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

Abstract

Abstract.To gain better understanding of auroral processes in Saturn's magnetosphere, we compare ultraviolet (UV) auroral images obtained by the Hubble Space Telescope (HST) with the position of the open-closed field line boundary in the ionosphere calculated using a magnetic field model that employs Cassini measurements of the interplanetary magnetic field (IMF) as input. Following earlier related studies of pre-orbit insertion data from January 2004 when Cassini was located ∼ 1300 Saturn radii away from the planet, here we investigate the interval 12-15 February 2008, when UV images of Saturn's southern dayside aurora were obtained by the HST while the Cassini spacecraft measured the IMF in the solar wind just upstream of the dayside bow shock. This configuration thus provides an opportunity, unique to date, to determine the IMF impinging on Saturn's magnetosphere during imaging observations, without the need to take account of extended and uncertain interplanetary propagation delays. The paraboloid model of Saturn's magnetosphere is then employed to calculate the magnetospheric magnetic field structure and ionospheric open-closed field line boundary for averaged IMF vectors that correspond, with appropriate response delays, to four HST images. We show that the IMF-dependent open field region calculated from the model agrees reasonably well with the area lying poleward of the UV emissions, thus supporting the view that the poleward boundary of Saturn's auroral oval in the dayside ionosphere lies adjacent to the open-closed field line boundary.

Open and partially closed models of the solar wind interaction with outer planet magnetospheres: the case of Saturn

“…The Alexeev et al (2006) model was used in papers by Belenkaya et al (2006bBelenkaya et al ( , 2007Belenkaya et al ( , 2010Belenkaya et al ( , 2011Belenkaya et al ( , 2013Belenkaya et al ( , 2014 to compare the high-latitude edge of Saturn's auroral oval with the model open-closed ionospheric field line boundary, the results of which supported the idea that these boundaries are located rather close to each other Cowley et al, 2004a;Bunce et al, 2008). The IMF data used in these studies were obtained from Cassini magnetometer data when available, while the auroral boundaries were determined from Hubble Space Telescope (HST) UV images.…”

“…9, in which for all cases from Table 1 The paraboloid model has been used for different magnetized planets and has given results that allowed us to explain a lot of observational data. In particular, it has been extensively applied to Saturn (Alexeev et al, 2006;Belenkaya et al, 2006bBelenkaya et al, , 2007Belenkaya et al, , 2010Belenkaya et al, , 2011Belenkaya et al, , 2013Belenkaya et al, , 2014. The advantages of the model relate to its block structure; i.e., the magnetic field of each magnetospheric current system is calculated using its own program (block).…”

“…The open paraboloid model of Saturn's magnetospheric magnetic field was constructed and described by Alexeev et al (2006) and has been used by Belenkaya et al (2006bBelenkaya et al ( , 2007Belenkaya et al ( , 2010Belenkaya et al ( , 2011Belenkaya et al ( , 2013Belenkaya et al ( , 2014 in a number of subsequent investigations. The sources of magnetic field in this model are (i) the planetary magnetic field, (ii) the tail current system, (iii) the ring current, and (iv) the magnetopause shielding currents.…”

“…We have selected several HST UV images as listed in Table 1, eight images for southward IMF and seven images for northward, for which the open-closed boundary in the ionosphere was calculated using the open paraboloid model and published in papers by Belenkaya et al (2007Belenkaya et al ( , 2010Belenkaya et al ( , 2014 2014]). The corresponding IMF vectors were determined in each case from Cassini magnetometer data when the spacecraft was located in the solar wind upstream of the planet.…”

Optimization of Saturn paraboloid magnetospheric field model parameters using Cassini equatorial magnetic field data

“…In this study, however, we focus on the global "paraboloid model" of Saturn's magnetosphere, in which the outer magnetopause boundary is taken to form a paraboloid of revolution about the planet-Sun line (Alexeev et al, 2006). This model has been used, in particular, to investigate the dependence of the magnetospheric magnetic structure and the origins of the aurorae on the direction and strength of the interplanetary mag-642 E. S. Belenkaya et al: Optimization of Saturn paraboloid magnetospheric field model parameters netic field (IMF), employing data from the Cassini spacecraft and the Hubble Space Telescope as inputs (Belenkaya et al, 2006a(Belenkaya et al, , 2007(Belenkaya et al, , 2010(Belenkaya et al, , 2011(Belenkaya et al, , 2013. In addition to the internal planetary field, taken to be the three-term axisymmetric model derived by Burton et al (2010) in the most recent work, this model contains representations of the field due to the ring current and the tail current, all contained within the paraboloid magnetopause by the surface current flowing on that boundary, to which a penetrating field due to the IMF can be added.…”