E. J. Bunce

Origin of the main auroral oval in Jupiter's coupled magnetosphere-ionosphere system

Abstract We show that the principal features of the main auroral oval in the jovian system are consistent with an origin in the magnetosphere–ionosphere coupling currents associated with the departure of the plasma from rigid corotation in the middle magnetosphere, specifically with the inner region of field-aligned current directed upwards from the ionosphere to the magnetosphere. The features we refer to include its location, its continuity in local time, its width, and the precipitating particle energy flux and auroral luminosity. A simple empirical model of the field and flow in the middle magnetosphere is used to estimate the field-aligned currents flowing into and out of the equatorial current sheet associated with the breakdown of corotation. The models indicate that the current flows outwards from the ionosphere into the current sheet through most of the middle magnetosphere. Mapped to the ionosphere, the upward field-aligned current density is of order ∼1 μA m −2 , confined to circumpolar annular rings around each pole of latitudinal width ∼1° (∼1000 km ), centred near ∼16° dipole latitude. The upward current is carried principally by downward-precipitating magnetospheric electrons from the tenuous hot plasma which extends outside the cooler denser equatorial plasma sheet to high latitudes. For reasonable observed values of the magnetospheric electron parameters it is found that such currents require the existence of field-aligned voltages of order ∼100 kV . The auroral primaries are thus ∼100 keV electrons, consistent with deep penetration of the jovian atmosphere and low-altitude auroras, as observed. The peak ionospheric energy flux associated with the accelerated precipitating electrons is of order ∼0.1– 1 W m −2 , sufficient to drive a UV aurora of 1–10 MR at ∼20% conversion efficiency. In addition, to produce the current, the acceleration region must extend in altitude typically above ∼3–4RJ. The spatially extended energetic auroral electron beams so formed are suggested to form a principal source of free energy for non-Io-related radio emissions. An important implication of the model is that the main oval auroras and radio emissions will respond principally to the dynamic pressure of the solar wind, in the sense of anticorrelation.

Response of Jupiter's and Saturn's auroral activity to the solar wind

[1] While the terrestrial aurorae are known to be driven primarily by the interaction of the Earth’s magnetosphere with the solar wind, there is considerable evidence that auroral emissions on Jupiter and Saturn are driven primarily by internal processes, with the main energy source being the planets’ rapid rotation. Prior observations have suggested there might be some influence of the solar wind on Jupiter’s aurorae and indicated that auroral storms on Saturn can occur at times of solar wind pressure increases. To investigate in detail the dependence of auroral processes on solar wind conditions, a large campaign of observations of these planets has been undertaken using the Hubble Space Telescope, in association with measurements from planetary spacecraft and solar wind conditions both propagated from 1 AU and measured near each planet. The data indicate a brightening of both the auroral emissions and Saturn kilometric radiation at Saturn close in time to the arrival of solar wind shocks and pressure increases, consistent with a direct physical relationship between Saturnian auroral processes and solar wind conditions. At Jupiter the correlation is less strong, with increases in total auroral power seen near the arrival of solar wind forward shocks but little increase observed near reverse shocks. In addition, auroral dawn storms have been observed when there was little change in solar wind conditions. The data are consistent with some solar wind influence on some Jovian auroral processes, while the auroral activity also varies independently of the solar wind. This extensive data set will serve to constrain theoretical models for the interaction of the solar wind with the magnetospheres of Jupiter and Saturn.

Reconnection in a rotation-dominated magnetosphere and its relation to Saturn's auroral dynamics

The first extended series of observations of Saturns auroral emissions, undertaken by the Hubble Space Telescope in January 2004 in conjunction with measurements of the upstream solar wind and interplanetary magnetic field (IMF) by the Cassini spacecraft, have revealed a strong auroral response to the interplanetary medium. Following the arrival of the forward shock of a corotating interaction region compression, bright auroras were first observed to expand significantly poleward in the dawn sector such that the area of the polar cap was much reduced, following which the auroral morphology evolved into a spiral structure around the pole. We propose that these auroral effects are produced by compression-induced reconnection of a significant fraction of the open flux present in Saturns open tail lobes, as has also been observed to occur at Earth, followed by subcorotation of the newly closed flux tubes in the outer magnetosphere region due to the action of the ionospheric torque. We show that the combined action of reconnection and rotation naturally gives rise to spiral structures on newly opened and newly closed field lines, the latter being in the same sense as observed in the auroral images. The magnetospheric corollary of the dynamic scenario outlined here is that corotating interaction region-induced magnetospheric compressions and tail collapses should be accompanied by hot plasma injection into the outer magnetosphere, first in the midnight and dawn sector, and second at increasing local times via noon and dusk. We discuss how this scenario leads to a strong correlation of auroral and related disturbances at Saturn with the dynamic pressure of the solar wind, rather than to a correlation with the north-south component of the IMF as observed at Earth, even though the underlying physics is similar, related to the transport of magnetic flux to and from the tail in the Dungey cycle.

