Xianzhe Jia 1 , Margaret Kivelson 1,2 , and, Tamas Gombosi 1

1. An Atmospheric Vortex as the Driver of Saturn’s Electromagnetic Periodicities:1. Global Simulations Xianzhe Jia1, Margaret Kivelson1,2, and, TamasGombosi1 1.Dept. of Atmospheric, Oceanic and Space Sciences, Univ. of Michigan, Ann Arbor, MI 2.Dept. of Earth and Space Sciences, Univ. of California at Los Angeles, Los Angeles, CA Magnetospheres of Outer Planets, Boston, July 11-15, 2011

2. Introduction: Magnetospheric periodicities • Characteristics: • Close to planetary rotation period • Drift of order 1% per year • Different in north and south • Appears to have seasonal dependence • The slowly drifting, seasonally varying period has led Gurnett et al. (2007, 2010), Don Mitchell (private communication), Southwood and Kivelson (2009) and others to suggest that the ionosphere/thermosphere is a likely source region. • If the ionosphere is to control the magnetosphere, field-aligned currents are needed. • Vorticity drives field-aligned currents. • Here we use a global MHD model to investigate what high-latitude ionospheric vortices can do to the magnetosphere. • Previously proposed drivers • Centrifugally driven convection system • (Goldreich and Farmer, 2007; Gurnett et al., 2007) • Plasma anomaly in the inner magnetosphere (Carbary et al., 2007) • Rotating asymmetry of energetic particle fluxes in inner/middle magnetosphere (Khurana et al., 2009) • Periodic plasma injections • (Paranicas et al., 2005; Carbary et al., 2008; • Mitchell et al., 2009; Brandt et al., 2010) • Thermospheric winds (C. G. A. Smith, 2011) An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

3. Global MHD model setup and input parameters • Use the BATSRUS MHD model with a mass source and a coupled ionosphere-magnetosphere • Simulation domain-: -576 RS < X < 96 RS,-192 RS < Y, Z < 192 RS;Inner boundary at 3 RS • Total mass-loading rate ~ 170 kg/s • Steady solar wind (400 km/s) flowing perpendicular to the spin/dipole axis • Southward IMF (0.5 nT) to minimize solar wind influence • Magnetosphere and ionosphere are coupled through field-aligned currents. • First run the global MHD model to create a quasi-steady state magnetosphere. • Then introduce a flow vortex in the ionosphere (see next slide) to see the response of the magnetosphere/ionosphere system. An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

4. A flow vortex in the southern ionosphere/thermosphere (Viewed from the north) • Flow vortex model: Use one cycle of Y15,1(q,f), centered at 70o latitude • Vortical flow drives field-aligned currents (see the southern ionosphere) • FACs flow from the ionosphere to the magnetosphere and to the opposite ionosphere • A weaker vortical flow develops in the passive hemisphere (SP,S= 3S, SP,N= 1S). An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

5. Periodic plasmoid releases in the tail (the movie shows color contours of plasma pressure and 3D field lines plotted at fixed locations at r=20 Rs; Inset shows FACs perturbation in the southern ionosphere) An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

6. Bow shock and magnetopause oscillations • The movie on the previous slide shows that both bow shock and the magnetopause oscillate in time. • The left plot shows the subsolar locations of BS and MP extracted from our model as function of time. • The two boundaries oscillate at the planetary rotation period, consistent with the Cassini observations of Clarke et al. (2010a & b, JGR). An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

7. Comparison with Cassini MAG observations Rev 32 (Nov. 2006): Inclined orbit • The field components of data and model generally oscillate in phase. • Sharp changes of Bf , produced by sheets of field-aligned current (“Cam” current), are well captured. • The model also captures the depression of Bq near closest approach resulting from the ring current and the oscillation of dBqresulting from the asymmetric ring current. • Inbound, the zero-crossing of dBr is delayed in the data because the warped current sheet is not modeled in the present simulation (SW flow is ⊥ to the dipole axis). • The depth of dBq minimum near closest approach is not captured, probably because of our mass-loading profile. (the contributions of a centered dipole have been subtracted from both sets of curves) An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

8. Comparison with Cassini MAG observations Rev 23 (Apr. 2006): Near-equatorial orbit • Near closest approach (between ~ 04/28 and 04/30), the depression of Bqresults from the ring current. • The oscillation in dBq results from the asymmetric ring current. • Excellent correspondence of phase of periodicity in all components. Amplitude slightly off near CA, close to inner bndy. of the simulation. An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

9. Comparison of phase relations of magnetic perturbations (for sources in the south and in the north) Source located in the SOUTH Source located in the NORTH (dBr,dBq,Bj) (dBr,dBq,Bj) An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

10. How about other forms of flow vortex? • We have run a test case with a single vortex, i.e., retain only the vortex driving upward FAC) • In response to the upward FACs imposed by the single vortex, the magnetosphere-ionosphere interaction also produces sheets of downward flowing FACs. Dual vortices Single vortex Imposed An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

11. The magnetospheric response to the single vortex appears qualitatively similar to that seen in the case with dual vortices. Dual vortices Single vortex An atmospheric vortex as the driver of Saturn's electromagnetic periodicities

12. Summary and Conclusions • We have examined the effects of a flow vortex fixed in the ionosphere/thermosphere by using a global MHD model that couples magnetosphere and ionosphere. • Our model reproduces a host of periodic properties of the magnetosphere observed during southern summer including: • A current system (the “Cam” current) flowing between the two ionospheres varying roughly sinusoidally with longitude • Periodic plasmoid releases in the tail • An asymmetric ring current linked to periodic variations of field magnitude • Periodic oscillations of the magnetopause and bow shock • More details about the magnetosphere/ionosphere responses in our model will be given in the next presentation by Margaret Kivelson. An atmospheric vortex as the driver of Saturn's electromagnetic periodicities