Jupiter's polar ionospheric flows: measured intensity and velocity variations poleward of the main auroral oval

Recent analysis of high-resolution spectra of Doppler-shifted H3+ emission from the auroral/polar regions of Jupiter revealed a complex wind system, with a persistent auroral electrojet and strong anti-sunward flows in a region of lesser intensity centred around the magnetic pole [ Stallard et al., 2001 ]. This region, which we have called the Dark Polar Region (DPR), is re-investigated, transforming the observed line-of-sight velocities into a frame of reference fixed with respect to the magnetic pole. The DPR is shown to include a region essentially stagnant in this frame of reference (the f-DPR). We identify it as a region coupled to open magnetotail field lines. There is also a transition region in which the ion velocity returns to corotation (the r-DPR)

Saturn’s northern auroras as observed using the Hubble Space Telescope

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°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

Cassini nightside observations of the oscillatory motion of Saturn's northern auroral oval

In recent years we have benefitted greatly from the first in-orbit multi-wavelength images of Saturn's polar atmosphere from the Cassini spacecraft. Specifically, images obtained from the Cassini UltraViolet Imaging Spectrograph (UVIS) provide an excellent view of the planet's auroral emissions, which in turn give an account of the large-scale magnetosphere-ionosphere coupling and dynamics within the system. However, obtaining near-simultaneous views of the auroral regions with in situ measurements of magnetic field and plasma populations at high latitudes is more difficult to routinely achieve. Here we present an unusual case, during Revolution 99 in January 2009, where UVIS observes the entire northern UV auroral oval during a 2 h interval while Cassini traverses the magnetic flux tubes connecting to the auroral regions near 21 LT, sampling the related magnetic field, particle, and radio and plasma wave signatures. The motion of the auroral oval evident from the UVIS images requires a careful interpretation of the associated latitudinally “oscillating” magnetic field and auroral field-aligned current signatures, whereas previous interpretations have assumed a static current system. Concurrent observations of the auroral hiss (typically generated in regions of downward directed field-aligned current) support this revised interpretation of an oscillating current system. The nature of the motion of the auroral oval evident in the UVIS image sequence, and the simultaneous measured motion of the field-aligned currents (and related plasma boundary) in this interval, is shown to be related to the northern hemisphere magnetosphere oscillation phase. This is in agreement with previous observations of the auroral oval oscillatory motion

Cassini multi-instrument assessment of Saturn's polar cap boundary

We present the first systematic investigation of the polar cap boundary in Saturn's high-latitude magnetosphere through a multi-instrument assessment of various Cassini in situ data sets gathered between 2006 and 2009. We identify 48 polar cap crossings where the polar cap boundary can be clearly observed in the step in upper cutoff of auroral hiss emissions from the plasma wave data, a sudden increase in electron density, an anisotropy of energetic electrons along the magnetic field, and an increase in incidence of higher-energy electrons from the low-energy electron spectrometer measurements as we move equatorward from the pole. We determine the average level of coincidence of the polar cap boundary identified in the various in situ data sets to be 0.34° ± 0.05° colatitude. The average location of the boundary in the southern (northern) hemisphere is found to be at 15.6° (13.3°) colatitude. In both hemispheres we identify a consistent equatorward offset between the poleward edge of the auroral upward directed field-aligned current region of ~1.5–1.8° colatitude to the corresponding polar cap boundary. We identify atypical observations in the boundary region, including observations of approximately hourly periodicities in the auroral hiss emissions close to the pole. We suggest that the position of the southern polar cap boundary is somewhat ordered by the southern planetary period oscillation phase but that it cannot account for the boundary's full latitudinal variability. We find no clear evidence of any ordering of the northern polar cap boundary location with the northern planetary period magnetic field oscillation phase

Axi-symmetric models of auroral current systems in Jupiter's magnetosphere with predictions for the Juno mission

