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Current sheet flapping and magnetotail activity

Current sheet flapping and magnetotail activity. V. А. Sergeev (St. Petersburg State University, Russia).

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Current sheet flapping and magnetotail activity

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  1. Current sheet flapping and magnetotail activity V. А. Sergeev (St. Petersburg State University, Russia) Thanks to : S. Dubyagin, D. Sormakov, S. Apatenkov (SPbU), Y.Asano, A. Runov, W. Baumjohann, R. Nakamura, T. Zhang (IWF), A.Petrukovich (IKI), P. Louarn , J.-A. Sauvaud (CESR), A. Fazakerley, C.Owen (MSSL), T. Nagai (TITECH), T. Mukai (ISAS) INTAS grant 03-51-3738 IUGG, Perugia, July 2007

  2. Characteristics of ‘flapping’ current sheets Relationship to BBFs and activity Flapping near the reconnection region Simulations and Interpretation C O N T E N T IUGG, Perugia, July 2007

  3. Flapping motions in the magnetotail Collaborative research(INTAS grant, 2003) : Properties of CS during fast crossings CS structure(s) Peculiarities near reconnection region Previous picture - nearly flat tail CS (~XY GSM), - CS structure – Harris sheet (Bn=0) as basic model, often generalized forBn>0 (but 1D) Most of results – for fast moving sheets Cluster, Geotail Tilt determination 1/ MinVarAnalysis (1SC)  n1 , n2(~j), n3 (~n) 2/ Current-based (4SC): nj = [n1 x jNS] / | n1 x jNS | 3/ Timing (4SC) B1 n j Geotail Cluster Objectives IUGG, Perugia, July 2007

  4. Multiple NS crossings during substorm, slow flow Vx,y , all time inside PS(Sergeev et al., 2004 GRL) Alternating signs jz(+B,-b, etc) and nzvariations under positive jy; |jz|  |jy| – large tilts n determinations from T, J and MVA – very large tilt toward Y !! Propagate duskward Large tilts, YZ Kink mode with strong folds Example : October 20, 2001 CL at [-13, 12,-1] Re Z Y j , n NS IUGG, Perugia, July 2007

  5. MVA normals Dawn Dusk Z X Dusk Y Dawn TC-1 CL Tail 20040805 Cluster & TC-1, Kink mode confirmed Cluster: [-15.4 -8.9 3.4] TC-1: [-10.7 -6.8 2.6] • 8 NS crossings, timescale ~150 – 500 s , weak substorms • Similar ~phased variations of Bx (1320-15 UT) • Cluster & TC-1  kinks with length >5 Re along the tail , move dawnward at 30km/s [Zhang et al., Ann Geo, 2005] • Geotail statistics (Sergeev et AnnGeo 2006): • 3 years, 15 – 30 Re ; • MVA for 480 crossings, (with 2/3>4 and N>15) • CS kinks with N swinging in YZ • rare cases with N in XZ (Volwerk et 2003) IUGG, Perugia, July 2007

  6. Propagation from center to flanks, perpendicular to ‘magnetic field plane’; Propagation speeds 20-300 km/s, median ~ 50 km/s; Larger speeds near MReconnection site and in BBFs; Propagation pattern reminds the pattern of cross-tail average flow, could be structures standing in the flow??? Timing : propagation direction & velocities VT B1 Runov et al., AG 2005 Sergeev et al., GRL 2004 IUGG, Perugia, July 2007

  7. ION FLOWS (from CIS) : Problems:effect of same order of magnitude as possible instrumental Z-offsets and errors; transient effects and changing tilts easily ruin any correlation; only Vn is available from timing Special subset : 42 events with nTnJnMVA within 20o RESULTS : Plasma flows in the same direction with same velocity along the normal; Most of flapping Kink structures – can be the structures nearly standing in the flow Large tangential velocities (>Vn) are frequent, especially in BBFs Kink Propagation and Ion flows along normal direction [ Dubyagin et al., AnnGeo, 2007 subm ] IUGG, Perugia, July 2007

