During Cassini's Grand Finale proximal orbits, the spacecraft traversed the nightside magnetotail to ∼21 Saturn radii. Clear signatures of Saturn's equatorial current sheet are observed in the ...magnetic field data. An axisymmetric model of the ring current is fitted to these data, amended to take into account the tilt of the current layer by solar wind forcing, its teardrop‐shaped nature and the magnetotail and magnetopause fringing fields. Variations in ring current parameters are examined in relation to external driving of the magnetosphere by the solar wind and internal driving by the two planetary period oscillations (PPOs), and compared with previous dawn and dayside observations. We find that the relative phasing of the PPOs determines the ring current's response to solar wind conditions. During solar wind compressions when the PPOs are in antiphase, a thick partial ring current is formed on the nightside, dominated by hot plasma injected by tail reconnection. This partial ring current should close partly via magnetopause currents and possibly via field‐aligned currents into the ionosphere. However, during solar wind compressions when the PPOs are in phase, this partial ring current is not detected. During solar wind rarefactions an equatorial “magnetodisc” configuration is observed in the dayside/dawn/nightside regions, with similar total currents flowing at these local times. During very quiet intervals of prolonged solar wind rarefaction, a thin current sheet with an enhanced current density is formed, indicative of a ring current dominated by cool, dense, Enceladus water group ions.
Plain Language Summary
We have studied Saturn's nightside ring current during Cassini's Grand Finale. We find that Saturn's ring current, like Saturn's magnetosphere as a whole, is driven by factors both internal and external to the magnetosphere. The external driver is the solar wind and the internal drivers are the planetary period oscillations. Magnetospheric storms, triggered by both drivers, result in a partial ring current of hot plasma on Saturn's nightside.
Key points
We study Saturn's nightside ring current during Cassini's Grand Finale
Saturn's nightside ring current is driven jointly by the external solar wind and by the internal planetary period oscillations
Magnetospheric storms, triggered by both the internal and external drivers, result in a partial ring current of hot plasma on the nightside
We evaluate the expected effects of magnetosphere‐ionosphere coupling at Jupiter along the Juno Perijove 1 (PJ1) trajectory using an axisymmetric physical model. As found at Saturn, the model ...predicts distributed downward field‐aligned currents over polar regions mapping to the tail and outer magnetosphere, closed principally through a ring of upward current mapping to the middle magnetosphere, which requires downward acceleration of magnetospheric electrons generating Jupiter's main auroral emission. Auroral location, width, intensity, electron energy, and current density are in accord with values derived from previous ultraviolet imaging, such that the model forms an appropriate baseline for comparison with Juno data. We evaluate the azimuthal field perturbations during six anticipated near‐planet encounters with middle magnetosphere field lines at radial distances between ~1.6 and ~16 Jovian radii, discuss the expected form of the accelerated electron distributions, and comment briefly on model expectations in relation to first results derived from Juno PJ1 data.
Key Points
A model of magnetosphere‐ionosphere coupling at Jupiter benchmarked against auroral data is evaluated on the Juno Perijove 1 trajectory
Expected plasma regimes on PJ1 include six near‐planet encounters with middle magnetosphere (main oval) field lines at radii ~1.6 to ~16 RJ
Field effects produced by expected distributed polar downward currents and main oval upward currents are quantified for comparison with data
We present high‐sensitivity Hubble Space Telescope (HST) Cosmic Origins Spectrograph and HST Space Telescope Imaging Spectrograph measurements of atmospheric OI 130.4‐nm and OI 135.6‐nm emissions at ...Ganymede, which exhibit significant spatial and temporal variability. These observations represent the first observations of Ganymede using HST Cosmic Origins Spectrograph and of both the leading and trailing hemispheres within a single HST campaign, minimizing the potential influence of long‐term changes in the Jovian plasma sheet or in Ganymede's atmosphere on the comparison of the two hemispheres. The mean disk‐averaged OI 135.6‐nm/OI 130.4‐nm observed intensity ratio was 2.72 ± 0.57 on the leading hemisphere and 1.42 ± 0.16 on the trailing hemisphere. The observed leading hemisphere ratios are consistent with an O2 atmosphere, but we show that an atomic oxygen component of ~10% is required to produce the observed trailing hemisphere ratios. The excess 130.4‐nm emission on the trailing hemisphere relative to that expected for an O2 atmosphere was ~11 R. The O column density required to produce this excess is determined based on previous estimates of the electron density and temperature at Ganymede and exceeds the limit for an optically thin atmosphere. The implication that the O atmosphere is optically thick may be investigated in future by observing Ganymede as it moves into eclipse or by determining the ratio of the individual components within the 130.4‐nm triplet.
