Benoit Lavraud

CESR/CNRS, Toulouse, France

Uppsala, May 2008

The altered solar wind – magnetosphere interaction at low Mach numbers:Magnetosheath and magnetopause

OUTLINE

• Introduction and motivation

• Magnetosheath β properties

• Asymmetric flows in the magnetosheath

• Asymmetric magnetopause shape

• Kelvin-Helmholtz instability

• Other expected effects

• Occurrence distribution of solar wind MA

• Conclusions

Note: we primarily use global MHD simulations here

INTRODUCTION :Mach numbers, shocks and plasma β

- Mach numbers:

AlfvénMA = VSW / VA with VA~|B|/√N

MagnetosonicMMS = VSW / √(VA

2 + VS2)

So that MA > MMS

- Plasma β:

- Rankine-Hugoniot shock jump conditions: Bdownstream = f(Bupstream) Ndownstream = f(Nupstream) etc.

2/

..2B

TkN B

MOTIVATION :An “unknown” or “over-looked” magnetosphere

- Pivotal role of magnetosheath - Implications for CME-driven storms

Lobes

Plasma sheet

Magnetopause

Solar wind

Magnetosheath

High Mach number = High-β magnetosheath

Cusp

Low Mach number = Low-β magnetosheath

Magnetosheath β as a function of SW Mach number

Low-β magnetosheath prevails during low Mach numbers:Magnetic forces become important

- Rankine-Hugoniot shock Jump conditions

- MMS = VSW / √(VA2 + VS

2)

- MA = VSW / VA

- MA > MMS

Varying IMF

Perpendicular shock case

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

Equatorial planes

Magnetosheath flow acceleration and asymmetry

Asymmetric flow acceleration, along the flanks only: a magnetic “slingshot” effect?

Global MHD simulation (BATS-R-US)

for low Mach number (MA = 2)

Equatorial plane X = -5 RE

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 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

See also: Rosenqvist et al. [2007]

Flow asymmetry: role of IMF direction

The enhanced flow location follows the IMF orientation

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

Magnetopause asymmetry: role of magnetic forces

The magnetopause is squeezed owing to enhanced magnetic forces in the magnetosheath

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

Magnetopause asymmetry: role of IMF direction

The magnetopause squeezing follows the IMF orientation

Current magnitude and sample field lines from MHD simulation (X = -5 RE)

Occurrence of the Kelvin-Helmholtz instability

May expect KH instability to grow faster with larger flows

- Used their list of such KH vortices to inspect their dependence on MA

- Rolled-up KH vortices may be identified in data Hasegawa et al. [2006]

N & V high

N & V low

Vx

Vx higher

Sheath

Sphere

Occurrence of giant spiral auroral features

KH instability, large flow and spiral aurora may be related

- Giant spiral auroral features have been identified during great storms (Dst < -250 nT) (courtesy of J. Kozyra) + case by Rosenqvist et al. [2007]

7 out of 8 occurred for MA < 5

See: Rosenqvist et al. [2007]

SOME OTHER LOW MA EFFECTS

• Changes in dayside reconnection rate

• Cross-polar cap potential saturation

• Sawtooth oscillations

• Plasma depletion layer (disappears at high MA)

• Heating at bow shock (Ti/Te and tracing)

• Drifts and losses to the magnetopause (radiation belts and ring current)

• Alfvén wings in sub-Alfvénic flow

• Bow shock acceleration and reflection

else …?

Occurrence distribution of solar wind Mach numbers

CMEs, and particularly the subset of magnetic clouds, have low Mach numbers

- Binning of OMNI dataset

- Lists from: CMEs: Cane and Richardson [2003] MCs: Lepping et al. [2006] HSS: Borovsky and Denton [2006]

See also:Gosling et al. [1987]Borovsky and Denton [2006]

CONCLUSIONS

• SW – magnetosphere interaction is significantly altered during low Mach number

• All these effects are thus important during CME-driven storms

• They must occur at other magnetospheres (Mercury: low MA and no ionosphere Moons, e.g., like Io in sub-Alfvenic flows)

Acknowledgments

Joseph E. Borovsky (Los Alamos, USA)Aaron J. Ridley (Univ. Michigan, USA)Janet Kozyra (Univ. Michigan, USA)Maria M. Kuznetsova and CCMC

(NASA GSFC, USA)and Cluster teams