The Ring Current of SaturnNick Sergis

Office of Space Research, Academy of Athens

Cassini/MIMI, MAG and CAPS colleagueswith special acknowledgements to C.M. Jackman, D.G. Mitchell,

D.C. Hamilton, N. Krupp, S.M. Krimigis, and M.K. Dougherty.

2011 MOP meeting, Boston, MA, USA, July 11-15, 2011

Since the early 1980ies, several studies have attempted to derive the radial profile of the current density in the Saturnian magnetosphere:

Connerney et al., 1983magnetic field data(Voyager I and II)

Mauk et al., 1985magnetic field and energetic particle data(Voyager I and II)

Beard and Gast, 1987magnetic field data(Pioneer 11, Voyager I and II)

mA/km2

However, the full particle energy distribution was not available then.

Sergis et al., 2009

2004-2006 Cassini equatorial coverage

Giampieri and Dougherty used a Connerney-like model to fit Pioneer 11 and Voyager 1 and 2 magnetic field data. An axisymmetric current sheet model is inadequate for describing the Saturnian ring current (local time and temporal variation not compatible with the assumption of an axisymmetric disk). Then Cassini got there…

Giampieri and Dougherty, AnGeo, 2004

Notice that the noon-to-dusk sector is missing…

Bunce et al., JGR, 2008

Inner radius (6.5 RS)

Outer radius (15 RS to 21 RS)

Thickness (5 RS)

As was later shown, when particle datawere also included in the analysis, this assumption proved incorrect. Jphi dropswith radial distance faster than 1/r.

Jphi~1/ρ

6 8 10 12 14 16 18 200.00

0.05

0.10

0.15

0.20

J/Jmax

range

//P

PA

inertial contribution

pressure gradientanisotropy

The radial, steady-state, force balance equation can be solved for the azimuthal current density:

Average ring current density

inertial inside 9 RS

pressure gradient driven beyond 10 RS

Jφ has a maximum and drops outwards faster than 1/r

Sergis et al., GRL, 2010

6 7 8 9 10 11 12 13 14 150.1

1

10

100

par

ticl

e b

eta

range (RS)

Equatorial orbits 2004-2006, total (CAPS+MIMI) particle pressure.

Thomsen et al., JGR, 2010.E<45 keV particle pressure.

Plasma β in the Saturnian magnetosphere

Saturn possesses a high-β magnetosphere with β>1 outside 8 RS, reaching an (average) maximum of 2-10 near 11 to 14 RS.

High-β values (~ 1) are maintained up to the dayside magnetopause

Kellett at al., 2011 analyzed the magnetic field and plasma data (2004-2006) per Local Time sector and found higher (x2) current densities in the nightside. Still, noon-to-dust is missing…

2004-2011 Cassini equatorial coverage and suprathermal pressure map.

Pre

ssur

e (d

yne/

cm2

)

All local times are now covered(but with a 7-year span…)

Occurrence probability map for the equatorial suprathermal plasma pressure in the Saturnian magnetosphere from

Cassini/MIMI, all local times, July 2004-April 2011.

6 8 10 12 14 16 18-12

-11

-10

-9

-8

range (RS)

LO

G[s

up

rath

erm

al p

ress

ure

(d

yne

/cm

2 )]

5

6

8

10

13

17

22

27

35

occ

urr

en

ce p

rob

ab

ility

(a

rb. u

nits

)

max PSUP

region max rPSUP

2004-2011, full MIMI data set (E>3 keV) NEW RESULTS!

Sergis et al., JGR, 2009

Old (2009) pressure probability map (2004-2008)

We can extend our analysis to include the new available data…

0 3 6 9 12 15 18 21 2410-11

10-10

10-9

10-8

Su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

Local Time (hr)

Distribution of the means for 8 to 11 RS

0 3 6 9 12 15 18 21 2410-11

10-10

10-9

10-8

Su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

Local Time (hr)

Distribution of the means for 8 to 11 RS

0 3 6 9 12 15 18 21 2410-11

10-10

10-9

10-8

Su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

Local Time (hr)

means medians

8 to 11 RS

We now look at the local time variation of the maximumsuprathermal pressure(plateau between 8 and 11 RS)

Clear local time variation, perfectly noon-to-midnight symmetric, with nightside (max) pressure being higher by a factor of ~4.

