Publications
403 results found
Sorba AM, Achilleos NA, Sergis N, et al., 2019, Local Time Variation in the Large-Scale Structure of Saturn's Magnetosphere, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 7425-7441, ISSN: 2169-9380
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- Citations: 7
Hunt G, Cowley S, Provan G, et al., 2019, Currents associated with Saturn's intra-D ring azimuthal field perturbations, Journal of Geophysical Research: Space Physics, Vol: 124, Pages: 5675-5691, ISSN: 2169-9380
During the final 22 full revolutions of the Cassini mission in 2017, the spacecraft passed at periapsis near the noon meridian through the gap between the inner edge of Saturn’s D ring and the denser layers of the planet’s atmosphere, revealing the presence of an unanticipated low-latitude current system via the associated azimuthal perturbation field peaking typically at ~10-30 nT. Assuming approximate axisymmetry, here we use the field data to calculate the associated horizontal meridional currents flowing in the ionosphere at the feet of the field lines traversed, together with the exterior field-aligned currents required by current continuity. We show that the ionospheric currents are typically~0.5–1.5 MA per radian of azimuth, similar to auroral region currents, while the field-aligned current densities above the ionosphere are typically ~5-10 nA m-2 , more than an order less than auroral values. The principal factor involved in this difference is the ionospheric areas into which the currents map. While around a third of passes exhibit unidirectional currents flowing northward in the ionosphere closing southward along exterior field lines, many passes also display layers of reversed northward field-aligned current of comparable or larger magnitude in the region interior to the D ring, which may reverse sign again on the innermost field lines traversed. Overall, however, the currents generally show a high degree of north-south conjugacy indicative of an interhemispheric system, certainly on the larger overall spatial scales involved, if less so for the smaller-scale structures, possibly due to rapid temporal or local time variations.
Sulaiman AH, Farrell WM, Ye S-Y, et al., 2019, A Persistent, Large-Scale, and Ordered Electrodynamic Connection Between Saturn and Its Main Rings, GEOPHYSICAL RESEARCH LETTERS, Vol: 46, Pages: 7166-7172, ISSN: 0094-8276
Provan G, Cowley SWH, Bunce EJ, et al., 2019, Variability of Intra-D Ring Azimuthal Magnetic Field Profiles Observed on Cassini's Proximal Periapsis Passes, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 124, Pages: 379-404, ISSN: 2169-9380
Brown P, Auster U, Bergman JES, et al., 2019, MEETING THE MAGNETIC EMC CHALLENGES FOR THE IN-SITU FIELD MEASUREMENTS ON THE JUICE MISSION, ESA Workshop on Aerospace EMC (Aerospace EMC), Publisher: IEEE
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- Citations: 5
Dougherty M, Christensen U, Cao H, et al., 2019, Saturn's Magnetic Field and Dynamo, Saturn in the 21st Century, Editors: Baines, Flasar, Krupp, Stallard, Publisher: Cambridge University Press, Pages: 69-96, ISBN: 978-1-107-10677-2
Guo RL, Yao ZH, Sergis N, et al., 2018, Reconnection Acceleration in Saturn's Dayside Magnetodisk: A Multicase Study with Cassini, ASTROPHYSICAL JOURNAL LETTERS, Vol: 868, ISSN: 2041-8205
Krupp N, Roussos E, Kollmann P, et al., 2018, Energetic Neutral and Charged Particle Measurements in the Inner Saturnian Magnetosphere During the Grand Finale Orbits of Cassini 2016/2017, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 10847-10854, ISSN: 0094-8276
Dougherty M, Buratti BJ, Seidelmann PK, et al., 2018, Enceladus as an active world: History and discovery. In Enceladus and the Icy, Enceladus and the Icy Moons of Saturn, Editors: Schenk, Clark, Howett, Verbiscer, Waite, Publisher: University of Arizona Press, Pages: 3-16, ISBN: 9780816537075
Dougherty M. K., Buratti B. J., Seidelmann P. K., and Spencer J. R. (2018) Enceladus as an active world: History and discovery. In Enceladus and the Icy Moons of Saturn (P. M. Schenk et al., eds.), pp. 3–16. Univ. of Arizona, Tucson, DOI: ...