Key Points
Ganymede's FUV oxygen emissions were observed on both orbital leading and trailing hemispheres in the same HST campaign for the first time
The observed hemispheric difference in the brightness of both emissions and in their ratio therefore cannot be caused by long‐term changes
The OI 135.6‐nm/OI 130.4‐nm ratio on the trailing hemisphere suggests that an optically thick O atmosphere is present there with O/O2 ~ 10%
During the Saturn orbit insertion (SOI) fly‐through of the Cassini spacecraft, Saturn's magnetosphere underwent a significant corotating interaction region (CIR) related compression. Such ...compressions have recently been suggested to produce rapid bursts of tail reconnection, enhanced Saturn kilometric radiation (SKR), and consequent auroral dynamics. On the outbound pass the spacecraft became engulfed by hot plasma, associated with a reduction in field strength, and a change in orientation indicative of a dipolarisation. Concurrently, a substantial enhancement in SKR emissions took place, together with a disruption of the typical planetary modulation. We suggest this is the first in situ evidence of compression‐related tail collapse via magnetic reconnection and hot plasma acceleration in Saturn's magnetotail.
•We discuss Saturn’s northern FUV auroras as observed using HST over 2011–2013.•The morphologies of the northern auroras are broadly consistent with the south.•The north exhibits increased prevalence ...of poleward patches in noon/afternoon.•The northern oval oscillates with the northern PPO phase, similar to the southern.•Overall, the dependence of the auroras on northern magnetic phase is somewhat weak.
We discuss the features of Saturn’s northern FUV auroras as observed during a program of Hubble Space Telescope observations which executed over 2011–2013 and culminated, along with Cassini observations, in a comprehensive multi-spectral observing campaign. Our 2011–2013 observations of the northern aurora are also compared with those from our 2007–2008 observation of the southern aurora. We show that the variety of morphologies of the northern auroras is broadly consistent with the southern, and determine the statistical equatorward and poleward boundary locations. We find that our boundaries are overall consistent with previous observations, although a modest poleward displacement of the poleward boundaries is due to the increased prevalence of poleward auroral patches in the noon and afternoon sectors during this program, likely due to the solar wind interaction. We also show that the northern auroral oval oscillates with the northern planetary period oscillation (PPO) phase in an elongated ellipse with semi-major axis ∼1.6° oriented along the post-dawn/post-dusk direction. We further show that the northern auroras exhibit dawn-side brightenings at zero northern magnetic PPO phase, although there is mixed evidence of auroral emissions fixed in the rotating frame of the northern PPO current system, such that overall the dependence of the auroras on northern magnetic phase is somewhat weak.
•We document pulsation at about a 1h period in Saturn’s aurora.•The B-field, energetic electrons, and auroral hiss at the magnetopause are modulated together.•The pulsating forms in the high latitude ...auroral zone rotate at about 40% of corotation speed.•A large fraction of the polar cap is involved in the pulsation, although phase varies with location.
Over the course of about 6h on Day 129, 2008, the UV imaging spectrograph (UVIS) on the Cassini spacecraft observed a repeated intensification and broadening of the high latitude auroral oval into the polar cap. This feature repeated at least 5 times with about a 1h period, as it rotated in the direction of corotation, somewhat below the planetary rotation rate, such that it moved from noon to post-dusk, and from roughly 77° to 82° northern latitudes during the observing interval. The recurring UV observation was accompanied by pronounced ∼1h pulsations in auroral hiss power, magnetic perturbations consistent with small-scale field aligned currents, and energetic ion conics and electrons beaming upward parallel to the local magnetic field at the spacecraft location. The magnetic field and particle events are in phase with the auroral hiss pulsation. This event, taken in the context of the more thoroughly documented auroral hiss and particle signatures (seen on many high latitude Cassini orbits), sheds light on the possible driving mechanisms, the most likely of which are magnetopause reconnection and/or Kelvin Helmholtz waves.
