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Magnetic Interconnection of Saturn's Polar Regions: Comparison of Modelling Results with Hubble Space Telescope UV Auroral

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Magnetic Interconnection of Saturn's Polar Regions: Comparison of Modelling Results with Hubble Space Telescope UV Auroral EGU Journal Logos (RGB) Open Access Open Access Open Access Ann. Geophys., 31, 1447–1458, 2013 www.ann-geophys.net/31/1447/2013/ Advances in Annales Nonlinear Processes doi:10.5194/angeo-31-1447-2013 © Author(s) 2013. CC Attribution 3.0 License.Geosciences Geophysicae in Geophysics Open Access Open Access Natural Hazards Natural Hazards and Earth System and Earth System Sciences Sciences Magnetic interconnection of Saturn’s polar regions: comparisonDiscussions of Open Access Open Access modelling results with HubbleAtmospheric Space Telescope UV auroralAtmospheric images Chemistry Chemistry E. S. Belenkaya1, S. W. H. Cowley2, V. V. Kalegaevand Physics1, O. G. Barinov1, and W. O. Barinova1 and Physics 1 Lomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics, 1(2), Leninskie gory, GSP-1, MoscowDiscussions Open Access 119991, Russian Federation Open Access 2Department of Physics & Astronomy, UniversityAtmospheric of Leicester, Leicester LE1 7RH, UK Atmospheric Correspondence to: E. S. Belenkaya ([email protected])Measurement Measurement Techniques Techniques Received: 14 May 2013 – Revised: 15 July 2013 – Accepted: 16 July 2013 – Published: 28 August 2013 Discussions Open Access Open Access Abstract. We consider the magnetic interconnection of Sat- 1 Introduction Biogeosciences urn’s northern and southern polar regionsBiogeosciences controlled by the Discussions interplanetary magnetic field (IMF), studying in particular the more complex and interesting case of southward IMF, In this paper we study Saturn’s ultraviolet (UV) auroral emis- when the Kronian magnetospheric magnetic field structure sions observed by the Hubble Space Telescope (HST) in Open Access one hemisphere, and their mapping along field lines intoOpen Access is the most twisted. The simpler case of northward IMF is Climate also discussed. Knowledge of the magnetospheric magneticClimatethe equatorial magnetosphere and the conjugate ionosphere. From Earth’s orbit, it is usually only possible to observe au- field structure is very significant, for example, forof investiga- the Past of the Past tion of the electric fields and field-aligned currents in Sat- roras in one hemisphere, the southern aurorasDiscussions prior to the urn’s environment, particularly those which cause the au- recent equinox in mid-August 2009, and the northern auro- ras afterwards. However, close to equinox, emissions from Open Access roral emissions. Here we modify the paraboloid magneto- Open Access spheric magnetic field model employed in previous related both hemispheres can be observed,Earth if obliquely, System with such studies by including higher multipole terms inEarth Saturn’s System inter- images being obtained in the interval January–March 2009 nal magnetic field, required for more detailed considerationsDynamics(Nichols et al., 2009). It is assumed that Saturn’sDynamics auroras are of inter-hemispheric conjugacy, together with inclusion of a caused by accelerated magnetospheric electronsDiscussions associated spheroidal boundary at the ionospheric level. The model is with strong upward-directed field-aligned currents. In order employed to map Southern Hemisphere auroral regions ob- to better understand the origins of these currents and their Open Access Open Access served by the Hubble Space Telescope (HST) inGeoscientific 2008 under location in the magnetosphere, brightGeoscientific auroral regions ob- known IMF conditions to both the equatorialInstrumentation plane and the served in HST images are mappedInstrumentation along field lines into Sat- northern ionosphere. It is shown that the brightestMethods auroral andurn’s magnetosphere and the conjugateMethods ionosphere and using a features map typically to the equatorial region between the newly refined version of the paraboloid magnetospheric mag- central ring current and the outer magnetosphere,Data and thatSystems au- netic field model that has been employedData in previousSystems related roral features should be largely symmetric between the two studies (e.g. Alexeev et al., 2006; BelenkayaDiscussions et al., 2006, Open Access 2007,Open Access 2008). These studies show that the mapping depends hemispheres, except for a small poleward displacement and Geoscientific latitudinal narrowing in the Northern HemisphereGeoscientific compared strongly on the interplanetary magnetic field (IMF) penetrat- with the Southern Hemisphere due to the quadrupole field ing the magnetosphere, withModel the most Development complex and interest- Model Development asymmetry. The latter features are in agreement with the con- ing case occurring when the IMF is directedDiscussions southward, in jugate auroras observed under near-equinoctial conditions in the same direction as the planet’s equatorial field. Unfortu- nately no contemporaneous IMF data are available during early 2009, when IMF data are not available. Open Access Open Access Hydrology andthe HST equinoctial campaign mentionedHydrology above, so and that here