510-10 + 310-10cos(0.262LT)

Mueller et al., JGR, 2010

Weighted distribution of the energetic particle injections in local time.

0

2

4

6

8

10

12

14

16

18

20

22

DAWN

DUSK

NOON suprathermal pressure

10-9

10-10

10-8

10-11

10-10

10-9

10-8

6 8 10 12 14 16 18

10-10

10-9

su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

range (RS)

0000-0600 0600-1200 1200-1800 1800-2400

Suprathermal pressure radial profile per Local Time sector

6 8 10 12 14 16 18

10-10

10-9

su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

range (RS)

0000-0600 0600-1200 1200-1800 1800-2400

6 8 10 12 14 16 18

10-10

10-9

su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

range (RS)

0000-0600 0600-1200 1200-1800 1800-2400

6 8 10 12 14 16 18

10-10

10-9

su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

range (RS)

0000-0600 0600-1200 1200-1800 1800-2400

6 8 10 12 14 16 18

10-10

10-9

su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

range (RS)

0000-0600 0600-1200 1200-1800 1800-2400

6 8 10 12 14 16 18

10-10

10-9

0000-0600 0600-1200 1200-1800 1800-2400

su

pra

the

rma

l pre

ssu

re (

dyn

e/c

m2 )

range (RS)

Fitting with 4th order polynomials

Very small current !

8 9 10 11 12 13 14 15

0.0

1.0x10-19

2.0x10-19

3.0x10-19

4.0x10-19

0000-0600 0600-1200 1200-1800 1800-2400

d

Psu

p/dr

(N/m

3 )

range (RS)

x7

Suprathermal pressure gradient (rPsup) along the radial direction per Local Time sector

8 9 10 11 12 13 14 150

5

10

15

20

25

30

35

40

0000-0600 0600-1200 1200-1800 1800-2400

J phi(p

A/m

2 )

range (RS)

By including the magnetic field measurements (courtesy of M. Dougherty and C. Jackman of the Cassini/MAG team), we can derive directly the average suprathermal contribution to the pressure grad-driven component of the Saturnian ring current, per local time sector.

Significant local time asymmetry. This ring current component almost vanishes in the noon-to-dusk sector!

Jphi=(1/B)(∂P/∂r)

average

-20 -10 0 10 20-20

-10

0

10

20

YS

ZS(R

S)

XSZS

(RS)

28.4%

Let us take a look at the coverage statistics:equatorial plane 6 to 18 RS (2004-2011)

21.3%26.9%

23.4%

TO SUN

28.4%23.4%

26.9% 21.3%

-20 -10 0 10 20-20

-10

0

10

20

YS

ZS(R

S)

XSZS

(RS)

Equatorial plane6 to 18 RS

(2004-2011)

28.4%23.4%

21.3%26.9%

2004 2005 2006 2007 2008 2009 2010 20110

10

20

30

40

50

60

70

80

perc

ent e

poch

ical

dis

trib

utio

n

year

2004 2005 2006 2007 2008 2009 2010 20110

10

20

30

40

50

60

70

80

perc

ent e

poch

ical

dis

trib

utio

n

year

2004 2005 2006 2007 2008 2009 2010 20110

10

20

30

40

50

60

70

80

pe

rce

nt e

po

chic

al d

istr

ibu

tion

year2004 2005 2006 2007 2008 2009 2010 20110

10

20

30

40

50

60

70

80pe

rcen

t epo

chic

al d

istr

ibut

ion

year

Seasonal effect?Probably not…

SUN

ConclusionsAnalysis of plasma, energetic particle and magnetic field data acquired by Cassini from the beginning of the mission to this day (2004-2011) show that:

2. The radial profile of the suprathermal pressure and the corresponding pressure gradient (-∂P/∂r) present significant dependence on local time being higher by a factor of 2 to 8 in the nightside, resulting a local time dependent ring current. The average ring current density appears as recently reported (Sergis et al., 2010 Kellett et al., 2011).