  8. SHAPE (reconstructed from probing small parts !): usually not exactly periodic waveforms, but periodic events exist (Petrukovich AG2006) some appear assolitary structures ; ’ Y‘  AZ  h CS ?1-3Re 0.5-2Re 0.2-1Re LIFETIME : Only lower estimates are possible in examples with slow propagation which give TL can be  5 min Appearance of Flapping Structures 5 min IUGG, Perugia, July 2007

  9. B1 n j Guide- versus normal B-field components B vector is averaged over |BX |<+3nT region. α - angle between n and Z GSM BJ/Bn Tilted sheets possess large guide B-field as distinct from sheets with standard tilt. (Always Bz>0 ! ) Standard Tilted CS [ Petrukovich AG, 2007, Dubyagin et al., AnnGeo, 2007 subm ] IUGG, Perugia, July 2007

  10. Vz integration – vertical PS/plasma motions: consistent variations Vp & Vc =dB1/dt / |gradB|; z~1-2 Re CS structure of individual crossings, from B (z*) (Vc and average tilts used) : different shapes (single peaks, flat, bifurcated, symm/asym, time-dependent?,…) ; CS scale-sizes vary between ~1000 km and ~4000 km; All crossings are different ! October 20, 2001:Oscillation amplitude and CS structure Thin embedded, Bifurcated, 1 asymmetric, etc Runov et al., PSS 2005 A B b C c D d E e IUGG, Perugia, July 2007

  11. Reconstruction of 30 suitable crossings (Runov et al., AnnGeo 2006) Non-Harris distributions most frequent !! j shape deviate from Harris; Density and j- profiles are different, flat density distributions ~55% central embedded sheets ; ~10% bifurcated sheets; ~35% asymmetric sheets iConfirmed in Asano et al GRL 2005 (any CS, not only flapping CS) No big difference in CS parameters in groups ! NOT very Thin sheets (1000-2000km)! Survey of structures , comparison to Harris distributions IUGG, Perugia, July 2007

  12. Current sheet structures • Are non-Harris features special for flapping sheets only (e.g. transient effect??) • Asano et al 2005 (GRL, L03108): • Other approach and method (no preference to flapping sheets); • Large tilts excluded; • Results: • Harris distributions – rare ! • Central embedded sheets- ~40% • Bifurcated/off-center peaks - ~17% • comparable to % in flapping sheet survey! • - % of bifurcated sheets slightly increases during BBFs Harris bifurcated B B embedded IUGG, Perugia, July 2007

  13. Characteristics of ‘flapping’ current sheets Relationship to BBFs and ground activity Flapping near the reconnection region Simulations and Interpretation IUGG, Perugia, July 2007

  14. GT CL Occurrence of Fast CS crossings and BBFs Flapping BBF • Geotail, 3 years of data • Occurrence frequency • increases downtail, has • a peak at Y~[-5..+10] Re • Consistent with flankward propagation, confirms magnetospheric origin • Resembles occurrence of BBFs and occurrence of Reconnection events at substorm onsets (Nagai et al.,1998) [Sergeev et al. AnnGeophys, 2006] IUGG, Perugia, July 2007

  15. Fast CS crossings and BBFs Fast crossings and BBFs • similar spatial occurrence ; • duration dT is shorter and propagation speed larger (>100km/s) when local fast flows; • V-dependence of occurrence - non-gaussian distribution with large V-wings; • However, flapping occurs under any possible PS parameters • ~30% of fast crossings under Vxy<100km/s – no rigid local relationship with BBFs: [Sergeev et al. AnnGeo, 2006] IUGG, Perugia, July 2007

  16. Flapping : Activity Dependence • Dependence on AE and auroral activity • Common during substorms, prefer SBS expansion phase (1100 events), • but sometimes met during quiet AE, • both features agree with previous results (Toyichi, Miyazaki,1976, Sergeev et 1998, etc) Comparison with IMAGE global auroras (Dubyagin, unpublished) : • Usually associated with auroral brightenings or poleward arcs (PC or streamer?), but clear one-to-one relationship rare • few events with NO auroral brightenings (all – CDPS events). IUGG, Perugia, July 2007