Keywords. Magnetospheric physics (planetary magneto- we further study images of the southern aurora obtained in spheres) Earth System2008 during intervals when the CassiniEarth spacecraft System was lo- Sciencescated just upstream of the near-noon bow shockSciences (Belenkaya Discussions Open Access Published by Copernicus Publications on behalf of the EuropeanOpen Access Geosciences Union. Ocean Science Ocean Science Discussions Open Access Open Access Solid Earth Solid Earth Discussions Open Access Open Access The Cryosphere The Cryosphere Discussions 1448 E. S. Belenkaya et al.: Magnetic interconnection of Saturn’s polar regions et al., 2010, 2011). The knowledge obtained can then be used 2 IMF dependence of the projection of Saturn’s polar to further consider the equinoctial images in future studies. zones to the magnetospheric equatorial plane Southward IMF for Saturn is analogous to northward IMF for Earth, since the magnetic dipole axes are oppositely di- We begin by discussing the magnetic mapping that corre- rected for these two planets. The interaction of the IMF with sponds to Southern Hemisphere HST “image A” obtained at the terrestrial magnetosphere has been studied for a long time ∼ 21:48 UT on day 43 of 2008 under southward IMF con- (e.g. Dungey, 1961; Cowley, 1973; Alexeev and Belenkaya, ditions, studied previously by Belenkaya et al. (2011). The 1983; Belenkaya, 1998a, b; Blomberg et al., 2005). In par- Kronian solar magnetospheric (KSM) (X, Y , Z) field com- ticular, Figs. 1 and 2 of Blomberg et al. (2005), based on the ponents measured by Cassini were (0.20, 0.85, 0.24) nT, this results of Alexeev and Belenkaya (1983), illustrate the differ- vector representing a one-hour average taking account of ence in the terrestrial magnetospheric magnetic field struc- the propagation delay between measurement at the space- ture for southward and northward IMF using a simple spher- craft and arrival of effects in the ionosphere (see Belenkaya ical magnetic field model. As in the case of northward IMF et al., 2010, for details). Here we use the same paraboloid for Earth, for southward IMF at Saturn two magnetic neutral magnetosphere field model as the latter authors (for details points occur in the cusp regions, as also demonstrated by the see Alexeev et al., 2006, and Belenkaya et al., 2006), but paraboloid model under these conditions (e.g. Belenkaya et provide calculations of the mapping not only for the south- al., 2011). As the IMF at Saturn rotates from northward to ern but also for the northern polar regions. The paraboloid southward, the two magnetic neutral points move from the model has been tested on several different planetary mag- low-latitude magnetopause to the cusp regions and are lo- netospheres. It has provided good agreement with observa- cated inside the magnetosphere, leading to reconfiguration tions not only for the Earth’s magnetosphere (e.g. Alexeev of the magnetic and electric field structure and convection, et al., 2001), but also at Saturn (Alexeev et al., 2006; Be- and modification of the interaction between the polar regions. lenkaya et al., 2006, 2007, 2008, 2010, 2011), Jupiter (e.g. Here we pay main attention to the latter point. Alexeev and Belenkaya, 2005) and Mercury (Alexeev et al., Kurth et al. (2009) noted that the processes generating Sat- 2008; Belenkaya et al., 2013). For the above case studied by urn’s auroras are not well known yet. Gerard´ et al. (2004) Belenkaya et al. (2010), the model parameters chosen cor- showed that Saturn’s auroral precipitation is usually stronger respond to a high solar wind dynamic pressure (∼ 0.1 nPa), in the morning hours, and noted that the degree of devel- with a distance from the planet’s centre to the subsolar mag- opment of the morning arc could be connected with the netopause of Rss = 17.5 RS. The distances to the outer and north–south orientation of the IMF. Cowley et al. (2004a, inner edges of the ring current are taken to be Rrc1 = 12.5 RS b) suggested that Saturn’s auroral oval is caused by upward- and Rrc2 = 6.5 RS, respectively, with the radial component directed field-aligned currents occurring at the open–closed of the ring current field just outside the thin current sheet field line boundary, thus being strongly connected with the at its outer edge being Brc1 = 3.62 nT. The distance to the IMF, with the dawn–dusk asymmetry being associated with inner edge of the tail current sheet is R2 = 14 RS, with the the effects of Dungey-cycle flow. The Kelvin–Helmholtz field due to the tail current system at this distance, just out- (KH) instability on the morning (mainly) and afternoon side the equatorial current sheet, being Bt/α0, where α0 = 1/2 flanks of the magnetopause represents an alternative mech- (1 + 2R2/Rss) and Bt = 8.7 nT. Here RS is Saturn’s 1-bar anism for auroral generation, as discussed, e.g., by Galo- equatorial radius equal to 60 268 km. The coefficient of IMF peau et al. (1995) in the context of Saturn kilometric radi- penetration appropriate to Saturn, kS, is not known with cer- ation (SKR) emission. The latter authors noted in particu- tainty, but is taken to be equal to 0.2, corresponding to val- lar that the pre-noon SKR sources, which are caused by en- ues determined directly at Earth (e.g.
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