The Source of Planetary Period Oscillations in Saturn’s magnetosphere

1. The Source of Planetary Period Oscillations in Saturn’s magnetosphere Krishan Khurana Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, 90095. Jonathan L. Mitchell Department of Physics, Westmont College, Santa Barbara. Ingo C. F. Mueller-Wodarg Department of Physics, Imperial College, London.

2. Saturn’s dual SKR clocks Winter Clock Summer Clock Gurnett et al. 2010

3. The correct clock mechanism must explain: • The rotation rate of SKR source in the summer hemisphere and its variations over the season. • “Cam” currents in the inner magnetosphere at the SKR period. A rotating uniform field in the equatorial plane with Br leading Bf. • Nearly in phase relationship between Bq and Br in the inner magnetosphere. Rotating partial ring current • Plasma density variations in the inner magnetosphere at the SKR period. • ENA rotations at the SKR period. • Current sheet tilt in a frame rotating at the SKR period. • The SKR clock has characteristics of both a rotating beam and a strobe.

4. Gurnett et al. (2007)

5. Core field (Feb 2006 – July 2006)

6. Field-aligned current system proposed by Southwood and Kivelson (2007) Equatorial view

7. Modeling the core field Khurana et al. 2017

8. The current system in 3-d (0.9 MA/radian)

9. The “Cam” current Khurana et al. 2017

10. The upper-stratospheric convection driven by auroral heating: 1 Khurana et al. 2017

11. The upper-stratospheric convection driven by auroral heating: 2

12. Generation of tangential and normal components by momentum impulses Khurana et al. 2017

13. Phase delays in Br and Bf Khurana et al. 2017

14. Distant Plasma currents

15. The “Cam” current Khurana et al. 2017

16. Core region currents + Distant plasma currents Khurana et al. 2017

17. Energetics • The best fit to the observations was obtained using I0 = 0.25 MA (corresponding to an average current of 0.91 MA/radian). It is instructive to evaluate the total torque applied by this current on the northern ionosphere segment of width df = : This torque should be compared to the total angular momentum of the subcorotating magnetosphere  1.7x1021 kg m2/s. Thus the torque is capable of increasing the rotational velocity by 10% in approximately 1 hour.

18. Energetics - 2 • The angular momentum flux at a radial distance r in the equatorial plane is given by: at r = 15 and a mass outflow rate of 300 kg/s. • Thus most of the torque received by the magnetosphere is lost to the outflowing plasma outside the core region.

19. Conclusions • We show that the observed oscillations are the manifestations of two global convectional conveyor belts excited below the northern and southern auroral zones of Saturn by auroral heating. • We demonstrate that it is likely that a feedback process develops in which the magnetosphere expends energy to drive convection in Saturn’s upper stratosphere but gains back an amplified share in the form of angular momentum. • Propagation phase delays are consistent with observations, unambiguously confirming that their source lies in the auroral-zone upper atmosphere. • The proposed model is consistent with the properties of magnetic field and currents at low and high latitudes.

20. Reserve slides follow

21. Explains current sheet tilt

22. Explains why the SKR is both a rotating beam and a strobe

23. A closer look at the SKR periodicities Gurnett et al. 2010

24. A Comparison of SKR and atmospheric rotation periods Faster Rotation

25. Field-aligned current system proposed by Southwood and Kivelson (2007) Equatorial view

26. Magnetosphere/ionosphere coupling and heating of upper stratosphere 0.01 J/m3 10mw/m2