[1] 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 openclosed-field line boundary.

We have identified seven periapsis passes during the first high‐latitude phase of the Cassini mission, from mid‐2006 to mid‐2007, in which the spacecraft traversed at intermediate altitudes the region between open field lines at highest latitudes and the region inside the inner edge of the ring current. Varying azimuthal magnetic fields indicative of the presence of field‐aligned currents were observed in both hemispheres on all these passes, corresponding to the dawn and prenoon sector in the summer southern hemisphere and the dusk to premidnight sector in the winter northern hemisphere. In the southern hemisphere, strongly “lagging” fields observed on open field lines are observed to decline rapidly across the open‐closed field line boundary, usually then reversing in sense to a “leading” configuration in a narrow layer of closed field lines, before declining to smaller values determined by the phase of the planetary period oscillation in the inner region. These observations suggest that the plasma flow in this sector increases sharply across the boundary from subcorotation on open field lines to a layer of supercorotation on closed field lines, accompanied by a major layer of upward‐directed field‐aligned current that is shown to be colocated with the statistical location of the southern auroral oval. Downward current then flows in the inner region as the leading field declines. In the northern hemisphere, however, only weak azimuthal fields are observed on open field lines, suggesting weak conductivity in the winter ionosphere if the plasma similarly subcorotates, while a layer of stronger lagging field indicative of subcorotation is observed immediately equatorward in the closed field region. The field‐aligned currents in this case are thus directed downward just inside the boundary and upward in the interior region, opposite to the southern hemisphere, but are again modulated by the field of the planetary period oscillations on either side.

A c c e p t e d m a n u s c r i p t remote sensing and will make measurements on spatial scales of less than 10 km for 57 major elements during solar flares, sufficient to isolate surface landforms, such as craters 58 and their internal structures. The spatial resolution achieved by MIXS-T is made possible 59 by novel, low mass microchannel plate X-ray optics, in a Wolter type I optical geometry. 60 61 MIXS measurements of surface elemental composition will help determine rock types, 62 the evolution of the surface and ultimately a probable formation process for the planet. In 63 this paper we present MIXS and its predicted performance at Mercury as well as 64 discussing the role that MIXS measurements will play in answering the major questions 65 about Mercury. 66 67

[1] During 2008, the Cassini spacecraft traversed Saturn's high-latitude field-aligned current systems on sequential north-south periapsis passes in the nightside magnetosphere. Two types of current systems have previously been identified, associated with antisymmetric azimuthal field signatures in the northern and southern hemispheres. The first exhibits exclusively "lagging" field morphology, while the second also includes an equatorward interval of "leading" field. Here we report the statistical characteristics of these currents, their strength, ionospheric location, and relationship to plasma boundaries. From high to low latitude, the first type comprises a downward current followed by an upward current, whose strengths are ∼0.5-3.5 MA per radian of azimuth. The downward current maps to ionospheric colatitudes of ∼13.5°and ∼16°in the north and south, respectively, usually centered in the outer magnetosphere, while the upward current maps to ∼16.5°and ∼19°in the north and south, located within the ring current region. The second type comprises a distributed downward current of ∼1-2 MA rad −1 flowing in the open field and outer magnetosphere regions, followed by an upward current of ∼2.5-5 MA rad −1 mapping to ∼15.5°and ∼18°in the north and south, corresponding to the outer magnetosphere and outer ring current, and a further downward current of ∼1-2.5 MA rad −1 mapping to ∼17.5°and ∼20°in the north and south, corresponding to the inner ring current.

We report first results of a survey of near‐simultaneous and near‐conjugate magnetic field perturbations observed over Saturn's northern and southern nightside auroral regions on ∼40 periapsis passes of the Cassini spacecraft during 2008. Structured azimuthal fields that are generally anti‐symmetric north and south were observed at auroral latitudes on all passes, indicative of the signatures of field‐aligned currents associated with magnetosphere‐ionosphere coupling. Two basic field patterns are discerned. One is associated exclusively with ‘lagging’ fields on high‐latitude field lines in both hemispheres, while the other includes a transition from ‘lagging’ to ‘leading’ fields with decreasing latitude in both hemispheres. The principal field‐aligned currents are found to span the region of the open‐closed field line boundary and the outer magnetosphere/ring current, with the region of upward current, potentially associated with ionospheric auroral emissions, usually being located on closed field lines just equatorward of the boundary.

Abstract. The first simultaneous observations of fields and plasmas in Saturn's high-latitude magnetosphere and UV images of the conjugate auroral oval were obtained by the Cassini spacecraft and the Hubble Space Telescope (HST) in January 2007. These data have shown that the southern auroral oval near noon maps to the dayside cusp boundary between open and closed field lines, associated with a major layer of upward-directed field-aligned current (Bunce et al., 2008). The results thus support earlier theoretical discussion and quantitative modelling of magnetosphereionosphere coupling at Saturn , that suggests the oval is produced by electron acceleration in the field-aligned current layer required by rotational flow shear between strongly sub-corotating flow on open field lines and near-corotating flow on closed field lines. Here we quantitatively compare these modelling results (the "CBO" model) with the Cassini-HST data set. The comparison shows good qualitative agreement between model and data, the principal difference being that the model currents are too small by factors of about five, as determined from the magnetic perturbations observed by Cassini. This is suggested to be principally indicative of a more highly conducting summer southern ionosphere than was assumed in the CBO model. A revised model is therefore proposed in which the heightintegrated ionospheric Pedersen conductivity is increased by a factor of four from 1 to 4 mho, together with more minor adjustments to the co-latitude of the boundary, the flow shear across it, the width of the current layer, and the properties of the source electrons. It is shown that the revised model agrees well with the combined Cassini-HST data, reCorrespondence to: S. W. H. Cowley (swhc1@ion.le.ac.uk) quiring downward acceleration of outer magnetosphere electrons through a ∼10 kV potential in the current layer at the open-closed field line boundary to produce an auroral oval of ∼1 • width with UV emission intensities of a few tens of kR.

[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.

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.