[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.