Abstract
Abstract. We determine the field-aligned mapping of Saturn's auroras into the magnetosphere by combining UV images of the southern dayside oval obtained by the Hubble Space Telescope (HST) with a global model of the magnetospheric magnetic field. The model is tailored to simulate prevailing conditions in the interplanetary medium, corresponding to high solar wind dynamic pressure and variable interplanetary magnetic field (IMF) strength and direction determined from suitably lagged field data observed just upstream of Saturn's dayside bow shock by the Cassini spacecraft. Two out of four images obtained in February 2008 when such simultaneous data are available are examined in detail, exemplifying conditions for northward and southward IMF. The model field structure in the outer magnetosphere and tail is found to be very different in these cases. Nevertheless, the dayside UV oval is found to have a consistent location relative to the field structure in each case. The poleward boundary of the oval is located close to the open-closed field boundary and thus maps to the vicinity of the magnetopause, consistent with previous results. The equatorward boundary of the oval then maps typically near the outer boundary of the equatorial ring current appropriate to the compressed conditions prevailing. Similar results are also found for related images from the January 2004 HST data set. These new results thus show that the mapped dayside UV oval typically spans the outer magnetosphere between the outer part of the ring current and the magnetopause. It does not encompass the region of primary corotation flow breakdown within the inner Enceladus torus.
Highlights
This paper is concerned with the structure of the magnetic field in the outer regions of Saturn’s magnetosphere, the mapping of field lines from the high-latitude ionosphere to the equatorial regions and magnetopause, and what one can learn from this about the mapping and origins of Saturn’s polar auroras
Belenkaya et al (2006b, 2007, 2008, 2010) have studied the relationship between Saturn’s aurora and the IMFdependent field structure of the outer magnetosphere using the paraboloid field model, examining the correspondence between the auroral oval and the boundary between open and closed field lines. Such studies require the simultaneous availability of both ultra violet (UV) auroral images obtained by the Hubble Space Telescope (HST) and interplanetary magnetic field (IMF) data upstream from Saturn obtained by Cassini, of which only two such joint campaign data sets exist at present
778 We provide a similar discussion of image A in Fig. 2, obtained by the HST at ∼22:00 UT on DOY 43 with a corresponding lagged kronocentric solar-magnetospheric (KSM) IMF vector of (0.20, −0.85, −0.24) nT, such that the Z-component was southward-directed in this case, opposite to case C (Belenkaya et al, 2010)
Summary
This paper is concerned with the structure of the magnetic field in the outer regions of Saturn’s magnetosphere, the mapping of field lines from the high-latitude ionosphere to the equatorial regions and magnetopause, and what one can learn from this about the mapping and origins of Saturn’s polar auroras. Belenkaya et al (2006b, 2007, 2008, 2010) have studied the relationship between Saturn’s aurora and the IMFdependent field structure of the outer magnetosphere using the paraboloid field model, examining the correspondence between the auroral oval and the boundary between open and closed field lines Such studies require the simultaneous availability of both UV auroral images obtained by the HST and IMF data upstream from Saturn obtained by Cassini, of which only two such joint campaign data sets exist at present. (RS is Saturn’s 1 bar equatorial radius equal to 60 268 km.) Subsequently, Belenkaya et al (2010) studied HST auroral images obtained on an approximately daily basis during a twoweek period in February 2008 (see Clarke et al, 2009, for further campaign details) For much of this interval Cassini was located inside Saturn’s magnetosphere, but it emerged into the solar wind near apoapsis on the dayside of the planet during DOY (day of year) 43 to 46, such that the upstream IMF could be monitored with much reduced propagation delay and uncertainty. We briefly compare our results with those derived from selected related images from the January 2004 data set
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