hot plasma, originating for example from magnetic reconnection sites, charge‐exchanges with the ambient neutral population surrounding the planet. The motion of ENAs is not governed by the magnetic field, allowing remote imaging. During the Cassini mission, the Ion and Neutral CAmera (INCA) of the Magnetosphere Imaging Instrument (MIMI) collected vast amounts of hydrogen and oxygen ENA observations of Saturn's magnetosphere from a variety of different viewing geometries. To enable investigations of the morphology and dynamics of Saturn's ring current, it is useful to re‐bin and re‐project the camera‐like views from the spacecraft‐based perspective into a common reference frame. We developed an algorithm projecting INCA's ENA observations into a regular grid in Saturn's equatorial plane. With most neutrals and ions being confined into an equatorial rotating disc, this projection is quite accurate in both spatial location and preservation of ENA intensity, provided the spacecraft is located at large enough elevations. Such projections were performed for all INCA ENA data from the Cassini Saturn tour; the data are available for download together with a Python routine flagging contaminated data and returning detailed spacecraft geometry information. The resulting data set is a good foundation for investigating for example the statistical properties of Saturn's ring current and its complicated dynamics in relation to other remote and in situ observations of, for example, auroral emissions and magnetotail reconnection events.

During its more than 13 years in orbit, the Cassini spacecraft detected a large number of plasma and energetic charged particle injections in Saturn's inner magnetosphere. In the corotating frame of the planet, the plasma contained within an injection moves radially inward with the component particles gaining energy. The highest energy particles in the injection experience stronger gradient‐curvature drifts in the longitudinal direction and can drift out of the main body of the injection. We have used these drift‐out effects to estimate the inflow speed of 20 injections by surveying cases from the available plasma data. We find that the average inflow speed from our sample is 15 km/s, and the values are well distributed between 0 and 50 km/s, with a few higher estimates. We have also computed the radial travel distance of interchange events and found that these are typically one to two Saturn radii. We discuss the implications of these quantifications on our understanding of transport.

Near the end of its mission, NASA's Cassini spacecraft performed several low-altitude passes across Saturn's auroral region. We present ultraviolet auroral imagery and various coincident particle and field measurements of two such passes, providing important information about the structure and dynamics of Saturn's auroral acceleration region. In upward field-aligned current regions, upward proton beams are observed to reach energies of several tens of keV; the associated precipitating electron populations are found to have mean energies of about 10 keV. With no significant wave activity being apparent, these findings indicate strong parallel potentials responsible for auroral acceleration, about 100 times stronger than at Earth. This is further supported by observations of proton conics in downward field-aligned current regions above the acceleration region, which feature a lower energy cutoff above similar to 50 keV-indicating energetic proton populations trapped by strong parallel potentials while being transversely energized until they can overcome the trapping potential, likely through wave-particle interactions. A spacecraft pass through a downward current region at an altitude near the acceleration region reveals plasma wave features, which may be driving the transverse proton acceleration generating the conics. Overall, the signatures observed resemble those related to the terrestrial and Jovian aurorae, the particle energies and potentials at Saturn appearing to be significantly higher than at Earth and comparable to those at Jupiter. Plain Language Summary NASA's Cassini spacecraft orbited closer to Saturn than ever before during the last stage of its mission, the "Grand Finale". This allowed the onboard instruments to measure charged particles and plasma waves directly above the auroral region while simultaneously providing high-resolution imagery of the ultraviolet aurorae. Based on observations of highly energetic ions streaming away from the planet in regions of low plasma wave activity, we infer the existence of strong electric fields which act to accelerate electrons down into the atmosphere, driving the bright auroral emissions. Our estimates of the average energy of the precipitating electrons support this finding. Charged ions sometimes seem to be energized by plasma waves above the aurorae before they can escape, but the exact process in which this happens is not fully understood. Most signatures presented here resemble those observed in relation to Earth's aurorae, suggesting that the mechanisms acting at both planets are quite similar although Saturn's acceleration mechanism is significantly stronger.