During the periapsis pass of Revolution 89, specifically on day 291 of 2008, the Cassini spacecraft observed unusual field‐aligned current signatures in Saturn's high‐latitude southern hemisphere in ...the midnight and dawn sector. The region of open field lines was found to be contracted close to the pole, and surrounded by an unusual region containing hot keV electrons and ‘leading’ field signatures indicative of super‐corotating flow. Usual ‘lagging’ fields indicative of sub‐corotating flow were also present at lower latitudes, though of unusual strength. Unique within the ∼40 similar nightside auroral region Cassini passes during 2008, the overall field‐aligned current system thus consisted of a central region of downward current flanked by two regions of upward current. This distinctive signature coincided with the first in situ encounter of Cassini with a source region of Saturn kilometric radiation, located within the unusual poleward region of upward current adjacent to the open‐closed field line boundary. We propose that these unusual conditions relate to a major open flux closure event in Saturn's tail, possibly triggered by solar wind compression of the magnetosphere.
The Dynamics of Saturn's Main Aurorae Bader, A.; Badman, S. V.; Cowley, S. W. H. ...
Geophysical research letters,
09/2019, Letnik:
46, Številka:
17-18
Journal Article
Recenzirano
Odprti dostop
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.
Plain Language Summary
Saturn's main aurorae are thought to be generated as a result of sheared plasma flows near the boundary between the rapidly rotating magnetosphere of Saturn and interplanetary space. It is often assumed that the steady flow of the solar wind away from the Sun has an impact on this flow shear; due to the direction of Saturn's rotation the aurorae would then have to be brighter at the planet's dawnside than on its duskside, which was observed in previous studies. Here we analyze a large set of auroral images taken by Cassini's ultraviolet camera, but we cannot find any sign of such an asymmetry. This indicates that the impact of the solar wind on Saturn's aurorae must be smaller than previously thought and that they must instead mainly be controlled from within the system. This assumption is supported by our observations of bright auroral patches at dawn, which are likely a signature of plasma being released from Saturn's magnetosphere and appear at quite regular periods corresponding to Saturn's rotation period.
Key Points
A dawn‐dusk asymmetry in Saturn's auroral emissions due to Dungey cycle activity is not observed under typical solar wind driving
The previously observed statistical intensity maximum near dawn is the result of large‐scale auroral plasma injections from Saturn's nightside
The phasing of these auroral injections indicates that magnetotail reconnection seems to partly be governed by planetary period oscillations
We overview the properties of the azimuthal magnetic fields observed during the periapsis passes of the final 23 orbits of the Cassini spacecraft, including the partial orbit at end of mission, on ...near equatorial field lines passing inside of Saturn's D ring. The signatures are variable in form and amplitude, though generally approximately symmetric about the point where the spacecraft trajectory lies tangent to a flux shell, corresponding to where the ionospheric field line feet map closest to the equator, consistent with the effect of interhemispheric field‐aligned currents. The perturbations usually begin and end near symmetrically at some point on field lines threading the D ring and extend into the interior region, but in no case do they clearly extend outward onto field lines passing through the C ring. About 35% of cases display a ~20‐40 nT single positive central field peak indicative of southward field‐aligned current flow, while a further ~30% display two or three weaker ~10‐20 nT positive peaks indicative of multiple sheets of northward and southward current. Significant smaller‐scale >5 nT peak‐to‐peak field fluctuations are commonly superposed. A further ~20% of cases exhibit unique profiles within the data set, including two with ~20‐30 nT negative fields and two with only <10 nT fluctuating fields. The variable nature of the signatures is not connected with the pass altitude, local time, planetary period oscillation phase, or D68 ringlet phase but may relate to variable structured thermospheric winds and/or ionospheric conductivities that suggest a significant dynamical role for D ring‐atmosphere interactions.
Key Points
Azimuthal magnetic fields observed on Saturn's intra‐D ring (IDR) field lines exhibit a range of profiles on the 23 Cassini proximal passes
One, two, or three near‐symmetrical positive peaks ~10‐40 nT are typical, but two passes have ~20‐30 nT negative peaks and two <10 nT fields
Variable IDR field signatures are not organized by periapsis radius, local time, planetary period oscillation phase, or D68 ringlet phase
Cassini magnetic field observations show that few‐nT oscillations near the planetary rotation period, first observed in Pioneer‐11 and Voyager‐1 and ‐2 fly‐by data, are essentially ubiquitous in ...Saturn's magnetosphere, though their character differs between the quasi‐dipolar ring current region and the dawn tail. Examination of data from the ring‐current region shows, however, that the observed oscillation period is not fixed at the planetary period, but has smaller values on the inbound pass of the spacecraft, increasing to larger values at and beyond periapsis. These variations are shown to be consistent with the Doppler shifts due to spacecraft motion expected in a model in which the wave phase fronts rotate with the planet, and radiate outward at a speed comparable with the equatorial Alfvén speed.
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