We develop two related models of magnetosphere-ionosphere coupling in the jovian system by combining previous models defined at ionospheric heights with magnetospheric magnetic models that allow system parameters to be extended appropriately into the magnetosphere. The key feature of the combined models is thus that they allow direct connection to be made between observations in the magnetosphere, particularly of the azimuthal field produced by the magnetosphere-ionosphere coupling currents and the plasma angular velocity, and the auroral response in the ionosphere. The two models are intended to reflect typical steady-state sub-corotation conditions in the jovian magnetosphere, and transient super-corotation produced by sudden major solar wind-induced compressions, respectively. The key simplification of the models is that of axi-symmetry of the field, flow, and currents about the magnetic axis, limiting their validity to radial distances within ~30 RJ of the planet, though the magnetic axis is appropriately tilted relative to the planetary spin axis and rotates with the planet. The first exploration of the jovian polar magnetosphere is planned to be undertaken in 2016–2017 during the NASA New Frontiers Juno mission, with observations of the polar field, plasma, and UV emissions as a major goal. Evaluation of the models along Juno planning orbits thus produces predictive results that may aid in science mission planning. It is shown in particular that the low-altitude near-periapsis polar passes will generally occur underneath the corresponding auroral acceleration regions, thus allowing brief examination of the auroral primaries over intervals of ~1–3 min for the main oval and ~10 s for narrower polar arc structures, while the "lagging" field deflections produced by the auroral current systems on these passes will be ~0.1°, associated with azimuthal fields above the ionosphere of a few hundred nT

Implications of rapid planetary rotation for the Dungey magnetotail of Saturn

Employing our current understanding of the structure and dynamics of Saturn's magnetosphere, we present a time-dependent model of the kronian Dungey cycle magnetotail, which is based upon a modification of a similar model developed for Earth's magnetotail (Milan, 2004a). The major difference arises due to the rapid rotation of Saturn and the partial corotation that this imposes on the open field lines threading the polar cap. This results in twisted tail lobes, with the form of concentric cylinders of oldest to newest open flux from the inside out. The oldest, and hence longest, open field lines form the backbone of a highly extended magnetotail. Surrounding this are bundles of field lines disconnected by tail reconnection, propagating down-tail at the solar wind speed. Owing to the twisted nature of the tail, these bundles remain entangled with the lobe cores to form "exterior flux ropes." In the limit that the addition and removal of open flux from the magnetosphere by magnetic reconnection can be treated as a last-in-first-out system, we formulate a description of the flux transport within the tail and drive this with estimated dayside reconnection voltages deduced from Cassini observations of the IMF made upstream of Saturn (Jackman et al., 2004).</p

Structure of the interplanetary magnetic field during the interval spanning the first Cassini fly-through of Saturn's magnetosphere and its implications for Saturn's magnetospheric dynamics

We examine the interplanetary magnetic field (IMF) data obtained by the Cassini spacecraft during a 5 month period spanning the first fly-through of Saturn's magnetosphere, this interval corresponding to six solar rotations at the spacecraft. It is shown that the structure of the interplanetary medium was consistent with expectations for the declining phase of the solar cycle, generally consisting of two IMF sectors and two corotating interaction region compressions during each solar rotation. Field strengths and consequent estimated reconnection voltages at Saturn's magnetopause were overall weaker by a factor of about two compared with those observed during the immediately preceding interval investigated by Jackman et al. (J. Geophys. Res., 109, A11203, doi:10.1029/2004JA010614, 2004). Specifically, during the four solar rotations immediately preceding the fly-through, it is estimated that the total open flux produced at Saturn's magnetopause was ∼60 GWb per solar rotation, compared with ∼100 GWb per solar rotation estimated similarly for the earlier interval. These values compare with estimates of ∼35 GWb of open magnetic flux typically present in Saturn's tail lobes and polar cap. However, in the solar rotation immediately following the fly-through, it is found that field and voltage values recovered to former overall values.</p

Cassini observations of the Interplanetary Medium Upstream of Saturn and their relation to the Hubble Space Telescope aurora data

We present Cassini magnetometer and plasma data for the January 2004 'solar wind campaign' in which the particles and fields instruments monitored the solar wind and interplanetary magnetic field, while the Hubble Space Telescope (HST) simultaneously observed the UV aurora in Saturn's southern ionosphere. Clear structuring is evident in the data which is associated with the highly developed nature of corotating interaction regions (CIRs) at this distance. The interplanetary medium during January consisted of four distinct types of behaviour. We see a 'major' compression region at the start of the interval followed by a rarefaction region, a 'minor' compression region, an 'intermediate' rarefaction region, and another major compression region at the end of the month. The highly dynamic nature of Saturn's aurora revealed by the HST observations appears to relate directly to the concurrent solar wind activity measured by Cassini. Collectively these data provide a unique insight into the solar wind driving of Saturn's magnetosphere and consequent auroral response.</p