Sorba AM, Achilleos NA, Guio P, et al., 2018, The Periodic Flapping and Breathing of Saturn's Magnetodisk During Equinox, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 8292-8316, ISSN: 2169-9380
Yao ZH, Radioti A, Grodent D, et al., 2018, Recurrent Magnetic Dipolarization at Saturn: Revealed by Cassini, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 123, Pages: 8502-8517, ISSN: 2169-9380
Khurana KK, Dougherty MK, Provan G, et al., 2018, Discovery of atmospheric-wind-driven electric currents in Saturn's magnetosphere in the gap between Saturn and its rings, Geophysical Research Letters, Vol: 45, Pages: 10068-10074, ISSN: 0094-8276
Magnetic field observations obtained by the Cassini spacecraft as it traversed regions inside of Saturn's D ring packed a genuine surprise. The azimuthal component of the magnetic field recorded a consistent positive perturbation with a strength of 15–25 nT near closest approach. The closest approaches were near the equatorial plane of Saturn and were distributed narrowly around local noon and brought the spacecraft to within 2,550 km of Saturn's cloud tops. Modeling of this perturbation shows that it is not of internal origin but is produced by external currents that couple the low‐latitude northern ionosphere to the low‐latitude southern ionosphere. The azimuthal perturbations diminish at higher latitudes on field lines that connect to Saturn's icy rings. The sense of the current system suggests that the southern feet of the field lines in the ionosphere leads their northern counterparts. We show that the observed field perturbations are consistent with a field‐aligned current whose strength is ~1 MA/radian, that is, comparable in strength to the planetary‐period‐oscillation‐related current systems observed in the auroral zone. We show that the Lorentz force in the ionosphere extracts momentum from the faster moving low‐latitude zonal belt and delivers it to the northern ionosphere. We further show that the electric current is generated when the two ends of a field line are embedded in zonal flows with differing wind speeds in the low‐latitude thermosphere. The wind‐generated currents dissipate 2 × 1011W of thermal power, similar to the input from the solar extreme ultraviolet flux in this region.
Dougherty MK, Cao H, Khurana KK, et al., 2018, Erratum for the Research Article “Saturn’s magnetic field revealed by the Cassini Grand Finale” by M. K. Dougherty, H. Cao, K. K. Khurana, G. J. Hunt, G. Provan, S. Kellock, M. E. Burton, T. A. Burk, E. J. Bunce, S. W. H. Cowley, M. G. Kivelson, C. T. Russell, D. J. Southwood, Science, Vol: 362, ISSN: 0036-8075
Dougherty MK, Cao H, Khurana KK, et al., 2018, Saturn's magnetic field revealed by the Cassini Grand Finale, Science, Vol: 362, Pages: 1-9, ISSN: 0036-8075
INTRODUCTIONStarting on 26 April 2017, the Grand Finale phase of the Cassini mission took the spacecraft through the gap between Saturn’s atmosphere and the inner edge of its innermost ring (the D-ring) 22 times, ending with a final plunge into the atmosphere on 15 September 2017. This phase offered an opportunity to investigate Saturn’s internal magnetic field and the electromagnetic environment between the planet and its rings. The internal magnetic field is a diagnostic of interior structure, dynamics, and evolution of the host planet. Rotating convective motion in the highly electrically conducting layer of the planet is thought to maintain the magnetic field through the magnetohydrodynamic (MHD) dynamo process. Saturn’s internal magnetic field is puzzling because of its high symmetry relative to the spin axis, known since the Pioneer 11 flyby. This symmetry prevents an accurate determination of the rotation rate of Saturn’s deep interior and challenges our understanding of the MHD dynamo process because Cowling’s theorem precludes a perfectly axisymmetric magnetic field being maintained through an active dynamo.RATIONALEThe Cassini fluxgate magnetometer was capable of measuring the magnetic field with a time resolution of 32 vectors per s and up to 44,000 nT, which is about twice the peak field strength encountered during the Grand Finale orbits. The combination of star cameras and gyroscopes onboard Cassini provided the attitude determination required to infer the vector components of the magnetic field. External fields from currents in the magnetosphere were modeled explicitly, orbit by orbit.RESULTSSaturn’s magnetic equator, where the magnetic field becomes parallel to the spin axis, is shifted northward from the planetary equator by 2808.5 ± 12 km, confirming the north-south asymmetric nature of Saturn’s magnetic field. After removing the systematic variation with distance from the spin axis, the peak-to-peak
Staniland N, Dougherty M, Masters A, 2018, Quantifying the stress of the Saturnian magnetosphere during the Cassini era, Geophysical Research Letters, Vol: 45, Pages: 8704-8711, ISSN: 0094-8276
We quantify the magnetospheric stress state of Saturn, revealing the nature of the planetary environment and its current systems. The complete magnetic field data set collected by the Cassini spacecraft is used to track the global behavior of the Saturnian magnetosphere during the Cassini era. Variations in the magnetodisc current model parameter μoIo determine when the system is stretched, compressed, or near the ground state. Of the 111 orbits that pass through our chosen region, 69 are well described by the model, indicating a steady state current sheet during this interval. While the stress state displays a dependence on local time, it is also shown to vary temporally. We conclude that the Saturnian magnetosphere remained in a quiet state for a significant period of the Cassini orbital mission at Saturn, with occasional large‐scale deviations observed.