Morphological differences between Saturn's ultraviolet aurorae and those of Earth and Jupiter

It has often been stated that Saturns magnetosphere and aurorae are intermediate between those of Earth, where the dominant processes are solar wind driven, and those of Jupiter, where processes are driven by a large source of internal plasma. But this view is based on information about Saturn that is far inferior to what is now available. Here we report ultraviolet images of Saturn, which, when combined with simultaneous Cassini measurements of the solar wind and Saturn kilometric radio emission, demonstrate that its aurorae differ morphologically from those of both Earth and Jupiter. Saturns auroral emissions vary slowly; some features appear in partial corotation whereas others are fixed to the solar wind direction; the auroral oval shifts quickly in latitude; and the aurora is often not centred on the magnetic pole nor closed on itself. In response to a large increase in solar wind dynamic pressure Saturns aurora brightened dramatically, the brightest auroral emissions moved to higher latitudes, and the dawn side polar regions were filled with intense emissions. The brightening is reminiscent of terrestrial aurorae, but the other two variations are not. Rather than being intermediate between the Earth and Jupiter, Saturns auroral emissions behave fundamentally differently from those at the other planets.

Origin of Saturn's aurora: Simultaneous observations by Cassini and the Hubble Space Telescope

Outer planet auroras have been imaged for more than a decade, yet understanding their physical origin requires simultaneous remote and in situ observations. The first such measurements at Saturn were obtained in January 2007, when the Hubble Space Telescope imaged the ultraviolet aurora, while the Cassini spacecraft crossed field lines connected to the auroral oval in the high-latitude magnetosphere near noon. The Cassini data indicate that the noon aurora lies in the boundary between open- and closed-field lines, where a layer of upward-directed field-aligned current flows whose density requires downward acceleration of magnetospheric electrons sufficient to produce the aurora. These observations indicate that the quasi-continuous main oval is produced by the magnetosphere-solar wind interaction through the shear in rotational flow across the open-closed-field line boundary.

Open flux estimates in Saturn's magnetosphere during the January 2004 Cassini-HST campaign, and implications for reconnection rates

During 8–30 January 2004, a sequence of 68 UV images of Saturns southern aurora was obtained by the Hubble Space Telescope (HST), coordinated for the first time with measurements of the upstream interplanetary conditions made by the Cassini spacecraft. Using the poleward edge of the observed aurora as a proxy for the open-closed field line boundary, the open flux content of the southern polar region has been estimated. It is found to range from ∼15 to ∼50 GWb during the interval, such a large variation providing evidence of a significant magnetospheric interaction with the solar wind, in particular with the interplanetary structures associated with corotating interaction regions (CIRs). The open flux is found to decline slowly during a rarefaction region in which the interplanetary magnetic field remained very weak, while decreasing sharply in association with the onset of CIR-related solar wind compressions. Such decreases are indicative of the dominating role of open flux closure in Saturns tail during these intervals. Increases in open flux are found to occur in the higher-field compression regions after the onsets, and in a following rarefaction region of intermediate field strength. These increases are indicative of the dominating role of open flux production at Saturns magnetopause during these intervals. The rate of open flux production has been estimated from the upstream interplanetary data using an empirical formula based on experience at Earth, with typical values varying from ∼10 kV during the weak-field rarefaction region, to ∼200 kV during the strong-field compression. These values have been integrated over time between individual HST image sets to estimate the total open flux produced during these intervals. Comparison with the changes in open flux obtained from the auroral images then allows us to estimate the amount of open flux closed during these intervals, and hence the averaged tail reconnection rates. Intermittent intervals of tail reconnection at rates of ∼30–60 kV are inferred in rarefaction regions, while compression regions are characterised by rates of ∼100–200 kV, these values representing averages over the ∼2-day intervals between HST image sequences. The forms of the aurorae observed are also discussed in relation to the deduced voltage values.

A multi-instrument view of tail reconnection at Saturn

Three instances of tail reconnection events at Saturn involving the ejection of plasmoids downtail have been reported by Jackman et al. (2007) using data from Cassini’s magnetometer (MAG). Here we show two newly discovered events, as identified in the MAG data by northward/southward turnings and intensifications of the field. We discuss these events along with the original three, with the added benefit of plasma and energetic particle data. The northward/southward turnings of the field elucidate the position of the spacecraft relative to the reconnection point and passing plasmoids, while the variability of the azimuthal and radial field components during these events indicates corresponding changes in the angular momentum of the magnetotail plasma following reconnection. Other observable effects include a reversal in flow direction of energetic particles, and the apparent evacuation of the plasma sheet following the passage of plasmoids.

