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Comparison of Cassini trajectories plotted in the ρ‐z plane, panels (a) and (b), and mapped in the northern and southern ionosphere, panels (c) and (d). (a) Comparison between a typical 2008 orbit (red) and Proximal orbit (black). Day of year markers are shown, and the circles are every 3 hr. The gray shaded region marks the typical field‐aligned current region. Model field lines are shown, the model is comprised of a 3‐degree planetary field and ring current. (b) Comparison between a typical F‐ring (blue) and Proximal (black) orbit in the same format as panel (a). (c, d) Ionospheric projections using the field model as above and in text are plotted with circles shown at every hour, with DOY:hh labels at first and last hour markers, and the orange section shows the observed upward current regions. Both are viewed from the north. Orbits have the same coloring as in panels (a, b). Statistical auroral boundaries (solid lines) as defined by the half power emission and peak emission (crossed centerline) determined from Cassini Ultraviolet Imaging Spectrograph observations are shown (Carbary, 2012).
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We examine the azimuthal magnetic field signatures associated with Saturn's northern hemisphere auroral field‐aligned currents observed in the dawn sector during Cassini's Proximal orbits (April 2017 and September 2017). We compare these currents with observations of the auroral currents from near noon taken during the F‐ring orbits prior to the Pr...
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We calculate the magnetospheric drag torques on Saturn's northern and southern polar thermospheres during late southern summer in 2008 and northern spring in 2012–2013 using previously derived profiles of ionospheric meridional coupling currents determined from high‐latitude Cassini magnetic field data. We show that the drag torques in the “winter”...
Citations
... The reconnection events appear in clusters at both Jupiter and Saturn (Smith et al. 2016;. Field-aligned particles accelerated by magnetic reconnection can form localized field-aligned currents (Hunt et al. 2020(Hunt et al. , 2022 and increase electron thermal anisotropy in the magnetodisk current sheet (Artemyev et al. 2023). ...
... In addition to removing mass from the magnetosphere, the reconnection process can trigger aurorae at the ionosphere by energizing particles and generating FACs. The Cassini spacecraft has observed high-energy protons in the dawn FACs, which are likely injected from tail reconnection (Hunt et al. 2020(Hunt et al. , 2022. The energetic ions can collide with the background cold atoms, producing energetic neutral atoms (ENA) in the magnetosphere. ...
Magnetic reconnection is crucial in understanding magnetospheric dynamics and aurorae processes at planets. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisk, where thin-current-sheet conditions are conducive to reconnection. At the Earth, the magnetopause and magnetotail current sheets are primarily controlled by the upstream solar wind. At Jupiter and Saturn, their fast rotation and internal mass sources lead to an additional current sheet that encircles the planet, forming a magnetodisk inside the magnetosphere. The reconnection processes in the magnetodisk current sheet are associated with centrifugal force-driven dynamics. The magnetodisk reconnection is not limited to the nightside but is discretely distributed at all local times inside the magnetosphere. The reconnection sites also rotate with the magnetosphere. These widely distributed small-scale reconnection sites can result in the global release of energy and mass from the magnetosphere.
... Fig. 5.5 for two examples), similar in intensity to the auroral region current (e.g.,Hunt et al., 2020).Hunt et al. (2019) then computed the current density of the associated FACs, ...
The last 22.5 orbits of the Cassini mission brought the spacecraft to less than 3000 km from Saturn's 1-bar surface. These close encounters offered an unprecedented view of Saturn's magnetic field, including contributions from the internal dynamo, the ionosphere, and the magnetosphere. In this chapter, we highlight the new picture of Saturn's magnetic field from the Cassini mission including the persistent yet time-varying low-latitude field-aligned currents, Alfv\'en waves planet-ward of the D-ring, extreme axisymmetry, and high-degree magnetic moments. We then discuss the implications and new questions raised for Saturn's innermost magnetosphere, equatorial ionosphere, and interior. We conclude this chapter with an outlook for the future exploration of Saturn and other giant planets.
... Field-aligned currents are fundamental in transferring momentum within a planetary magnetosphere by coupling the planet's ionosphere to its magnetosphere. For Saturn's magnetosphere these current systems have been extensively studied through Cassini magnetometer data Bunce et al., 2008;Cowley et al., 2008;Dougherty et al., 2004;Hunt et al., 2014Hunt et al., , 2015Hunt et al., , 2016Hunt et al., , 2018Hunt et al., , 2020Southwood & Kivelson, 2007;Talboys et al., 2009aTalboys et al., , 2009bTalboys et al., , 2011. Observations indicate that two principal large-scale field-aligned current systems are present. ...