  17. CLUSTER : Flapping is very common near flow reversals, often produce turbulent-like appearance Crossings can be very short (seconds), partly due to high propagation velocities (>100 km/s) Thin sheets ~1 c/pi (400-800km) directly confirmed, j ~ 30-50 (up to 120nA/m2) large tilts are common Runov et al.,PSS 2005, Wygant et al., JGR 2005 , Nakamura et al., JGR 2006, Laitinen et al., AG 2007 ~jzgsm 10sec CSheet in ~XZ gsm plane • Current Sheet near the Reconnection Region flapping-like? IUGG, Perugia, July 2007

  18. large tilts are common (Nakamura et al.,2006, Runov et al.,2003, Wygant et al.,2005, Laitinen et 2007) Hall ‘Ez’& ‘By’are rotated with tilted sheets producing Ez, Bz signatures (Laitinen et 2007) Flapping produce spikes in Ey (large Vz * Bx) looking as intermittent component of turbulence… : Reconnection Current Sheet is often strongly corrugated (kinked)  Obstruction for reconnection analyses , -? Role in reconnection ~jzgsm 10sec CSheet in ~XZ gsm plane • Current sheet near the reconnection region, 2 IUGG, Perugia, July 2007

  19. Large-amplitude Nonlinear Structures (Waves or Solitary structures); Very large tilts common(>45o from Z in ~50% of events), upto >90o; Fast CS crossings due to YZ kink ‘waves’with normals rotated B (in ~YZ); Propagate ‘centerflanks’ at speeds 20-80 (Q), ~>100 km/s (near MR); Occur more frequenly with incr.distance, peaks premidnight, like BBFs No strict local dependence on BBFs or plasma parameters; generally CS is not as thin locally (h~1000-2000km) [kinetic or MHD?], occur with any possible PS parameters; CS is extremally thin, fast-moving and very corrugatednear XL; Often associated with auroral brightenings and substorms, but not exclusively; Summary of Flapping-related structures WORKING HYPOTHESIS : Flapping structures – of inner origin, most possibly generated in association with midtail reconnection and BBFs, - can include structures slowly propagating (or slowly transported by plasma convection) from the source IUGG, Perugia, July 2007

  20. Characteristics of ‘flapping’ current sheets Relationship to BBFs and ground activity Flapping near the reconnection region Simulations and Interpretation IUGG, Perugia, July 2007

  21. - Is ‘Flapping’ kinetic or MHD effect? Could variety of non-Harris CS structures be 2-3D or temporal effect or 1D structures? With no hopes to reproduce exactly temporal and spatial structures during very dynamic states (e.g. SBS) – MHD-simulations offer possibility to see analogues and investigate possible source processes Preliminary results of our study (in agreement with those from El-Alaoui et al., 2001, 2007, Harnett et al. GRL 2006, Thompson et al.,2006..) : wavy CS structures of correct temp/spatial scales are seen in 3D MHD; generally ‘CS kinks’ is associated with localized/distributed reconnection coexistence of different CS structures at one time (even at nearby locations) – can be related to reconnection acting at different distances Flapping in 3d MHD Simulations Cluster IUGG, Perugia, July 2007

  22. Mechanisms of Flapping ? Kinetic or MHD modes? Instability or propagated modes? (Conditions of instability ?? Drivers of propagated modes?? Could waves propagate in Y??) • Kinetic instabilities (provide YZ kinks, can propagate in Y) • Yoon et al., 1998: DKI; Karimabadi et al., 2003: Ion-Ion Kink Instability, • Nakagawa-Nishida, 1989, Yoon et al., 1996; Ricci et al., 2004: KH Instability • Sitnov et al., 2004, 2005: Kinks standing or propagating with Vφ ~0.1 Vth in PIC simulatins Difficulty : not as thin CS, propagation direction ?? • Standing MHD modes in (neutral) Harris sheet(Fruit et 2002, Volverk et al., 2003) – have different properties • MHD effects of localized reconnection in presence of By • YZ tilt of current sheet at leading front of Alfvenic jet (Shirataki et al., 2006) • Surface waves in current sheet (Semenov et al.,1994, ..) • special MHD modes on closed plasma sheet field lines (Bn>0) • Propagation in Y is possible at small speeds (Golovchanskaya, Maltsev, 2004) IUGG, Perugia, July 2007

