Possible mechanisms of Saturn's aurora generation

1. Possible mechanisms of Saturn's aurora generation E.S. Belenkaya1, S.W.H. Cowley2, J.D. Nichols2, V.V. Kalegaev1, and M.S. Blokhina1 1Institute of Nuclear Physics, Moscow State University, Vorob’evy Gory, 119992 Moscow, Russia 2Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

2. UV images of the southern dayside oval were obtained by the Hubble Space Telescope (HST) in February 2008. • Simultaneously Cassini observed IMF upstream of Saturn’s dayside bow shock. • Using a global paraboloid model of the magnetospheric magnetic field we mapped Saturn’s auroras into the magnetosphere. • The model took into account the observed high solar wind dynamic pressure and IMF measured by Cassini and suitably lagged (~7h: ~6 h for the ionospheric reaction to solar wind conditions and 1 h for light propagation to Earth). • UV images were examined for northward and southward IMF. • The magnetospheric field structure was very different in these cases, however, the dayside UV oval has a consistent location relative to the field structure in each case. • The poleward boundary of the oval is located close to the open-closed field line boundary and thus maps to the vicinity of the magnetopause. • The equatorward boundary of the oval maps typically near the outer boundary of the equatorial ring current appropriate to the compressed conditions prevailing. Abstract

3. Although the magnitude of the IMF (~0.3 nT) is significantly lower than magnetic field of Saturn(BS0=21160) nT, the reconnection is very significant in forming the global structure and dynamics of the kronian magnetosphere. • We study an interval, unique to date, in which HST imaging was coordinated with simultaneous interplanetary observations by Cassini located immediately upstream of Saturn’s bow shock. • While the dynamics of Saturn’s magnetosphere is driven mainly by the planet’s rotation (e.g. Badman and Cowley, 2007),the auroras and related radio emissions are also respond to increases in solar wind dynamic pressure(Clarke et al., 2005, 2009; Crary et al., 2005; Kurth et al., 2005; Jackman et al., 2005; Bunce et al., 2006; Badman et al., 2008). • Changes in the IMF lead to variations in the size and position of the open field region (calculated in the paraboloid model) that are reflected in the aurora (Belenkaya et al., 2007, 2008, 2010). Interaction of the solar wind with Saturn’s magnetosphere

4. Magnetopause:x/Rss =(y2+z2)/2Rss2 • Bm = Bd(BS0,RS) + Bsd(BS0,RS,Rss) + BTS(Rss,R2,Bt) + • + Brc(Brc1,Rrc1,Rrc2) + Bsrc(Brc1,Rrc1,Rrc2,Rss) + b(kS,BIMF). • Paraboloid model includes planetary magnetic field, magnetic field of magnetospheric current systems shielded by magnetopause currents, and an IMF partially penetrated into the magnetosphere. divB=0; divj=0. • Time-dependent input model parameters: • Rss ismagnetopause subsolar distance; • R2isdistance to the inner edge of the tail current sheet; • Bt/0isfield at the inner edge of the tail current sheet; • 0 = (1 + 2 R2/Rss)1/2; • Brc1isradial component of the field at the outer edge of the ring current; • Rrc1isdistance to the outer edge of the ring current; • Rrc2isdistance to the inner edge of the ring current; • BIMFisIMF; • kSiscoefficient of IMF penetration into magnetosphere. Magnetospheric magnetic field in the paraboloid model (Alexeev et al., 2006)

5. The angular velocity of Saturn:S=1.638·10−4 s−1 • Magnetic moment of Saturn:МS=4.6·1013 G·km3=0.2G·RS3 • Magnetic field at Saturn’s equator:BS0=21160nT • Model parameters • Compressed magnetosphere (Pioneer-11) psw~0.08 nPa(Belenkaya et al., 2006) • Rss=17.5RS, Rrc1=12.5RS , Rrc2=6.5RS, Brc1 =3.62nT, R2=14RS, Bt=8.7nT • Intermediate magnetosphere (Voyager-1) psw~0.03 nPa(Belenkaya et al., 2008) • Rss=22RS, Rrc1=15RS , Rrc2=6.5RS, Brc1 =3nT, R2=18RS, Bt=7nT • Expanded magnetosphere (Cassini SOI orbit) psw~0.01 nPa(Alexeev et al., 2006) • Rss=28RS, Rrc1=24.5RS , Rrc2=6.5RS, Brc1 =2.2nT, R2=22.5RS, Bt=5.3nT Dependence of Rsson Psw • For Earth:Rss~ Psw-1/6(e.g., Shue et al., 1997), • forJupiter:Rss~ Psw-1/4 –Psw-1/5(Huddleston et al., 1998), • for Saturn:Rss~ Psw-1/4.3(Arridge et al., 2006) Saturn