Sulaiman AH, Kurth WS, Hospodarsky GB, et al., 2018, Enceladus auroral hiss emissions during Cassini's grand finale, Geophysical Research Letters, Vol: 45, Pages: 7347-7353, ISSN: 0094-8276
Cassini's Radio and Plasma Wave Science (RPWS) instrument detected intense auroral hiss emissions during one of its perikrone passes of the Grand Finale orbits. The emissions were detected when Cassini traversed a flux tube connected to Enceladus' orbit (L‐shell = 4) and at a time when both the spacecraft and the icy moon were in similar longitudes. Previous observations of auroral hiss related to Enceladus were made only during close flybys and here we present the first observation of such emissions close to Saturn. Further, ray‐tracing analysis shows the source location at a latitude of 63°, in excellent agreement with earlier UVIS observations of Enceladus' auroral footprint by Pryor et al. (2011, https://doi.org/10.1038/nature09928). The detection has been afforded exclusively by the Grand Finale phase, which enabled sampling of Enceladus' high‐latitude flux tube near Saturn. This result provides new insight into the spatial extent of the electrodynamic interaction between Saturn and Enceladus.
Guo RL, Yao ZH, Wei Y, et al., 2018, Rotationally driven magnetic reconnection in Saturn's dayside, Nature Astronomy, Vol: 2, Pages: 640-645, ISSN: 2397-3366
Magnetic reconnection is a key process that explosively accelerates charged particles, generating phenomena such as nebular flares1, solar flares2 and stunning aurorae3. In planetary magnetospheres, magnetic reconnection has often been identified on the dayside magnetopause and in the nightside magnetodisc, where thin-current-sheet conditions are conducive to reconnection4. The dayside magnetodisc is usually considered thicker than the nightside due to the compression of solar wind, and is therefore not an ideal environment for reconnection. In contrast, a recent statistical study of magnetic flux circulation strongly suggests that magnetic reconnection must occur throughout Saturn’s dayside magnetosphere5. Additionally, the source of energetic plasma can be present in the noon sector of giant planetary magnetospheres6. However, so far, dayside magnetic reconnection has only been identified at the magnetopause. Here, we report direct evidence of near-noon reconnection within Saturn’s magnetodisc using measurements from the Cassini spacecraft. The measured energetic electrons and ions (ranging from tens to hundreds of keV) and the estimated energy flux of ~2.6 mW m–2 within the reconnection region are sufficient to power aurorae. We suggest that dayside magnetodisc reconnection can explain bursty phenomena in the dayside magnetospheres of giant planets, which can potentially advance our understanding of quasi-periodic injections of relativistic electrons6 and auroral pulsations7.
Sergis N, Achilleos N, Guio P, et al., 2018, Mapping Saturn's nightside plasma sheet using Cassini's proximal orbits, Geophysical Research Letters, Vol: 45, Pages: 6798-6804, ISSN: 0094-8276
Between April and the end of its mission on 15 September, Cassini executed a series of 22 very similar 6.5‐day‐period proximal orbits, covering the mid‐latitude region of the nightside magnetosphere. These passes provided us with the opportunity to examine the variability of the nightside plasma sheet within this time scale for the first time. We use Cassini particle and magnetic field data to quantify the magnetospheric dynamics along these orbits, as reflected in the variability of certain relevant plasma parameters, including the energetic ion pressure and partial (hot) plasma beta. We use the University College London/Achilleos‐Guio‐Arridge magnetodisk model to map these quantities to the conjugate magnetospheric equator, thus providing an equivalent equatorial radial profile for these parameters. By quantifying the variation in the plasma parameters, we further identify the different states of the nightside ring current (quiescent and disturbed) in order to confirm and add to the context previously established by analogous studies based on long‐term, near‐equatorial measurements.