We present an investigation into the currents within the Jovian magnetodisc using all available spacecraft magnetometer data up until 28 July 2018. Using automated data analysis processes as well as the most recent intrinsic field and current disk geometry models, a full local time coverage of the magnetodisc currents using 7,382 lobe traversals over 39 years is constructed. Our study demonstrates clear local time asymmetries in both the radial and azimuthal height‐integrated current densities throughout the current disk. Asymmetries persist within 30 R J where most models assume axisymmetry. Inward radial currents are found in the previously unmapped dusk and noon sectors. Azimuthal currents are found to be weaker in the dayside magnetosphere than the nightside, in agreement with global magnetohydrodynamic simulations. The divergence of the azimuthal and radial currents indicates that downward field‐aligned currents exist within the outer dayside magnetosphere. The presence of azimuthal currents is shown to highly influence the location of the field‐aligned currents, which emphasizes the importance of the azimuthal currents in future magnetosphere‐ionosphere coupling models. Integrating the divergence of the height‐integrated current densities, we find that 1.87 MA R J−2 of return current density required for system closure is absent.

Cassini's mission exploring the Saturn system ended with the Grand Finale, a series of orbits bringing the spacecraft closer to the planet than ever before and providing unique opportunities for observations of the ultraviolet aurorae. This study presents a selection of high‐resolution imagery showing the aurorae's small‐scale structure in unprecedented detail. We find the main arc to vary between a smooth and a rippled structure, likely indicating quiet and disturbed magnetospheric conditions, respectively. It is usually accompanied by a diffuse and dim outer emission on its equatorward side which appears to be driven by wave scattering of hot electrons from the inner ring current into the loss cone. The duskside is characterized by highly dynamic structures which may be signatures of radial plasma injections. This image set will be the only high‐resolution data for the foreseeable future and hence forms an important basis for future auroral research on Saturn.

Maria-Theresia Walach, Greg Hunt, Alexandra Fogg and Alexander Bader report on the rescheduled Autumn MIST Meeting for 2019, organized by the MIST Council, which took place in January 2020.

Saturn's morningside auroras consist mainly of rotating, transient emission patches, following periodic reconnection in the magnetotail. Simultaneous responses in global energetic neutral atom (ENA) emissions have been observed at similar local times, suggesting a link between the auroras and large-scale injections of hot ions in the outer magnetosphere. In this study, we use Cassini's remote sensing instruments to observe multiple plasma injection signatures within coincident auroral and ENA imagery, captured during 9 April 2014. Kilometric radio emissions also indicate clear injection activity. We track the motion of rotating signatures in the auroras and ENAs to test their local time relationship. Two successive auroral signatures-separated by similar to 4 hr UT-form postmidnight before rotating to the dayside while moving equatorward. The first has a clear ENA counterpart, maintaining a similar local time mapping throughout similar to 9 hr observation. Mapping of the ionospheric equatorward motion post-dawn indicates a factor similar to 5 reduction of the magnetospheric source region's radial speed at a distance of similar to 14-20 R-S, possibly a plasma or magnetic boundary. The second auroral signature has no clear ENA counterpart; viewing geometry was relatively unchanged, so the ENAs were likely too weak to detect by this time. A third, older injection signature, seen in both auroral and ENA imagery on the nightside, may have been sustained by field-aligned currents linked with the southern planetary period oscillation system, or the re-energization of ENAs around midnight local times. The ENA injection signatures form near magnetic longitudes associated with magnetotail thinning.

We investigate the average configuration and structure of Saturn's magnetosphere in the nightside equatorial and high‐latitude regions. Electron data from the Cassini Plasma Spectrometer's Electron Spectrometer (CAPS‐ELS) is processed to produce a signal‐to‐noise ratio for the entire CAPS‐ELS time of operation at Saturn's magnetosphere. We investigate where the signal‐to‐noise ratio falls below 1 to identify regions in the magnetosphere where there is a significant depletion in the electron content. In the nightside equatorial region, we use this to find that the most planetward reconnection x‐line location is at 20–25 RS downtail from the planet in the midnight to dawn sector. We also find an equatorial dawn‐dusk asymmetry at a radial distance of >20 RS, which may indicate the presence of plasma‐depleted flux tubes returning to the dayside after reconnection in the tail. Furthermore, we find that the high‐latitude magnetosphere is predominantly in a state of constant plasma depletion and located on open field lines. We map the region of high‐latitude magnetosphere that is depleted of electrons to the polar cap to estimate the size and open flux content within the polar caps. The mean open flux content for the northern and southern polar caps are found to be 25 ± 5 and 32 ± 5 GWb, respectively. The average location of the open‐closed field boundary is found at invariant colatitudes of 12.7 ± 0.6° and 14.5 ± 0.6°. The northern boundary is modulated by planetary period oscillations more than the southern boundary.