6. Trajectory of the Cassini spacecraft in KSM coordinates from near the periapsis of Rev 58 to near the periapsis of Rev 59 • The three panels show the Cassini trajectory projected onto the X-Y, X-Z, and Y-Z planes. • The Cassini position is marked by star at the beginning of each day. • The intersections of the magnetopause and bow shock with these planes are shown by the solid and dashed red lines, respectively, obtained from the models of Kanani et al. (2010)andMasters et al. (2008) for a psw=0.1 nPa. • The blue segment of the trajectory corresponds to the interval (12-15 Febr 2008) for which Cassini measured IMF(Belenkaya at al., 2010).

7. Magnetic field for DOY 43 (12 Febr) to 46 (15 Febr) of 2008 • Three components of the magnetic field in KSM coordinates (Bx, By, Bz), and the field magnitude|B| innT. • Green - magnetosphere, red - magnetosheath, and blue - solar wind. The blue vertical stripes labelled “A” to “D” correspond to the suitably lagged times of HST images. • The spacecraft was located in the solar wind just upstream of Saturn’s dayside bow shock from the middle of DOY 43 (12 Febr) to the end of DOY 45 (14 Febr). For this interval we have simultaneous observations of the IMF (1nT) and UV images of southern aurora.

8. Cassini data from 12-15 February 2008. Propagation and response time effects • We have averaged the IMF data over a 1 h interval centred on this time in order to obtain a reasonable representative value of the IMF relating to each image (Belenkaya et al., 2010). • Saturn’s magnetosphere was compressed by the solar wind, with the dynamic pressure peaking at 0.1 nPa on DOY 44 and 45. • The corresponding model parameters are: • Rss = 17.5 RS, Rrc1 = 12.5 RS, • Rrc2 = 6.5 RS, Brc1 = 3.62 nT, • R2 =14 RS, Bt= 8.7 nT; = -8.4о • It was found (Belenkaya et al., 2010) that for the studied cases the delay from solar wind observation to ionosphere is ~7 h 10 min. • This time includes three time delays: • 1. The one-way light travel time between Saturn at the time of emission and the HST position at the time the image was obtained (~1 h 10 min). • 2. The solar wind propagation time between Cassini and the reconnection regions on Saturn’s magnetopause (30 min). • 3. The auroral and flow response time in Saturn’s ionosphere resulting from changes in the IMF following their arrival at the magnetopause (330 min) (Belenkaya et al., 2010).

9. Field lines emerging from Saturn’s ionosphere in the noon-midnight meridian for IMF vectors corresponding to HST images A, B, C, and D • For ks = 0.2, the 2D meridianal noon- midnight magnetic field structure is shown. Field lines on the nightside terminate where they intersect the magnetopause. • For southward and northward IMF the magnetospheric magnetic field structures are quite different even for IMF magnitudes less than 1 nT. • As a consequence, corresponding changes in the shape and size of the open field line region in the ionosphere should also be expected. Bz<0 Bz>0

10. The poleward and equatorward boundaries of the emission are shown by red crosses. Solid orange line shows the open field line region boundary for ks = 0.2. • A: IMF={0.20, -0.85, -0.24} nT • С: IMF={-0.11, 0.28, 0.25} nT • HST UV images of Saturn’s southern auroras are projected onto a spheroidal surface 1100 km above the 1 bar level. At the top are given DOY and the start time of the 20 min combined exposure time. A and C UV images of Saturn’s southern auroras

11. Open field line «window» at the southern magnetopausefor northward IMF (image C) Intersection of the open magnetic field lines with the southern magnetopause for the case C with ks = 0.2. Solid curves show projections along the field lines of constant ionospheric latitude from ‑74° to ‑90° at steps of 4°, while dashed lines show projections of lines of constant LT (with step 1h). The principal meridians (noon, dusk, dawn, midnight) are shown by the green lines. IMF={-0.11, 0.28, 0.25} nT

