A magnetospheric vortex as the source of periodicities in Saturn’s magnetosphere

1. A magnetospheric vortex as the source of periodicities in Saturn’s magnetosphere Krishan Khurana Institute of Geophysics and Planetary Physics, UCLA, Los Angeles, CA, 90095.

2. The correct clock mechanism must explain: • The rotation rate of SKR source in the summer hemisphere and its variations over the season. • Plasma density variations in the inner magnetosphere at the SKR period. • “Cam” currents in the inner magnetosphere at the SKR period. A rotating uniform field in the equatorial plane with Br leading Bf. • Nearly out of phase relationship between Bq and Bf in the inner magnetosphere. Rotating partial ring current • 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.

3. A successful model must explain … Winter Clock Summer Clock Gurnett et al. 2010

4. The correct clock mechanism must explain: • The “Rotation rates” of SKR sources in the summer and winter hemispheres and their variations over the season. • Plasma density variations in the inner magnetosphere at the SKR period. • “Cam” currents in the inner magnetosphere at the SKR period. A rotating uniform field in the equatorial plane with Br leading Bf. • Nearly out of phase relationship between Bq and Bf in the inner magnetosphere. Rotating partial ring current • 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.

5. Gurnett et al. (2007)

6. The correct clock mechanism must explain: • The “Rotation rates” of SKR sources in the summer and winter hemispheres and their variations over the season. • Plasma density variations in the inner magnetosphere at the SKR period. • “Cam” currents in the inner magnetosphere at the SKR period. A rotating uniform field in the equatorial plane with Br leading Bf. • Nearly out of phase relationship between Bq and Bf in the inner magnetosphere. Rotating partial ring current • 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.

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

9. The correct clock mechanism must explain: • The “Rotation rates” of SKR sources in the summer and winter hemispheres and their variations over the season. • Plasma density variations in the inner magnetosphere at the SKR period. • “Cam” currents in the inner magnetosphere at the SKR period. A rotating uniform field in the equatorial plane with Br leading Bf. • Nearly out of phase relationship between Bq and Bf in the inner magnetosphere. Rotating partial ring current. • ENA rotations at the SKR period (partial ring current). • Current sheet tilt in a frame rotating at the SKR period. • The SKR clock has characteristics of both a rotating beam and a strobe.

10. Paranicas et al. 2005, GRL Khurana et al. 2009, JGR

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

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

13. Saturn’s summer clock • For the summer clock, the plasma containing the SKR sources must be subcorotational and the field-aligned currents should be distributed in a sinusoidal fashion to account for the SKR’s periodic modulation. • A plasma convection system with these essential properties was mooted by Gurnett et al. (2007) and Goldreich and Farmer (2007) to explain the plasma density periodicities in the inner magnetosphere. • I will now show that the summer clock is a manifestation of the inner magnetospheric two cell convection system proposed by these authors.

14. Two-cell plasma convection system Assume a mass outflow rate of 300 kg/s, the width of outflow sector = p/2, D= 2 Rs. At r = 6 RS where the local plasma density is 30 cm-3 (Persoon et al. 2006), we get an average Vr ~1.35 km/s. Convection cycle time = 28 days or 60 rotations. Long memory Inertial currents driving the system would be too weak to detect directly. After Gurnett et al 2007

15. Current system associated with the convection system

16. The effect of enhanced conductivity in southern ionosphere “Cam” currents can be generated without a sinusoidal conductivity distribution in the magnetosphere.

19. Why are the middle magnetospheric SKR sources quiescent? • SKR generation requires accelerated electrons created by strong field-aligned electric potentials (~ 10 keV and higher). • Large field-aligned potentials develop in regions just above the ionosphere to facilitate the MI coupling when the ambient plasma population is unable to support the large currents required (Knight, 1973). • In fast-rotating magnetospheres, the high atomic-mass ions are confined tightly to the magnetodisc by the centrifugal forces. Ambipolar potentials develop which also confine most of the lighter ionic species and electrons to the magnetodisc and only hotter electrons with temperatures ~ 100 eV and higher are able to overcome the ambipolar potential and participate in current closure at high latitudes (Ray et al. 2009).

20. Schippers et al. 2008

22. Explains electron density modulation Explains why ne and Bf are in phase (The outflow region (max Bf) has maximum density. Gurnett et al. (2007)

23. Explains “cam” currents Does not require sinusoidal variation of ionospheric conductivity

24. ENA and partial ring current periodicities The inflow region would act like a giant suction hose which gathers and funnels hotter plasma of the middle magnetosphere towards the inner magnetosphere. At the mouth of the inflow region (8-12 Rs), the plasma is hot and tenuous. In the outflow region, the plasma is cold and dense forming a partial ring current. The plasma in the ring current region may not be corotational but the pressure peak would be.

25. Conclusions • The summer clock is a manifestation of the inner magnetospheric two cell convection system proposed by Gurnett et al. (2007) and Goldreich and Farmer (2007). • The postulated mechanism explains the “rotation rate” of the summer clock and its variation over the season. • It explains the plasma density variations in the inner magnetosphere at the SKR period. • It explains the “cam” currents in the inner magnetosphere at the SKR period. A rotating uniform field in the equatorial plane with Br leading Bf. • The magnetospheric vortex would imprint itself on the ionosphere and possibly the thermosphere (the flywheel) explaining its persistence and longevity.

26. Reserve slides follow

27. The conceptual model explains seasonal variations of the clock periods.

28. Why are the middle magnetospheric SKR sources quiescent? • Current density required in the ionosphere for corotation enforcement in the equatorial plane is given by: (Nichols and Cowley, 2004) • Thus at L = 6, assuming an outflow rate of 300 kg/s average current density in each hemisphere is 2.5 nA/m2. Because mainly the summer hemisphere is supplying most of the torque, the summer hemisphere current should be doubled or = 5 nA/m2 • Also, because the southern hemisphere exerts torque on the northern hemisphere. The average current density may be 10 nA/m2 and the peak current density may be 20 nA/m2. • This should be compared to the electron thermal current density given by qnve,thermal. At L =6, the hot electrons have a temperature of only 50 eV and a density of ~ 0.003 cm-3 for an estimated Js = 2 nA/m2 . • At L =12, the density of 1 keV electrons is 0.1 cm-3 for Js (L=12) = 300 nA/m2.

30. Explains current sheet tilt

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

33. Sittler et al. 2006