Real shocks: the Earth s bow shock

Transcription

1 Real shocks: the Earth s bow shock Quasi-perpendicular shocks Real shock normals/speeds Substructure within the ramp Shock variability Source of ions beams upstream Quasi-parallel shocks Ion acceleration How the parallel shock works... How does the parallel shock work? 1

2 Properties of the shock transition Relate upstream and downstream conditions using conservation/mhd equations Rankine-Hugoniot relations Do not describe the processes occurring at the shock Aim to understand shock processes Inherently 3D and time varying Solar wind Figure 1 How? Simulations and theory Observations 1 & 2 spacecraft Cluster THEMIS X Z Figure 2 2

3 Quasi-perpendicular shock orientation and motion Horbury et al, J. Geophys. Res., 2002 Bow shock models Average position How do individual crossings compare? BN~86 ; V n ~ 26 km/s When spacecraft data are similar assume: Planar surface No time evolution Constant speed Calculate n and v.n from timing MHD relations across shock Coplanarity theorem n, B u and B d lie in same plane perpendicular to shock plane Use to estimate n for comparison magnetosheath solar wind 3

4 More complex crossings Profile changes Consistent with rapid acceleration Dunlop et al [2002] found examples of acceleration using DA method Partial shock crossings Upstream / downstream monitor Transmission of solar wind features Detailed time variations in transition Fields Particles 4

5 Y (R E ) Large scale shock orientation Horbury et al, J. Geophys. Res., 2002 Timing n sh and V.n sh Velocities: typically ~35 km/s, maximum ~150 km/s Stable direction over variety of upstream conditions Good agreement with model normal: ~80% within 10 Model is good estimate for quasiperpendicular shocks Comparison: timing (k) with model (r) 48 shocks Selection effect: can only analyse well behaved crossings X (RE) (Horbury et al, JGR, 2002) 5

6 Comparison with coplanarity (Horbury et al, J. Geophys. Res., 2002) Coplanarity theorum B u, B d, n all lie in one plane, perpendicular to the shock plane Use to calculate n (single spacecraft method) Large error for BN 90 Poor agreement with coplanarity (even for BN ~70 ) Only 20% within 10 ` Angle between timing n and shock model Angle between timing n and coplanarity 6

7 Electric field structure within the shock ramp Walker et al, Ann. Geophys., 2004 What about E scales? Scale important for energy transfer processes V sh from timing of B ramp Electric field spikes Amplitude ~ 70 mv/m Scale: majority < 10 c/w pe ~10s km < ramp scale Orientation parallel to plane of ramp Layered structure? Role in particle acceleration/scattering? Moderate Mach number What about Mach number dependence? 7

8 Shock variability Simulations show non-stationary structure Important for electron heating? Variability on a variety of timescales Reformation of the ramp Rippling of the shock front Does Cluster see such variations? Lembege and Savioni, 1992 Lembege et al,

9 Shock stability and ramp variability Horbury et al, Ann. Geophys., 2001 BN ~89 M A ~ 5.6 B signature different at each spacecraft Separated by only few seconds Do the variations occur in the shock ramp? Average four shock profiles to produce average shock structure 9

10 Average shock profile and variation about average Horbury et al, Ann. Geophys., 2001 Average profile Difference from average Average profile has signature of a shock foot Large variations in the foot; small variations in the ramp in this case Amplitude of variation in foot increases with Mach number 10

11 Generation of ion beams What is the source of ion beams? Magnetosheath leakage? Shock generated? Compare ion distributions made Downstream In ramp Upstream Some multi-spacecraft, simultaneous observations 11

12 Generation of ion beams Kucharek et al, Ann. Geophys., 2004 Ion beams originate in the shock ramp they do not leak from the magnetosheath The density of the ion beam is higher for high Mach number shocks Gyrating ions and ion beams come from same population Specular reflection not sufficient for ions to escape scattering/multiple shock interactions? Predicted by simulations [Burgess, Ann. Geophys., 1987] 12