12. Mappings of field lines into the KSM equatorial plane for image C with ks = 0.2 • Equatorial projection of the equatorwardauroral oval boundary is shown by the blue squares and of the poleward boundary by the purple squares. The black two circles show the inner and outer boundaries of the equatorial ring current. The black curve across the tail indicates the inner edge of the tail current sheet. The pale blue lines show projections of the fixed southern ionospheric latitudes of ‑70o, ‑74o, and ‑78o. The red curve corresponds to the boundary of the closed field lines.ks = 0.2. • The dayside auroral oval maps from the dayside magnetopause (i.e. close to the boundary between open and closed field lines), to close to the outer edge of the ring current at ~12 RS. IMF={-0.11, 0.28, 0.25} nT

13. Open field line «windows» at the southern and northernmagnetopausefor southward IMF (image A) b) IMF={0.20, -0.85, -0.24 } nT

14. Top panel shows the near-planet region, while bottom panel shows a larger region extending into the magnetospheric tail. • The closed field line region is contained between the red and green lines, where the former is projected from the southern hemisphere and the latter from the northern. • The boundaries of the auroral ovalare shown by the blue and purple squares for the equatorward and poleward boundaries, respectively. The black two circles show the inner and outer boundaries of the equatorial ring current, while the black curve across the tail indicates the inner edge of the tail current sheet. The pale blue curves show projections of fixed southern ionospheric latitudes of ‑70o, ‑74o, ‑78o, and ‑82o; ks = 0.2. • The oval maps between the outer region of the ring current and the open-closed field boundary. Mappings of field lines into the KSM equatorial plane for image A with ks = 0.2

15. Earth’s magnetosphere for southward and northward IMF Southward IMF. Open field lines do not intersect equatorial plane. The last closed field lines form a single curve for the southern and the northern polar caps. Northward IMF. Open field lines of the southern and the northern polar caps intersect the equatorial plane forming two different curves representing open-closed boundary.

16. For southward and northward IMF, measured by Cassini, the mapping along magnetic field lines of the observed dayside UV oval and calculated open-closed boundary was performed to the equatorial plane and magnetopause. • It occurs that even a rather moderate IMF (<1nT) is very significant for the magnetospheric magnetic field structure of Saturn possessing strong intrinsic magnetic field (BS0=21160nT)and located at 9.5 AU from the Sun. • To date a few simultaneous data sets of the UV oval and IMF exist for Saturn, one obtained in January 2004 during Cassini approach at distances from the planet of ~1300 RS, resulting in significant IMF propagation time uncertainties, the second in February 2008 when four UV image were obtained when Cassini was in the solar wind just upstream from the dayside bow shock. • Calculations were undertaken using the paraboloid magnetosphere model appropriate to the compressed magnetospheric conditions prevailing and suitably lagged and averaged Cassini IMF data. • Mapping in the parabolod model of the UV oval boundaries allowed us to understand the connection between the southern polar ionosphere and domains within the equatorial magnetosphere and/or magnetopause, thus helping to clarify the relationship between the observed emissions and the main mechanisms of auroral generation(Hill, 2005; Sittler et al., 2006; Cowley et al., 2004). Results of mapping in the Saturn’s paraboloid model of the observed by the HST dayside UV auroral oval

17. 1. Field-aligned currents and auroras could be associated with the corotation breakdown in the inner and middle magnetosphere (Hill, 2005). • 2. The precipitating particles (~10 keV) reside in the outer boundary of the dayside plasma sheet and ring current, where the plasma is highly turbulent due to the enhanced wave activity. This mechanism of the auroral generation is referred to as the centrifugal instability model (Sittler et al., 2006). • 3. A ring of upward-directed currents should flow in the vicinity of the boundary between open and closed field lines due to the sheer in the azimuthal velocity (Cowley et al., 2004). Alternative mechanisms of the auroral generation at Saturn

18. 1. The corotation breakdown at Saturn occurs in the Enceladus torus at 3 - 4 RS, mapping to about ‑62° in the southern ionosphere. The corresponding field-aligned currents(Hill, 2005)could be associated with an auroral oval in IR emission at Saturn (a ‘secondary oval’), lying equatorward of the main UV oval investigated here(Stallard et al., 2008). • 2. The equatorward boundary of the oval was found to map typically to distances of ~12 RS in the equatorial plane, near the outer boundary of the model ring current appropriate to the compressed conditions prevailing. Thus,the mechanism suggested by Sittler et al.(2006) could act in this region. • 3. The poleward edge of the UV oval, was found typically to be located near the boundary between open and closed field lines, in agreement with the mechanism suggested by Cowley et al. (2004a,b) and with the previous conclusions of Belenkaya et al. (2006, 2007, 2008, 2010). Conclusions