Abstract
Abstract. We study the dependence of Saturn's magnetospheric magnetic field structure on the interplanetary magnetic field (IMF), together with the corresponding variations of the open-closed field line boundary in the ionosphere. Specifically we investigate the interval from 8 to 30 January 2004, when UV images of Saturn's southern aurora were obtained by the Hubble Space Telescope (HST), and simultaneous interplanetary measurements were provided by the Cassini spacecraft located near the ecliptic ~0.2 AU upstream of Saturn and ~0.5 AU off the planet-Sun line towards dawn. Using the paraboloid model of Saturn's magnetosphere, we calculate the magnetospheric magnetic field structure for several values of the IMF vector representative of interplanetary compression regions. Variations in the magnetic structure lead to different shapes and areas of the open field line region in the ionosphere. Comparison with the HST auroral images shows that the area of the computed open flux region is generally comparable to that enclosed by the auroral oval, and sometimes agrees in detail with its poleward boundary, though more typically being displaced by a few degrees in the tailward direction.
Highlights
The first unambiguous detections of polar ultraviolet (UV) auroras at Saturn were made by the Voyager spacecraft in 1980 and 1981 (e.g. Sandel and Broadfoot, 1981), and were later followed by observations from the Hubble Space Telescope (HST) (Gerard et al, 1995; Trauger et al, 1998)
Direct evidence of the strong connection between Saturn’s UV auroras and the solar wind was first obtained during the joint HST-Cassini campaign in January 2004, when the Cassini spacecraft was upstream of the planet, en route to Saturn orbit insertion (Clarke et al, 2005; Crary et al 2005; Bunce et al, 2006)
In this paper we have investigated the interplanetary magnetic field (IMF)-dependence of the open field line region in Saturn’s ionosphere using the paraboloid model of the magnetosphere, combined with IMF data obtained by the Cassini spacecraft during its approach to Saturn in January 2004
Summary
The first unambiguous detections of polar ultraviolet (UV) auroras at Saturn were made by the Voyager spacecraft in 1980 and 1981 (e.g. Sandel and Broadfoot, 1981), and were later followed by observations from the Hubble Space Telescope (HST) (Gerard et al, 1995; Trauger et al, 1998). The parameters which define Saturn’s magnetospheric magnetic field in the model are as follows: (i) Rss is the distance from Saturn’s centre to the subsolar point on the magnetopause; (ii) Rrc and Rrc are the distances to the outer and inner edges of the ring current, respectively; (iii) R2 is the distance from the planet’s centre to the inner edge of the magnetospheric tail current sheet; (iv) the field magnitude of the tail currents at the inner edge of the tail current sheet is Bt /α0, where α0=(1+2R2/Rss )1/2; (v) is the tilt angle between the magnetic dipole direction and the KSM Z axis (∼25◦ during the January 2004 interval, corresponding to Northern Hemisphere winter conditions); (vi) Brc is the radial component of the ring current magnetic field at the outer edge of the ring current; (vii) the effect of the IMF inside the magnetosphere is given by the uniform field ksBIMF, where BIMF is the IMF vector and kS is the coefficient of its penetration into the magnetosphere With regard to the latter model assumption, we note that Alexeev (1986) obtained a finite-conductivity solution for the magnetic field in the magnetosheath, in which magnetic field diffusion results in only a partial screening of the IMF by the magnetopause. We calculate the size and location of the open flux region in Saturn’s ionosphere using this model, employing parameter values determined by Belenkaya et al (2006b) appropriate for compressed active conditions
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