Tracing solar wind plasma entry into the magnetosphere using Ti/Te ratio

Benoit Lavraud CESR/CNRS/Université de Toulouse, France

and the Cluster teams

“remote” contribution to the ISSI meeting of Feb. 2009

Tracing solar wind plasma entry into the magnetosphere using Ti/Te ratio

MOTIVATION: solar wind Mach number

At low SW Mach number the magnetosphere behaves very differently; effects include:

• Magnetosheath β properties

• Strong/asymmetric flows in magnetosheath

• Asymmetric magnetopause shape

• Cross Polar Cap Potential saturation

• Changes to dayside reconnection rate

• Alfvén wings/Sawtooth oscillations, …

And lower ion-to-electron temperature ratio in the magnetosheath

MOTIVATION: Tracing plasma entry

Because magnetosheath ion-to-electron temperature ratio is lower during low Mach number (<6), I’ve been wondering whether one might use this property to trace SW plasma entry into the magnetosphere.

This presentation shows preliminary Cluster results that suggest we might indeed be able to do so…

Magnetosheath flow acceleration and asymmetry

Asymmetric flow acceleration, along the flanks only: somewhat similar to a magnetic “slingshot”

Larger flows along flanks might enhance KH transport

Equatorial plane X = -5 RE

Observation of such magnetosheath flow jets

Flows not associated with reconnection and 60% > SW

- Solar wind observations: IMF large and north SW density low

- Cluster observations: Flows B field outside MP Up to 1040 km/s while SW is only 650 km/s

Electrons

Ions

Sheath

sheathshock

dusk

Cluster

SW speed

Magnetopause

Cluster overview of MP/sheath during the same day

Three clear boundary layer intervals with SW-like plasma inside the magnetopause

- Solar wind observations: Long low SW MA interval during a CME/MC

- Cluster observations: One clear instance of KH activity at MP/BL

Three clear boundary layer intervals in total

sheathshock

dusk

Cluster

Magnetopause

KH wave activityNo clear (N-Vx)roll-up

Boundary layerInside MP

Large sheath flows

Temperature ratio observations for this interval

Ti/Te appears conserved for SW plasma in BL, instances where it is not conserved resemble pre-existing PS

Ti/Te NOT conserved

Ti/Te conserved

CONCLUSIONS

• In this event, that BL contain SW plasma is obvious, although confirmed using this test as compared to inner PS plasma

• Possible future implications I would seek:- Determine which processes

(reconnection, KH, else?), if any, conserve this ratio during entry

- Trace entry into inner mag. regions (time-scales)

- How does this info get lost/destroyed? (drifts in inner regions, etc.)

Note: Cluster/THEMIS case around CME of Nov. 20, 2007

EXTRA MATERIAL

Magnetosheath flow dependence on Mach number

Strong flow acceleration : increasing for decreasing MA

Global MHD simulations (BATS-R-US)

for high and low Mach numbers

Illustration of strong sheath flows from Lavraud et al. [2007]

See also Chen et al. [1993], Rosenqvist et al. [2007]

Movie of the field line « slingshot »

Mechanism of magnetosheath flow acceleration

We can estimate the contribution of each force:J x B acceleration dominates at low Mach numbers

- Steady state momentum equation:

- Magnetic forces

- Integration of forces:

Selection of streamline

∂s

MHD simulation for low MA

Y (

RE)

Z (RE)

Bjpvv ).(

)2

().(1 2

B

BBBj

CurvBBp AAAs

v

Note: Not a simple analogy to a “slingshot”, magnetic pressure gradient as important as tension force(~10% 45% 45%)

Observation of such magnetosheath flow jets

Flows not associated with reconnection and 60% > SW

- Solar wind observations: IMF large and north SW density low

- Cluster observations: Flows B field outside MP Up to 1040 km/s while SW is only 650 km/s

Electrons

Ions

Sheath

sheathshock

dusk

Cluster

SW speed

Magnetopause

Flow asymmetry: role of IMF direction

The enhanced flow location follows the IMF orientation+ additional anomalous flow deflections [Nishino et al., 2008]

Flow magnitude and sample field lines from MHD simulations (X = -5 RE)

Acknowledgments

Aaron J. Ridley (Univ. Michigan, USA)Janet Kozyra (Univ. Michigan, USA)Maria M. Kuznetsova and CCMC

(NASA GSFC, USA)and Cluster teams