An Atmospheric Vortex as the Driver of Saturn’s Electromagnetic Periodicities: 1. Global Simulations Xianzhe Jia 1, Margaret Kivelson 1,2, and, Tamas Gombosi 1 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

Introduction: Magnetospheric periodicitiesIntroduction: Magnetospheric periodicities Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 2  Characteristics:  Close to planetary rotation period  Drift of order 1% per year  Different in north and south  Appears to have seasonal dependence PPPPreviously proposed drivers CCCCentrifugally driven convection system (Goldreich and Farmer, 2007; Gurnett et al., 2007) PPPPlasma anomaly in the inner magnetosphere (Carbary et al., 2007) RRRRotating asymmetry of energetic particle fluxes in inner/middle magnetosphere (Khurana et al., 2009) PPPPeriodic plasma injections (Paranicas et al., 2005; Carbary et al., 2008; Mitchell et al., 2009; Brandt et al., 2010) TTTThermospheric winds (C. G. A. Smith, 2011)  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.

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

A flow vortex in the southern ionosphere/thermosphereA flow vortex in the southern ionosphere/thermosphere Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 4 (Viewed from the north) Flow vortex model: Use one cycle of Y 15,1 (  ), centered at 70 o latitude Flow vortex model: Use one cycle of Y 15,1 (  ), centered at 70 o latitude Vortical flow drives field-aligned currents (see the southern ionosphere) Vortical flow drives field-aligned currents (see the southern ionosphere) FACs flow from the ionosphere to the magnetosphere and to the opposite ionosphere FACs flow from the ionosphere to the magnetosphere and to the opposite ionosphere A weaker vortical flow develops in the passive hemisphere (  P,S = 3S,  P,N = 1S). A weaker vortical flow develops in the passive hemisphere (  P,S = 3S,  P,N = 1S).

Periodic plasmoid releases in the tailPeriodic plasmoid releases in the tail Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 5 (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)

Bow shock and magnetopause oscillations Bow shock and magnetopause oscillations Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 6 The movie on the previous slide shows that both bow shock and the magnetopause oscillate in time. 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 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). The two boundaries oscillate at the planetary rotation period, consistent with the Cassini observations of Clarke et al. (2010a & b, JGR).

Comparison with Cassini MAG observationsComparison with Cassini MAG observations Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 7 Rev 32 (Nov. 2006): Inclined orbit Inbound, the zero-crossing of dB r is delayed in the data because the warped current sheet is not modeled in the present simulation (SW flow is ⊥ to the dipole axis). Inbound, the zero-crossing of dB r 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 dB  minimum near closest approach is not captured, probably because of our mass-loading profile. The depth of dB  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) The field components of data and model generally oscillate in phase. The field components of data and model generally oscillate in phase. Sharp changes of B , produced by sheets of field-aligned current (“Cam” current), are well captured. Sharp changes of B , produced by sheets of field-aligned current (“Cam” current), are well captured. The model also captures the depression of B  near closest approach resulting from the ring current and the oscillation of dB  resulting from the asymmetric ring current. The model also captures the depression of B  near closest approach resulting from the ring current and the oscillation of dB  resulting from the asymmetric ring current.

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

Comparison of phase relations of magnetic perturbations (for sources in the south and in the north) Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 9 Source located in the SOUTH ( d B r, dB   B   Source located in the NORTH ( d B r, dB   B  

How about other forms of flow vortex?How about other forms of flow vortex? Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 10 We have run a test case with a single vortex, i.e., retain only the vortex driving upward FAC) 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. 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

Xianzhe Jia 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. The magnetospheric response to the single vortex appears qualitatively similar to that seen in the case with dual vortices. Dual vortices Single vortex

Summary and ConclusionsSummary and Conclusions Xianzhe Jia An atmospheric vortex as the driver of Saturn's electromagnetic periodicities 12 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. 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: 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 A current system (the “Cam” current) flowing between the two ionospheres varying roughly sinusoidally with longitude Periodic plasmoid releases in the tail Periodic plasmoid releases in the tail An asymmetric ring current linked to periodic variations of field magnitude An asymmetric ring current linked to periodic variations of field magnitude Periodic oscillations of the magnetopause and bow shock 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. More details about the magnetosphere/ionosphere responses in our model will be given in the next presentation by Margaret Kivelson.