Sulaiman AH, Kurth WS, Hospodarsky GB, et al., 2018, Auroral Hiss Emissions During Cassini's Grand Finale: Diverse Electrodynamic Interactions Between Saturn and Its Rings, GEOPHYSICAL RESEARCH LETTERS, Vol: 45, Pages: 6782-6789, ISSN: 0094-8276
The Cassini Grand Finale orbits offered a new view of Saturn and its environment owing to multiple highly inclined orbits with unprecedented proximity to the planet during closest approach. The Radio and Plasma Wave Science instrument detected striking signatures of plasma waves in the southern hemisphere. These all propagate in the whistler mode and are classified as (1) a filled funnel‐shaped emission, commonly known as auroral hiss. Here however, our analysis indicates that they are likely associated with currents connected to the rings. (2) First observations of very low frequency saucers directly linked to the planet on field lines also connected to the rings. The latter observations are unique to low altitude orbits, and their presence at the Earth and Saturn alike shows that they are fundamental plasma waves in planetary ionospheres. Our results give an insight, from a unique perspective, into the dynamic and diverse nature of Saturn's environment.
Dougherty MK, Spilker LJ, 2018, Review of Saturn's icy moons following the Cassini mission, Reports on Progress in Physics, Vol: 81, ISSN: 0034-4885
We review our knowledge of the icy moons of Saturn prior to the Cassini orbital mission, describe the discoveries made by the instrumentation onboard the Cassini spacecraft.
Hunt GJ, Provan G, Bunce EJ, et al., 2018, Field-aligned currents in Saturn’s magnetosphere: Observations from the F-ring orbits, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 3806-3821, ISSN: 2169-9402
We investigate the azimuthal magnetic field signatures associated with high‐latitude field‐aligned currents observed during Cassini's F‐ring orbits (October 2016–April 2017). The overall ionospheric meridional current profiles in the northern and southern hemispheres, that is, the regions poleward and equatorward of the field‐aligned currents, differ most from the 2008 observations. We discuss these differences in terms of the seasonal change between data sets and local time (LT) differences, as the 2008 data cover the nightside while the F‐ring data cover the post‐dawn and dusk sectors in the northern and southern hemispheres, respectively. The F‐ring field‐aligned currents typically have a similar four current sheet structure to those in 2008. We investigate the properties of the current sheets and show that the field‐aligned currents in a hemisphere are modulated by that hemisphere's “planetary period oscillation” (PPO) systems. We separate the PPO‐independent and PPO‐related currents in both hemispheres using their opposite symmetry. The average PPO‐independent currents peak at ~1.5 MA/rad just equatorward of the open closed field line boundary, similar to the 2008 observations. However, the PPO‐related currents in both hemispheres are reduced by ~50% to ~0.4 MA/rad. This may be evidence of reduced PPO amplitudes, similar to the previously observed weaker equatorial oscillations at similar dayside LTs. We do not detect the PPO current systems' interhemispheric component, likely a result of the weaker PPO‐related currents and their closure within the magnetosphere. We also do not detect previously proposed lower latitude discrete field‐aligned currents that act to “turn off” the PPOs.
Hunt GJ, Provan G, Cowley SWH, et al., 2018, Saturn's planetary period oscillations during the closest approach of Cassini's ring grazing orbits, Geophysical Research Letters, Vol: 45, Pages: 4692-4700, ISSN: 0094-8276
Saturn's planetary period oscillations (PPOs) are ubiquitous throughout its magnetosphere. We investigate the PPO's azimuthal magnetic field amplitude interior to the field‐aligned currents, during the closest approaches of Cassini's ring‐grazing orbits (October 2016 to April 2017), with periapses at ~2.5 RS. The amplitudes of the northern and southern PPO systems are shown to vary as a function of latitude. The amplitude ratio between the two PPO systems shows that the northern system is dominant by a factor of ~1.3 in the equatorial plane, and it is dominant to ~ −15° latitude in the southern hemisphere. The dayside amplitudes are approximately half of the 2008 nightside amplitudes, which agree with previous local time‐related amplitude observations. Overall, there is clear evidence that the PPOs are present on field lines that map to the outer edge of Saturn's rings, closer to Saturn than previously confirmed.