... The proximal orbits of the Cassini mission's Grand Finale in 2017 provided the opportunity to examine the auroral field-aligned currents in the dawn sector, 06-08 hr local time (LT), of Saturn's magnetosphere. The initial observations were reported by Hunt et al. (2020), such that we only provide a brief summary here. To begin with they identified the main field-aligned current sheets in the auroral region, namely the main upward current associated with the aurora and a downward current equatorward of this. ...
... To begin with they identified the main field-aligned current sheets in the auroral region, namely the main upward current associated with the aurora and a downward current equatorward of this. On a statistical level Hunt et al. (2020) separated the non-PPO and PPO field-aligned current systems. By comparing the proximal orbits with the F-ring orbits it was shown that there was an enhanced non-PPO upward current within the dawn sector (proximal orbits) compared with the noon sector (F-ring orbits). ...
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.
... Data from sufficient such passes must also be obtained that the mean current profile associated with plasma subcorotation, of main interest here, can be separated from the comparably large rotating oscillatory currents of the PPO systems. In addition to the optimal orbit intervals outlined above, current profiles have also been derived spanning the dawn, dusk, and dayside sectors from further sequences of highly inclined orbits during the final phase of the Cassini mission spanning northern summer solstice in May 2017, specifically by Hunt et al. (2018) for the F ring orbits (Revs 251-270) between October 2016 and April 2017, and by Provan, Lamy, et al. (2019) and Hunt et al. (2020) for the proximal orbits (Revs 271-293) between April 2017 and end of mission in September 2017. In regions of latitudinal overlap these profiles generally show similar properties to those derived by Hunt et al. (2014Hunt et al. ( , 2015 and Bradley et al. (2018), apart from small latitude shifts associated with the solar wind-related daynight asymmetry about the pole. ...
... with and superposed on the primary subcorotation field-aligned currents in the auroral region (Bradley et al., 2018;Hunt et al., 2014Hunt et al., , 2015Hunt et al., , 2018Hunt et al., , 2020Provan, Lamy, et al., 2019). They are also of comparable magnitude. ...
We calculate the magnetospheric drag torques on Saturn's northern and southern polar thermospheres during late southern summer in 2008 and northern spring in 2012–2013 using previously derived profiles of ionospheric meridional coupling currents determined from high‐latitude Cassini magnetic field data. We show that the drag torques in the “winter” and “summer” auroral regions are near equal at ~2.3 × 10¹⁶ N m, contrary to the recent discussion of Brooks et al. (2019, https://doi.org/10.1002/2019JA026870) who suggest that significant seasonal differences should occur in these regions. Instead, seasonally dependent torques occur in the adjacent polar open field regions, where the “winter” and “summer” torques are ~0.3 × 10¹⁶ and ~1.8 × 10¹⁶ N m, respectively. We derive a simple rotating disc model of the polar thermosphere and estimate the speed of the poleward flow from midlatitudes required to balance these torques in steady state, finding values of tens of m s⁻¹ consistent with previous numerical modeling. Comparison of the calculated torques with concurrent periods of the northern and southern planetary period oscillations (PPOs) does not suggest a direct connection between these quantities as proposed by Brooks et al., 2019, showing at the least that significant additional factors must be involved. We further note some issues with their scenario for dual modulation of radio emissions, previous observations having shown that the principal oscillatory PPO field‐aligned currents that modulate the emissions rotate in the auroral region with periods ~10.7 ± 0.1 hr, propagating through the more slowly rotating ~15–20 hr period outer magnetospheric plasma, with implications for the proposed “atmospheric flywheel” picture.
Non-thermal radio emissions from Saturn, known as Saturn Kilometric Radiation (SKR), are analyzed for the Faraday rotation effect detected in Cassini RPWS High Frequency Receiver (HFR) observations. This phenomenon, which mainly affects the lower-frequency part of SKR below 200 kHz, is characterized by a rotation of the semi-major axis of the SKR polarization ellipse as a function of frequency during wave propagation through a birefringent plasma medium. Faraday rotation is found in 4.1% of all HFR data recorded by Cassini above 20 degrees northern and southern magnetic latitude, from mid-2004 to late 2017. A statistical visibility analysis shows that elliptically polarized SKR from the dawn source regions, when beamed towards high latitudes into the noon and afternoon local time sectors, is most likely to experience Faraday rotation along the ray path. The necessary conditions for Faraday rotation are discussed in terms of birefringent media and sharp plasma density gradients, where SKR (mostly R-X mode) gets split into the two circularly polarized modes R-X and L-O. By means of a case study we also demonstrate how Faraday rotation provides an estimate for the average plasma density along the ray path.