Modulation of Jupiter's main auroral oval emissions by solar wind induced expansions and compressions of the magnetosphere

Abstract It has recently been suggested that the jovian ‘main auroral oval’ is associated with the region of upward-directed field-aligned currents in the circuit formed through angular momentum exchange between the atmosphere–ionosphere and the equatorial iogenic magnetospheric plasma. It has also been suggested that the luminosity of these emissions should be modulated primarily by the dynamic pressure of the solar wind, which causes expansions and compressions of the magnetosphere and thereby changes the angular velocity of the magnetospheric plasma and the strength of the coupling currents. Here we present a quantitative demonstration of these effects. We consider an initial empirically based middle magnetosphere field and plasma configuration extending to 50RJ, which expands outwards to 70RJ, or is compressed inwards to 30RJ, due e.g. to sudden changes in the dynamic pressure of the solar wind. Changes in the angular velocity of the plasma are computed from the flux-preserving motions of the field lines using conservation of angular momentum. The initial configuration produces a band of aurora in which the luminosity falls over ∼1° latitude from ∼200 kR at the poleward boundary to ∼20 kR . Expansion of the current sheet to 70RJ produces a significant increase in luminosity over this range, with the largest effect occurring at the poleward boundary where a thin (∼50– 100 km ) arc-like structure of MR intensity is formed. The initial emission is reduced by about an order of magnitude for a compression to 40RJ, and essentially to zero for a compression to 35RJ, when the middle magnetosphere plasma is brought to near rigid corotation over the whole radial range such that the coupling current system is essentially switched off. Further compression to 30RJ then induces significant super-corotation, with a reversal in sense of the magnetosphere–ionosphere coupling current system. In this case the middle magnetosphere field-aligned currents reverse to downward throughout, such that we would not expect them to be associated with bright electron-induced auroras. However, upward-directed ‘return’ currents then flow in the poleward region outside of the middle magnetosphere, which may be associated with an auroral oval lying poleward of the usual ‘main oval’ location. A ‘Hill-type’ equilibrium will re-form on time scales of a few tens of hours, and with it, the main oval auroras will re-emerge, corresponding to the changed size of the magnetospheric cavity.

Saturn's equinoctial auroras

Received 23 October 2009; accepted 24 November 2009; published 23 December 2009. [1] We present the first images of Saturn’s conjugate equinoctial auroras, obtained in early 2009 using the Hubble Space Telescope. We show that the radius of the northern auroral oval is � 1.5 smaller than the southern, indicating that Saturn’s polar ionospheric magnetic field, measured for the first time in the ionosphere, is � 17% larger in the north than the south. Despite this, the total emitted UV power is on average � 17% larger in the north than the south, suggesting that field-aligned currents (FACs) are responsible for the emission. Finally, we show that individual auroral features can exhibit distinct hemispheric asymmetries. These observations will provide important context for Cassini observations as Saturn moves from southern to northern summer. Citation: Nichols, J. D., et al. (2009), Saturn’s equinoctial auroras, Geophys. Res. Lett., 36, L24102, doi:10.1029/2009GL041491.

Divergence of the equatorial current in the dawn sector of Jupiter's magnetosphere: analysis of Pioneer and Voyager magnetic field data

Abstract Averaged values of the azimuthal component of the magnetic field observed outside the jovian middle magnetosphere equatorial current sheet have been used to derive radial profiles of the radial current intensity over the jovicentric distance range 20–50RJ. Data from four spacecraft flybys have been used, spanning the dawn sector from ∼0100 to ∼0900 MLT (i.e. inbound Pioneer-11, and outbound Pioneer-10, and Voyagers-1 and -2). These profiles have been combined with a recent empirical model of the azimuthal current intensity to estimate the total divergence of the current in the current sheet along the trajectory, and hence the density of the field-aligned current that couples the current sheet and the ionosphere. For the Voyager passes the inferred field-aligned current flows from the ionosphere into the current sheet, with (j||/B)≈2– 10×10 −13 A m −2 nT −1 . The inferred field-aligned currents at the ionosphere require significant field-aligned acceleration of thermal magnetospheric electrons (of 2–3 keV thermal energy), through voltages of ∼40–150 kV. The latitude of these currents, together with the estimated electron energy flux deposited in the ionosphere (0.01–0.1 W m−2), suggests a direct connection with the main jovian auroral oval. Related currents are present on the Pioneer passes as well, but lie equatorially inside of ∼20RJ. Between 20 and 50RJ, the inferred field-aligned currents on these passes are either small (Pioneer-11), or reversed in sense with similar densities (Pioneer-10). The radial profiles of the radial current associated with magnetosphere–ionosphere coupling have also been used to derive radial profiles of the angular velocity of the magnetospheric plasma, for given values of the effective Pedersen conductivity of the jovian ionosphere. Reasonable profiles are obtained for effective conductivities of several tenths of a mho. The Voyager data then indicate slowly falling values from near-rigid corotation at ∼20RJ, to ∼50% of rigid corotation at ∼50RJ. For the same values of the conductivity the Pioneer data indicate smaller angular velocities in the inner region (∼70% of rigid corotation), remaining either constant or even increasing with distance. This behaviour may either indicate the presence of some magnetospheric dynamic process operating in the outer magnetosphere on these passes, or could alternatively be due to falling conductivities with distance in the conjugate ionosphere.