3. As Cassini continues its equatorial plane orbits, more data will be analyzed and better coverage will be obtained. By early 2012 (perhaps EGU 2012) we will be able to have a complete picture of the ring current structure. Also, conditions symmetric to equinox (e.g. 2007-2011) will be available for direct comparison (seasonal change?)

1. The equatorial plane of the Saturnian magnetosphere is being progressively covered in all local time sectors for a wide range of radial distances (6-18 RS), making possible the mapping of the particle pressure and the study of the overall structure of the planetary ring current.

Thank you!

-20 -10 0 10 20-20

-10

0

10

20 10-9

10-10

YS

ZS(R

S)

XSZS

(RS)

1.00E-11

1.00E-10

1.00E-9

10-11

supr

athe

rmal

pre

ssur

e (d

yne/

cm2 )

SUN

A preliminary map of the energeticparticle pressure, based on roughmodeling using simple functions applied per local time x range bin(just a suggestion…)

-20 -15 -10 -5 0 5 10 15 20

10-10

10-9

supr

athe

rmal

pre

ssur

e (d

yne/

cm2)

x (RS)

MODEL DATA

Evaluation test example:midnight-to-noon line

midnight noon

The suprathermal imprint of the Saturnian ring current has also been captured by the ENA camera of Cassini

Sergis et al., JGR, 2009

Dione 6.3 RS

Rhea 8.7 RS

Titan 20.2 RS

s/c positionLat=58°LT=1902R=27.9 RS

-20 -15 -10 -5 0 5 10 15 20-20

-15

-10

-5

0

5

10

15

20

-20 -10 0 10 20-20

-10

0

10

20 10-9

10-10

YS

ZS(R

S)

XSZS

(RS)

1.00E-11

1.00E-10

1.00E-9

10-11

supr

athe

rmal

pre

ssur

e (d

yne/

cm2 )

6 8 10 12 14 16 18

-12

-11

-10

-9

-8

range (RS)

LO

G[s

up

rath

erm

al p

ress

ure

(d

yne

/cm

2 )]

5

6

8

10

13

17

22

27

35

occ

urr

en

ce p

rob

ab

ility

(a

rb. u

nits

)

6 8 10 12 14 16 18-12

-11

-10

-9

-8

range (RS)

LO

G[s

up

rath

erm

al p

ress

ure

(d

yne

/cm

2 )]

5

6

8

10

13

17

22

27

35

occ

urr

en

ce p

rob

ab

ility

(a

rb. u

nits

)

6 8 10 12 14 16 18-12

-11

-10

-9

-8

range (RS)

LO

G[s

up

rath

erm

al p

ress

ure

(d

yne

/cm

2 )]

8

10

13

17

22

27

35

occ

urr

en

ce p

rob

ab

ility

(a

rb. u

nits

)

6 8 10 12 14 16 18-12

-11

-10

-9

-8

range (RS)

LO

G[s

up

rath

erm

al p

ress

ure

(d

yne

/cm

2 )]

11

14

18

23

29

35

occ

urr

en

ce p

rob

ab

ility

(a

rb. u

nits

)

6 8 10 12 14 16 18-12

-11

-10

-9

-8

range (RS)

LO

G[s

up

rath

erm

al p

ress

ure

(d

yne

/cm

2 )]

18

23

29

35

occ

urr

en

ce p

rob

ab

ility

(a

rb. u

nits

)

Let us focus to the highest probabilities (most often conditions)