Evidence at Saturn for an Inner Magnetospheric Convection Pattern, Fixed in Local Time

M. F. Thomsen(1), R. L. Tokar(1),E. Roussos(2), M. Andriopoulou(2), C. Paranicas(3), P. Kollman(2), and

C. S. Arridge(4) (also thanks to Don Gurnett)(1)Los Alamos National Laboratory, Los Alamos, NM

(2)Max Planck Institute for Solar System Research, Lindau, Germany(3)Johns Hopkins University Applied Physics Laboratory, Laurel, MD

(4)Mullard Space Science Laboratory, University College London, Holmbury St. Mary, Surrey, UK

Magnetospheres of the Outer PlanetsBoston, MA

11-15 July 2011

New Survey of Local Time Dependence of CAPS/IMS Ion Plasma

Moments(Follow-up to Thomsen et al., JGR, 2010; extended

to 30 Sep 2010)

Same filters:• Actuator operating• Spacecraft not rotating• Moments-calculation iteration converges• Corotation in FOV

Significant scatter, but trend is evident: Temperatures are lower near noon

H+

H2+

W+

6 < r < 7 7 < r < 8 8 < r < 9 9 < r < 10

(low latitude; corotation in FOV)

Tem

pera

ture

(eV)

Simple cos(LT) fits

Why is the plasma temperature higher on the nightside than on the dayside?

Adiabatic variations in plasma circulating asymmetrically?

T ~ B ~ r-3

High B, high T

Low B, low T

Offset needed to map midnight T(r) curve to noon T(r) curve

Other evidence for asymmetric drift orbits: Day-night

asymmetry in cold electron temperature

(CAPS/ELS)

e-

[see also DeJong et al., GRL, 2011, for similar asymmetries in 12-100 eV electrons.]

Day-night asymmetry in cold electron temperature

e-

Other evidence for asymmetric drift orbits: Satellite microsignature

locations

Microsignatures observed OUTSIDE satellite orbit on dayside, INSIDE of

satellite orbit on nightside.

Roussos et al., J. Geophys. Res., 112, A06214, 2007.

Tethys4.9 Rs

Dione6.2 Rs

Absorption microsignatures trace the radial component of particle drifts

Satellite orbitMicrosignature created at midnight

Microsignature created at noon

Tethys(L=4.9)

Tethys

Dione

Dione(L=6.2)

(Radial offsets of this magnitude could also arise from gradient drift effects if the dayside magnetic field is ~30-40% higher than the nightside field, but no such field asymmetries are observed [e.g., Kollmann et al., JGR, 2011].)

e-

TethysDione

Other evidence for asymmetric drift orbits:Day-Night Asymmetries in Energetic Particle Fluxes

[Paranicas et al., JGR, A09214, 2010.]

41-60 keV electrons

Day-Night Asymmetries in Energetic Particle Fluxes: Phase-Space Densities

21 < LT < 3

9 < LT < 15

21 < LT < 3

9 < LT < 15

Day-Night Asymmetries in Energetic Particle Fluxes: Phase-Space Densities

DAYSIDE

NIGHTSIDE

e-

Tethys

Dione

Day-Night Asymmetries in Energetic Particle Fluxes: Phase-Space Densities

ElectronsMu=0.6Mu=2.0Mu=5.0

ProtonsMu=1.2Mu=2.3Mu=4.6Mu=8.0Mu=12.0Mu=20.0

Other evidence for asymmetric drift orbits:Day-night asymmetries in A-Ring absorption signatures

Paranicas, JGR, 115, A07216, 2010.

Ee~220-485 keV

12.5 LT

23.2 LT23.2 LT

12.5 LT

Similar offset in total electron density

[after Gurnett et al., Science, 2005 (courtesy, Don Gurnett)]

R=2.442

R=2.342

R=2.247

R=2.239

Comparison of drop-offs of electron density and energetic particles at A-Ring edge

=> Offsets exist all the way to the rings.

Inbound(12.5 LT)

Outbound(23.2 LT)

DR(noon-midnight)~0.1 Rs

Noon-midnight asymmetry of drift orbits requires a net outflow from midnight through dawn to noon, i.e., a net noon-to-midnight electric field, in addition to the corotational field.

For a uniform noon-to-midnight electric field, the azimuthal component is

where f is the local time in degrees.

The radial displacement in drifting from f1=0 (midnight) to f2=180 (noon) is

FLOW

E

So, for a dipole magnetic field:

Comparison with other estimates

Paranicas et al. [2010]:“To reproduce our data, we require the drift paths to be shifted toward noon (not dawn or dusk). Furthermore, a shift of 0.09 RS in our calculation corresponds to an electric field of at least 5 × 10−4 V/m pointing from noon to midnight.”

0.5 mV/mnoon to midnight at L~2.7

Roussos et al. [2007]:“The … electric field … should have a strength of more than 0.1 mV/m to account for the observed displacements.”

0.1 mV/mnoon to midnight at L~4.9

Roussos et al. [2010]: Azimuthal electric field strengths <~1 mV/m were derived from the energy dependence of the relative displacements of microsignatures of Tethys.

See also Roussos et al., this meeting

Summary

• Evidence for asymmetric drift paths (rnoon>rmidnight)– Day/night ion temperature variation (Tnoon<Tmidnight)

– Day/night cold-electron temperature variation (Tnoon<Tmidnight)– Radial offsets in satellite absorption microsignatures– Day/night energetic-particle flux differences– Day/night asymmetries in A-Ring absorption signatures (energetic

particles and total electron density)

• Inferred day/night radial offsets are consistently ~0.1-1 Rs• These displacements, affecting low-energy particles as well as

high-energy ones, are consistent with drifts in a noon-to-midnight electric field (L<10 in the equatorial plane).

• Necessary electric field magnitudes ~0.1-1 mV/m, possibly decreasing with radial distance.