Provan G, Cowley SWH, Bradley TJ, et al., 2018, Planetary period oscillations in Saturn's magnetosphere: Cassini magnetic field observations over the northern summer solstice interval, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 3859-3899, ISSN: 2169-9380
We determine properties of Saturn's planetary period oscillations from Cassini magnetic measurements over the ~2‐year interval from September 2015 to end of mission in September 2017, spanning Saturn northern summer solstice in May 2017. Phases of the northern system oscillations are derived over the whole interval, while those of the southern system are not discerned in initial equatorial data due to too low amplitude relative to the northern, but are determined once southern polar data become available from inclined orbits beginning May 2016. Planetary period oscillation periods are shown to be almost constant over these intervals at ~10.79 hr for the northern system and ~10.68 hr for the southern, essentially unchanged from values previously determined after the periods reversed in 2014. High cadence phase and amplitude data obtained from the short‐period Cassini orbits during the mission's last 10 months newly reveal the presence of dual modulated oscillations varying at the beat period of the two systems (~42 days) on nightside polar field lines in the vicinity (likely either side) of the open‐closed field boundary. The modulations differ from those observed previously in the equatorial region, indicative of a reversal in sign of the radial component oscillations, but not of the colatitudinal component oscillations. Brief discussion is given of a possible theoretical scenario. While weak equatorial beat modulations indicate a north/south amplitude ratio >5 early in the study interval, polar and equatorial region modulations suggest a ratio ~1.4 during the later interval, indicating a significant recovery of the southern system.
Bradley TJ, Cowley SWH, Provan G, et al., 2018, Field-aligned currents in Saturn's nightside magnetosphere: subcorotation and planetary period oscillation components during northern spring, Journal of Geophysical Research: Space Physics, Vol: 123, Pages: 3602-3636, ISSN: 2169-9380
We newly analyze Cassini magnetic field data from the 2012/2013 Saturn northern spring interval of highly inclined orbits and compare them with similar data from late southern summer in 2008, thus providing unique information on the seasonality of the currents that couple momentum between Saturn's ionosphere and magnetosphere. Inferred meridional ionospheric currents in both cases consist of a steady component related to plasma subcorotation, together with the rotating current systems of the northern and southern planetary period oscillations (PPOs). Subcorotation currents during the two intervals show opposite north‐south polar region asymmetries, with strong equatorward currents flowing in the summer hemispheres but only weak currents flowing to within a few degrees of the open‐closed boundary (OCB) in the winter hemispheres, inferred due to weak polar ionospheric conductivities. Currents peak at ~1 MA rad−1 in both hemispheres just equatorward of the open‐closed boundary, associated with total downward polar currents ~6 MA, then fall across the narrow auroral upward current region to small values at subauroral latitudes. PPO‐related currents have a similar form in both summer and winter with principal upward and downward field‐aligned currents peaking at ~1.25 MA rad−1 being essentially collocated with the auroral upward current and approximately equal in strength. Though northern and southern PPO currents were approximately equal during both intervals, the currents in both hemispheres were dual modulated by both systems during 2012/2013, with approximately half the main current closing in the opposite ionosphere and half cross field in the magnetosphere, while only the northern hemisphere currents were similarly dual modulated in 2008.
Carbary JF, Mitchell DG, Kollmann P, et al., 2018, Energetic electron pitch angle distributions during the Cassini final orbits, Geophysical Research Letters, Vol: 45, Pages: 2911-2917, ISSN: 0094-8276
Pitch angle distributions (PADs) of very energetic electrons (110–365 keV) are examined during the ring‐grazing and proximal orbits of the Cassini spacecraft, from day 320 2016 (15 November) to day 257 2017 (14 September). These repeating orbits allowed a statistical evaluation of the PADs within the magnetopause on the nightside of Saturn. Along L‐shells (i.e., equatorial crossing distances of magnetic field lines) near and outside that of Titan and north of the equator, the electron fluxes were unidirectionally field‐aligned going away from Saturn. Along L‐shells inside Titan's and south of the equator, the electrons had bidirectional or pancake (trapping) PADs. This behavior suggests that the field lines within Titan's L‐shell are generally closed, while those outside of that L‐shell are generally open. This result strictly applies only to the nightside local times sampled during the final Cassini orbits, but one may infer a similar behavior at other times.
