Magnetospheric magnetic field modelling for the 2011 and 2012 HST Saturn aurora campaigns – implications for auroral source regions

Abstract

Abstract.A unique set of images of Saturn's northern polar UV aurora was obtained by the Hubble Space Telescope in 2011 and 2012 at times when the Cassini spacecraft was located in the solar wind just upstream of Saturn's bow shock. This rare situation provides an opportunity to use the Kronian paraboloid magnetic field model to examine source locations of the bright auroral features by mapping them along field lines into the magnetosphere, taking account of the interplanetary magnetic field (IMF) measured near simultaneously by Cassini. It is found that the persistent dawn arc maps to closed field lines in the dawn to noon sector, with an equatorward edge generally located in the inner part of the ring current, typically at ∼ 7 Saturn radii (R S ) near dawn, and a poleward edge that maps variously between the centre of the ring current and beyond its outer edge at ∼ 15 R S , depending on the latitudinal width of the arc. This location, together with a lack of response in properties to the concurrent IMF, suggests a principal connection with ring-current and nightside processes. The higher-latitude patchy auroras observed intermittently near to noon and at later local times extending towards dusk are instead found to straddle the model open-closed field boundary, thus mapping along field lines to the dayside outer magnetosphere and magnetopause. These emissions, which occur preferentially for northward IMF directions, are thus likely associated with reconnection and open-flux production at the magnetopause. One image for southward IMF also exhibits a prominent patch of very high latitude emissions extending poleward of patchy dawn arc emissions in the pre-noon sector. This is found to lie centrally within the region of open model field lines, suggesting an origin in the current system associated with lobe reconnection, similar to that observed in the terrestrial magnetosphere for northward IMF.

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

“…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 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.…”

“…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.…”

“…For southward IMF (Fig. S2) the situation is more complicated (e.g., Belenkaya et al, 2014), since the neutral line does not exist either at the dayside magnetopause or in the tail, but neutral points occur in the cusps in the open model. For southward IMF, the tail length determined by the intersection of the last open field line with the magnetopause could be at an even greater distance than for northward IMF.…”