IMF dependence of the open-closed field line boundary in Saturn's ionosphere, and its relation to the UV auroral oval observed by the Hubble Space Telescope

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.

IMF dependence of Saturn's auroras: modelling study of HST and Cassini data from 12–15 February 2008

“…In this paper we have further studied the relationship between the dynamics of Saturn's auroral emissions and variations in the IMF in the upstream solar wind, following discussions of the January 2004 joint Cassini-HST imaging campaign data by Belenkaya et al (2007Belenkaya et al ( , 2008. While the latter campaign provided an important initial data set for study, the large ∼ 1300 R S displacement between the spacecraft and the planet with a related ∼ 17 h radial propagation delay results in considerable uncertainties in the identification of detailed correspondences between the interplanetary data and the auroral images.…”

“…The effect of the IMF inside the magnetosphere is given by adding the uniform field k s B IMF , where B IMF is the IMF vector and k s is the coefficient of its penetration into the magnetosphere. The latter coefficient is not well determined for Saturn, but here we consider values of 0.2 and 0.8 which span the likely range, as discussed previously by Belenkaya et al (2007).…”

“…As indicated above, the location of the open-closed field line boundary in the ionosphere for given interplanetary conditions is determined here via field line calculations using the Saturn magnetospheric model described by Alexeev et al (2006) and Belenkaya et al (2006bBelenkaya et al ( , 2007Belenkaya et al ( , 2008, in which the magnetopause is taken to be a paraboloid of revolution about the Saturn-Sun line. The model is consequently expressed in KSM coordinates.…”

“…Since according to the results presented by Clarke et al (2009) the interval studied corresponds overall to one in which Saturn's magnetosphere was subject to compression by the solar wind, with the dynamic pressure peaking at ∼ 0.1 nPa on DOY 44 and 45, here we choose to use a relatively compressed model throughout. Specifically we employ the model derived originally by Belenkaya et al (2006b) to describe the field during the Pioneer11 flyby, corresponding to a dynamic pressure of ∼ 0.08 nPa, which was subsequently successfully employed by Belenkaya et al (2007) to describe Saturn's magnetosphere under high solar wind dynamic pressure conditions. The corresponding set of model parameters is R ss = 17.5 R S , R rc1 = 12.5 R S , R rc2 = 6.5 R S , B rc1 = 3.62 nT,R 2 = 14 R S , and B t = 8.7 nT.…”

“…These auroral brightenings can last for hours to days, in some cases longer than the duration of the solar wind pressure increase that caused it. Belenkaya et al (2007Belenkaya et al ( , 2008 used magnetospheric magnetic field models incorporating lagged Cassini magnetic field measurements to study the January 2004 events, the results indicating that changes in the IMF also lead to variations in the size and position of the open field region in the southern polar ionosphere that are reflected in the aurora. However, determination of the detailed correspondence between auroral images and related interplanetary conditions using this data set presents challenges, due to the considerable distance between the spacecraft and the planet during the interval.…”