« Chocs sans collisions : étude d’objet astrophysique par les satellites Cluster »

1. «  Chocs sans collisions : étude d’objet astrophysique par les satellites Cluster » Vladimir Krasnoselskikh + équipe Plasma Spatial LPCE / CNRS-University of Orleans, and Cluster colleagues S. Bale, M. Balikhin, P. Decreau, T. Horbury, H. Kucharek, V. Lobzin, M. Dunlop, M. Scholer, S. Schwartz, S. Walker and others

2. Collisionless shocks : new results from Cluster Plan • Shocks in space plasmas and in astrophysics • Opened questions in shock physics • Simulations and theory • Multi-point measurements, what can they add to single satellite studies in space: Cluster mission • Small scale structure of the electric fields • Problem of stationarity • Problem of particle acceleration.

3. Collisionless shocks: new results from Cluster Supernova remnant in Magellan cloude

5. Collisionless shocks : new results from ClusterEarth’s bow shock Tsurutani and Rodriguez, 1981

6. MHD BLAST WAVES FROM POINT AND CYLINDRICAL SOURCES: COMPARISON WITH OBSERVATIONS OF EIT WAVES AND DIMMINGS

9. Collisionless shocks : new results from Cluster From Giacalone et al.,

12. Collisionless shocks : new results from Cluster

13. Notion de 2 nombre de Mach critique • 1985: Krasnoselskikh, Nonlinear motions of a plasma across a magnetic field, Sov. Phys. JETP • 1986: Arefiev, Krasnoselskikh, Balikhin, Gedalin, Lominadze, Influence of reflected ions on the structure of quasi-perpendicular collisionless shock waves, Proceesings of the Jiunt Varenna-Abastumani International School-Workshop on Plasma Astrophysics, ESA SP-251 • 1988:Galeev, Krasnoselskikh, Lobzin, Sov. J. of Plasma Physics • 2002:Krasnoselskikh, Lembege, Savoini, Lobzin, Physics of Plasmas

15. Second critical Mach number

16. Conséquences: • Pour les nombres de Mach « avant critiques » apparition des structures de petites échelles • Variation des amplitudes des élements de la structure : « overshoot », « downshoot » et cetera • Apparition des multiples « fronts» • Différence de la structure vus par différents satellites

17. Courtesy of Manfred Scholer

18. Courtesy of Manfred Scholer

19. Courtesy of Manfred Scholer

20. ‘four points’ derived vectors (1) Analysis methods for Multi-Spacecraft data G.Pashman and P. Daly, Eds. • Velocity of a planar boundary (normal vector n) from individual SC times and positions at the crossings (ra – r4 ) n = V (ta - t4) na 24 / 08 / 01 7/23

21. ‘four points’ derived vectors (2) • Spatial gradient of density Least square estimation, from the four positions ra,and the four density values na at a given time na 24 / 08 / 01 7/23

22. Shock questions • Reformation • Variability • Details of the shock transition • How do scales of parts of the shock vary with shock parameters (Mach number, BN, etc)? • Which parts of the shock transition are variable? Cluster: • Timings  shock orientation and speed • Multiple encounters with same shock  average profile, variability

23. Small scale electric field structuresData Sources Electric field from EFW • Sampling 25 Hz • 2 components in the spin plane Magnetic field from FGM • Resolution 5s-1 • Timing normals Density from WHISPER

24. Small scale electric field structureNormal Incidence Frame Walker et al., 2005 Shock frame moves with a velocity VNIF in the plane tangential to the shock such that the upstream flow is directed along the shock normal

25. Vsh=115kms-1 n=(0.96, -0.23, 0.13) θBn~77 deg Ma~2.8

26. Vsh=49kms-1 n=(0.94, -0.17, 0.29) θBn~77 deg

28. Walker et al., 2005 Scale size of spike-like features

29. Scale size V Ma Walker et al., 2005

30. Walker et al., 2005 ΔE V θBn

31. Problem of Stationarity

32. Horbury et al., 2001

33. Horbury et al. 2001 A typical shock • Select several shocks • Must have similar profiles at all four spacecraft • No nearby solar wind features • Feb-May 2001 • 600 km separations • 33 shocks in set

34. Horbury et al., 2001 Averaging the profile • Synchronise at four spacecraft  normal, speed • Plot in shock coordinates • Some variability between spacecraft, but large scale structure similar • MA~3.9 • BN~87º • Mcrit1=4.3; Mcrit2=6.1

36. Undershoot Peak Up Down Courtesy of Tim Horbury Enhancement of |B| • |B| for shock, at peak and downstream, relative to upstream value • Dependence of peak value on MA

37. Undershoot Peak Up Down Courtesy of Tim Horbury Shock overshoot and undershoot • How big are the overshoot and undershoot amplitudes? • Plotted relative to downstream |B| • Uses average profile

38. Courtesy of Tim Horbury Shock ramp scale • MA~1.9 • BN~88º • Average ramp profile often well described by exponential rise • Fit  scale of ramp • Note: fitted “scale” is not total size of shock • 6 of 33 shocks do not have “good” ramps

39. Courtesy of Tim Horbury Shock ramp scale • Ramp scale increases with MA and with less perpendicular shocks • Note: absolute values uncertain

40. Courtesy of Tim Horbury Regions of variability • MA~3.2 • BN~75º • Critical MA ~ 1.7, 2.4 • Measurements up to 18s apart • Variability in foot amplitude, peak waves • Different undershoot scale

41. Courtesy of Tim Horbury Variability of the shock ramp • Cross-correlate profiles through shock ramp • Poor statistics • Significant: normal-perpendicular field components decorrelate with time, not space: waves? • Field magnitude does not significantly decorrelate on these time and space scales

42. Undershoot Peak Up Down Courtesy of Tim Horbury Variability of the peak |B| • Peak |B| for each spacecraft, relative to peak |B| in averaged profile • Higher variability at larger MA • Evidence of reformation

43. Courtesy of Tim Horbury Summary for problem of non-stationarity • Measurements at 600 km separations • Four profiles  “average” shock profile • Variability of overshoot and undershoot amplitudes • Exponential ramp, scale ~c/pi, increases with Mach number • Variability of peak |B|, higher with higher Mach number • Evidence for temporal, rather than spatial, variability of shock front Future: • Compilation of shock list (CIS/FGM/EFW/WHISPER, …)  better statistics • Variability of parts of the shock

45. Courtesy of Steve Schwartz

46. Courtesy of Steve Schwartz

47. Courtesy of Steve Schwartz

48. Problem of energetic particles acceleration

49. Collisionless shocks: new results from Cluster(from Kis et al., 2004) Vsw (km/sec) 0 -400 -800 18 February 2003 20 0 -20 B (nT) Bx,By,Bz 0.02 0.01 0 N(cm-3) 12 14 16 18 20 22

50. Collisionless shocks:new results from Cluster Energetic particles (from Kis et al., 2004) Distance from the shock (RE) 10-1 10-2 10-3 10-4 energetic particles density (cm-3) 24-32 keV 0 2 4 6 8 10