  23. Mechanisms of Flapping ? -2 special MHD modes on closed plasma sheet field lines • Revisited and developed by Erkaev and Semenov (2007), ‘double-gradient waves/instability’ • dBz/dx >0 - Waves (Vx~0, propagate in Y at few 10km/s), can be excited by Y-localised BBF • dBz/dx < 0 - Instability /linear/ IUGG, Perugia, July 2007

  24. Summary/Future work • A variety of possibilities, different mechanisms can be involved Path to close problem : • Identify flapping drivers (or instab?) in 3d MHD simulations; • Evaluate propagation velocity in plasma frame (kinetic versus MHD); • THEMIS, MagCon (good Y- and Z- coverage required) • . . . IUGG, Perugia, July 2007

  25. 78 rapid crossings of the magnetotail current sheet during July - October 2001: CS orientation and motion, timing analysis: Most of the crossings are due to kink-like transients passing over the spacecraft The kinks propagate from the near-midnight sector (0<Y<8 RE) toward flanks Typical velocity of kinks in respect to SC ~ 30 – 100 km/s ~ 0.1 Va Kink amplitude 2000 – 10000 km with average value of 1 RE Electric currents and magnetic field geometry, curlometer: j rotates in +Y,±Z half-plane The kink fronts are often close to vertical with |jz|>jy In 78 % of cases the magnetic field curvature is positive, 10 cases of Cx<0 are at 0<Y<8 RE CS structures: In 55 % of cases the CS has a single peak near Bx=0; 10 % -bifurcated (double peak) CS; in 35 % - off-central, asymmetric CS Total pressure is fairly conserved The half-thickness of the CS during flapping varies from <1500 to 4000 km. The minimum curvature radius varies between 500 and 10000 km In 73% of the crossings the adiabaticity parameter κ< 1 Summary of observations IUGG, Perugia, July 2007 EGU General Assembly, Vienna, April 29, 2005

  26. duration <30 - >300s, depends on local flows (short in BBFs and near MR) in the neutral sheet Bns ~4 nT (1..18nT), usually |Bj*|>Bn* curvature –Earthward (closed FL-~90%) in CL data set; B-curvature radius Rc ~ 5 Lcp(Lcp -gyroradius in BL field) halfthickness h~10 Lcp (1..20 Lcp), weak correlation with Rc adiabaticity parameter  = {Rc BL/ (Lcp Bns)}1/2 ~0.6 (0.2..2) - non-adiabatic ions any local (density/temperature/velocity) conditions plasma velocity: BBFs only in ~25%, >60% Vp<100km/s Parameters of flapping current sheets Runov et al. AnnGeo, 2005, p.1391, also Shen et al JGR, 2003, p.1168 IUGG, Perugia, July 2007

  27. До этих работ – ТС представлялся плоским образованием, 1d слой Харриса (Bn=0), обобщенный на случай Bn>0 Основой 1d гипотезы обычно считается /z >>/x ; однако при Bx>>Bn  0 X>>Z (‘Tail approximation’) распределения Р(z), j(z) определяются их Х-распределениями Flapping in 3d MHD Simulations X В эксперименте(преимущ.lapping ТС) : -структура\масштаб последовательных слоев изменчивы (20\10\01, Nakamura et 2006), -возможна NS-асимметрия (t -? или структура-?), -вероятны уединенные структуры 3D MГД моделирование: - частое сосуществование структур, разного типа (по X и Y!!); быстрые изменения во времени; МГД-система производит многообразие структур преимущ. за счет 2d (+нестац.) эффектов IUGG, Perugia, July 2007

  28. Double-gradient waves and instabilityErkaev N. V., Semenov V.S. erkaev@rambler.ru sem@geo.phys.spbu.ru IUGG, Perugia, July 2007

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