13 Quasi-parallel shocks ( Bn ~0, β 1) Shock Flow Magnetic field ~ parallel to shock normal Significant flux threads shock surface Disturbed transition in B: not shock motion Extended spatial transition Particles escape upstream Generate ULF waves Waves carried back to shock Pulsations (SLAMS) grow by interacting with hot ions Shock intrinsically time varying n Energetic ions escape upstream Self-consistent system of ions, accelerated ions, upstream waves, nonlinear interactions hard to untangle 13

14 Parallel shock schematic Schwartz and Burgess,1991 Flow Steepened pulsations What we knew Pulsations are fast mode structures (correlated B and n i ) Pulsations grow from ULF waves in presence of hot ions Large pulsations start to stand in flow Act locally as a Q-perp shock What we didn t know Are pulsations essential? Schematic based on dual-spacecraft observations What do they look like? How many pulsations do you need to make a shock? 14

15 Hot ions upstream of the bow shock First order Fermi-acceleration: Away from shock Particle history: 1. Crosses shock 2. Scatters off density irregularities (waves, tubulence) in sheath 3. Moves upstream 4. Scatters again from pulsations? 5. Sees upstream and downstream irregularities are converging at u u -u d 6. Gains energy u u u d ion shock 15

16 Direct measurement of hot ion density gradient Kis et al, Geophys. Res. Lett., 2004 Upstream diffuse ion population Isotropic; E up to 150 kev; V<V sw 1st order Fermi acceleration, not magnetospheric leakage Steady state diffusive theory predicts N(E) decreases exponentially with distance from the shock Supported by earlier statistical results Direct measurement Fermi Acc N? Two spacecraft separation ~1-1.5 R E stable conditions Calculate distance from shock parallel to B using model Compare partial ion densities N(E) Gradient as function of Energy Inbound pass BN =

17 Quantification of hot ion density gradient Kis et al, Geophys. Res. Lett., 2004 Plot density gradient against distance from bow shock B Good exponential fit for 4 energy channels kev Recently extended to higher energies Plot e-folding distance as function of Energy Consistent with diffusion Fermi acceleration occurring Mean free path ~ R E for kev ions Lower than from statistical studies: factor of 6 (10 kev); factor of 2 (30 kev) Why is this relevant? Pulsations get their energy from these ions 17

18 Pulsation growth and hot ions ULF waves convected Earthward into gradient in energetic particle pressure (E~ kev) Fig 5, Giacalone et al, 1993 Pulsations grow rapidly (~few seconds) [Giacalone et al, 1993; Dubouloz & Scholer,1995] 18

19 What do pulsations look like? 100 km Scale ~ 100 km Smallest scale length ~ km Rippled Reflection of ions seen cf shock Scale ~ 250 km Grow very fast Scale ~ 600 km Size > 600 km Internal structure Scale ~ km Overall size: km Consistent with simulations Refract to lie near expected shock surface 250 km 600 km 1000 km 19

20 How thick is the quasi-parallel shock? Thick: lots of pulsations contribute to thermalisation? Thin: one or few pulsation(s) dominate? Cluster data Tetrahedron ~ 1000 x 3000 km Sun C4 C3 C1 20

21 Shock layer thickness Thickness of shock <2700 km Shock lies between C1 and C4 several times, few minutes each time Thickness of shock can be < 1200 km Only one/few pulsations within transition? Consistent with observations of hot ions Much more work to, do, but current working model of shock looks like 21

22 Evolving picture of the quasi-parallel shock Pulsation grow rapidly as convected Earthwards, gaining energy from hot ions As pulsations grow: Become filamentary shorter scale lengths They become refracted to lie parallel to expected shock front They start to reflect ions Single or few pulsations form the shock, cause plasma thermalisation V sw Away from shock n Ion density How long before new pulsations replace old ones? What is the filling factor of SLAMS important for ion acceleration Upstream perturbations Pulsations 22