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

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.

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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 B.

• Nearly out of phase relationship between B and B 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.2

A successful model must explain …

Gurnett et al. 2010 3

Summer Clock

Winter Clock

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 B.

• Nearly out of phase relationship between B and B 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.4

5

Gurnett et al. (2007)

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 B.

• Nearly out of phase relationship between B and B 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.6

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Field-aligned current system proposed by Southwood and Kivelson (2007)

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Equatorial view

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 B.

• Nearly out of phase relationship between B and B 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.9

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Paranicas et al. 2005, GRL

Khurana et al. 2009, JGR

A closer look at the SKR periodicities

Gurnett et al. 201011

A Comparison of SKR and atmospheric rotation periods

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Faster Rotation

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.

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Two-cell plasma convection system

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After Gurnett et al 2007

rDmM ri V2

Assume a mass outflow rate of 300 kg/s, the width of outflow sector = /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.

Current system associated with the convection system

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The effect of enhanced conductivity in southern ionosphere

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“Cam” currents can be generated without a sinusoidal conductivity distributionin the magnetosphere.

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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).

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Schippers et al. 2008

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Explains electron density modulation

22Gurnett et al. (2007)

Explains why ne and B are in phase (The outflow region (max B has maximum density.

Explains “cam” currents

23Does not require sinusoidal variation of ionospheric conductivity

ENA and partial ring current periodicities

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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.

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 B.

The magnetospheric vortex would imprint itself on the ionosphere and possibly the thermosphere (the flywheel) explaining its persistence and longevity.

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Reserve slides follow

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The conceptual model explains seasonal variations of the clock periods.

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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. 28

2p

4

||R

6

pB

LMj

||j

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Explains current sheet tilt

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

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33Sittler et al. 2006