Krupp N, Roussos E, Mitchell DG, et al., 2018, Charged particle measurements in the Saturnian magnetosphere during the Cassini era 2004-2017, Symposium Celebrating Prof. Wing-Huen Ip's 70th Birthday - Serendipities in the Solar System and Beyond, Publisher: ASTRONOMICAL SOC PACIFIC, Pages: 163-175, ISSN: 1050-3390
Sergis N, Bunce EJ, Carbary JF, et al., 2018, The Ring Current of Saturn, AGU Chapman Conference on Currents in Geospace and Beyond, Publisher: AMER GEOPHYSICAL UNION, Pages: 139-154, ISSN: 0065-8448
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Davies E, Masters A, Dougherty M, et al., 2017, Swept Forward Magnetic Field Variability in High-Latitude Regions of Saturn's Magnetosphere, Journal of Geophysical Research: Space Physics, Vol: 122, Pages: 12328-12337, ISSN: 2169-9380
Swept forward field is the term given to configurations of magnetic field wherein the field lines deviate from the meridional planes of a planet in the direction of its rotation. Evidence is presented for swept-forward field configurations on Cassini orbits around Saturn from the first half of 2008. These orbits were selected on the basis of high inclination, spatial proximity, and temporal proximity, allowing for the observation of swept-forward field and resolution of dynamic effects using data from the Cassini magnetometer. Nine orbits are surveyed; all show evidence of swept-forward field, with typical sweep angle found to be 23°. Evidence is found for transient events that lead to temporary dramatic increases in sweep-forward angle. The Michigan Solar Wind Model is employed to investigate temporal correlation between the arrivals of solar wind shocks at Saturn with these transient events, with two shown to include instances corresponding with solar wind shock arrivals. Measurements of equatorial electron number density from anode 5 of the Cassini Plasma Spectrometer instrument are investigated for evidence of magnetospheric compression, corresponding with predicted shock arrivals. Potential mechanisms for the transfer of momentum from the solar wind to the magnetosphere are discussed.
Yao ZH, Radioti A, Rae IJ, et al., 2017, Mechanisms of Saturn's Near-Noon Transient Aurora: In Situ Evidence From Cassini Measurements, GEOPHYSICAL RESEARCH LETTERS, Vol: 44, Pages: 11217-11228, ISSN: 0094-8276
Although auroral emissions at giant planets have been observed for decades, the physical mechanisms of aurorae at giant planets remain unclear. One key reason is the lack of simultaneous measurements in the magnetosphere while remote sensing of the aurora. We report a dynamic auroral event identified with the Cassini Ultraviolet Imaging Spectrograph (UVIS) at Saturn on 13 July 2008 with coordinated measurements of the magnetic field and plasma in the magnetosphere. The auroral intensification was transient, only lasting for ∼30 min. The magnetic field and plasma are perturbed during the auroral intensification period. We suggest that this intensification was caused by wave mode conversion generated field-aligned currents, and we propose two potential mechanisms for the generation of this plasma wave and the transient auroral intensification. A survey of the Cassini UVIS database reveals that this type of transient auroral intensification is very common (10/11 time sequences, and ∼10% of the total images).
Omidi N, Sulaiman AH, Kurth W, et al., 2017, A Single Deformed Bow Shock for Titan-Saturn System, JOURNAL OF GEOPHYSICAL RESEARCH-SPACE PHYSICS, Vol: 122, Pages: 11058-11075, ISSN: 2169-9380
During periods of high solar wind pressure, Saturn’s bow shock is pushed inside Titan’s orbitexposing the moon and its ionosphere to the solar wind. The Cassini spacecraft’s T96 encounter with Titanoccurred during such a period and showed evidence for shocks associated with Saturn and Titan. It alsorevealed the presence of two foreshocks: one prior to the closest approach (foreshock 1) and one after(foreshock 2). Using electromagnetic hybrid (kinetic ions and fluid electrons) simulations and Cassiniobservations,we showthat the origin of foreshock 1 is tied to the formation of a single deformed bow shock forthe Titan-Saturn system. We also report the observations of a structure in foreshock 1 with properties consistentwith those of spontaneous hot flow anomalies formed in the simulations and previously observed at Earth,Venus, and Mars. The results of hybrid simulations also show the generation of oblique fast magnetosonicwaves upstream of the outbound Titan bow shock in agreement with the observations of large-amplitudemagnetosonic pulsations in foreshock 2. We also discuss the implications of a single deformed bow shock fornew particle acceleration mechanisms and also Saturn’s magnetopause and magnetosphere.
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