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Beno i t Lavraud CESR/CNRS, Toulouse, France Uppsala, May 2008

Beno i t Lavraud CESR/CNRS, Toulouse, France Uppsala, May 2008. The altered solar wind – magnetosphere interaction at low Mach numbers: Magnetosheath and magnetopause. Introduction and motivation Magnetosheath β properties Asymmetric flows in the magnetosheath

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Beno i t Lavraud CESR/CNRS, Toulouse, France Uppsala, May 2008

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  1. Benoit LavraudCESR/CNRS, Toulouse, FranceUppsala, May 2008 The altered solar wind – magnetosphere interaction at low Mach numbers:Magnetosheath and magnetopause

  2. Introduction and motivation • Magnetosheath βproperties • 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 OUTLINE

  3. Mach numbers: • Alfvén • MA = VSW / VA with VA~|B|/√N • Magnetosonic • MMS = VSW / √(VA2 + VS2) • So thatMA > MMS • Plasma β: • Rankine-Hugoniot shock jump conditions: • Bdownstream = f(Bupstream) • Ndownstream = f(Nupstream) • etc. INTRODUCTION :Mach numbers, shocks and plasma β

  4. MOTIVATION :An “unknown” or “over-looked” magnetosphere Magnetosheath Lobes High Mach number = High-β magnetosheath Plasma sheet Solar wind Cusp Magnetopause - Pivotal role of magnetosheath - Implications for CME-driven storms Low Mach number = Low-β magnetosheath

  5. Magnetosheath β as a function of SW Mach number Perpendicular shock case • - Rankine-Hugoniot shock Jump conditions • MMS = VSW / √(VA2 + VS2) • MA = VSW / VA • MA > MMS Varying IMF  Low-β magnetosheath prevails during low Mach numbers:Magnetic forces become important

  6. Magnetosheath flow dependence on Mach number Equatorial planes Global MHD simulations (BATS-R-US) for high and low Mach numbers • Strong flow acceleration : increasing for decreasing MA

  7. Magnetosheath flow acceleration and asymmetry Equatorial plane X = -5 RE Global MHD simulation (BATS-R-US) for low Mach number (MA = 2) • Asymmetric flow acceleration, along the flanks only: a magnetic “slingshot” effect?

  8. Mechanism of magnetosheath flow acceleration MHD simulation for low MA • Steady state momentum equation: • Magnetic forces • Integration of forces: Selection of streamline Y (RE) ∂s Z (RE) Note:Not a simple analogy to a “slingshot”, magnetic pressure gradient as important as tension force (~10% 45% 45%)  We can estimate the contribution of each force:J x B acceleration dominates at low Mach numbers

  9. Observation of magnetosheath flow jets See also: Rosenqvist et al. [2007] dusk Sheath Cluster Electrons shock sheath Ions • Solar wind observations: • IMF large and north • SW density low • Cluster observations: • Flows B fieldoutside MP • Up to 1040 km/s while • SW is only 650 km/s SW speed Magnetopause  Flows not associated with reconnection and 60% > SW

  10. Flow asymmetry: role of IMF direction Flow magnitude and sample field lines from MHD simulations (X = -5 RE)  The enhanced flow location follows the IMF orientation

  11. Magnetopause asymmetry: role of magnetic forces Current magnitude and sample field lines from MHD simulations (X = -5 RE)  The magnetopause is squeezed owing to enhanced magnetic forces in the magnetosheath

  12. Magnetopause asymmetry: role of IMF direction Current magnitude and sample field lines from MHD simulation (X = -5 RE)  The magnetopause squeezing follows the IMF orientation

  13. Occurrence of the Kelvin-Helmholtz instability N & V low Sphere Vx higher Vx Sheath N & V high • Rolled-up KH vortices may be identified in dataHasegawa et al. [2006] • Used their list of such KH vortices to inspect their dependence on MA  May expect KH instability to grow faster with larger flows

  14. Occurrence of giant spiral auroral features • 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]  KH instability, large flow and spiral aurora may be related

  15. 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 …? SOME OTHER LOW MA EFFECTS

  16. Occurrence distribution of solar wind 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]  CMEs, and particularly the subset of magnetic clouds, have low Mach numbers

  17. SW – magnetosphere interaction is significantlyaltered 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 ionosphereMoons, e.g., like Io in sub-Alfvenic flows) CONCLUSIONS

  18. 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

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