Saturn's aurora represents the ionospheric response to plasma processes occurring in the planet's entire magnetosphere. Short-lived similar to 1-hr quasiperiodic high-energy electron injections, frequently observed in in situ particle and radio measurements, should therefore entail an associated flashing auroral signature. This study uses high time-resolution ultraviolet (UV) auroral imagery from the Cassini spacecraft to demonstrate the continuous occurrence of such flashes in Saturn's northern hemisphere and investigate their properties. We find that their recurrence periods of order 1 hr and preferential occurrence near dusk match well with previous observations of electron injections and related auroral hiss features. A large spread in UV auroral emission power, reaching more than 50% of the total auroral power, is observed independent of the flash locations. Based on an event observed both by the Hubble Space Telescope and the Cassini spacecraft, we propose that these auroral flashes are not associated with low-frequency waves and instead directly caused by recurrent small-scale magnetodisc reconnection on closed field lines. We suggest that such reconnection processes accelerate plasma planetward of the reconnection site toward the ionosphere inducing transient auroral spots while the magnetic field rapidly changes from a bent-back to a more dipolar configuration. This manifests as a sawtooth-shaped discontinuity observed in magnetic field data and indicates a release of magnetospheric energy through plasmoid release.

It is well known that Saturn's magnetospheric dynamics are greatly influenced by the so‐called planetary period oscillations (PPOs). Based on Cassini Ultraviolet Imaging Spectrograph (UVIS) imagery, it has been shown previously that the UV auroral intensity is clearly modulated in phase with rotating field‐aligned current (FAC) systems associated with the PPOs. Here we expand upon this investigation by using the same data set to examine the PPO‐induced spatial modulation of the main auroral oval. We present a robust algorithm used for determining the location of the main emission in Cassini‐UVIS images. The location markers obtained are then used to calculate the statistical location of the auroral oval and its periodic displacement due to the PPO FACs and the related ionospheric flows. We find that the largest equatorward displacement of the main arc lags behind the PPO‐dependent statistical brightening of the UV aurora by roughly 45–90° in both hemispheres and is not colocated with it as the present model based on magnetometer observations suggests. We furthermore find the center of the auroral oval by fitting circles to the main emission and analyze its elliptic motion as the entire oval is displaced in phase with the PPO phases. It is demonstrated that the periodic displacements of both the auroral oval arc and its center are larger when the two PPO systems rotate in relative antiphase than when they are in phase, clearly indicating that interhemispheric PPO FAC closure modulates not only the intensity but also the location of the main UV auroral emission.

Saturn's main aurorae are thought to be generated by plasma flow shears associated with a gradient in angular plasma velocity in the outer magnetosphere. Dungey cycle convection across the polar cap, in combination with rotational flow, may maximize (minimize) this flow shear at dawn (dusk) under strong solar wind driving. Using imagery from Cassini's Ultraviolet Imaging Spectrograph, we surprisingly find no related asymmetry in auroral power but demonstrate that the previously observed "dawn arc" is a signature of quasiperiodic auroral plasma injections commencing near dawn, which seem to be transient signatures of magnetotail reconnection and not part of the static main aurorae. We conclude that direct Dungey cycle driving in Saturn's magnetosphere is small compared to internal driving under usual conditions. Saturn's large-scale auroral dynamics hence seem predominantly controlled by internal plasma loading, with plasma release in the magnetotail being triggered both internally through planetary period oscillation effects and externally through solar wind compressions.