Z. Němeček and J. Šafránková Charles University Faculty of Mathematics and Physics

1. Reconnection and its relation to the LLBL formation Z. Němeček and J. Šafránková Charles University Faculty of Mathematics and Physics Prague, Czech Republic IAFA 2011, Alpbach, Austria, June 21, 2011

2. The contribution is based on results of the Prague group but many others should be mentioned: Introduction Discovery of the LLBLmore than 30 years ago Hones et al. (1972) – VELA Eastman et al. (1976) – IMP 6 Formation of the LLBLsouthward IMF - dayside reconnection - Luhmann et al. (1984) northward IMF - reconnection poleward of both cusps (e.g., Song and Russell, 1992; Reiff, 1984; Onsager et al., 2001; Fuselier et al., 2002; Nemecek et al., 2003) Thickness of the LLBLincreases with increasing distance from subsolar point (e.g., Haerendel et al., 1978; Eastman and Hones, 1979; Paschmann et al., 1993) very thick LLBL – Sauvaud et al. (1997) changes of thickness(e.g., Phan and Paschmann, 1996; Hapgood and Lockwood, 1995; Faruggia et al., 2000; Safrankova et al., 2007; Rossolenko et al., 2008) New observations (Cluster, Double Star) Formation of the LLBLcombination of anti-parallel geometry and weak-KHI Nakamura et al., (2006); Hasegawa et al. (2006); Foullon et al. (2008); Bogdanova et al. (2008) Contribution to study of the LLBL thickness – Rossolenko et al., (2008), Bogdanova et al. (2008); Foullon et al. (2008)

3. Outline • LLBL definition • Observations of the LLBL • LLBL formation • LLBL thickness • N–T plots and the LLBL profile • Structure of the LLBL • Implication for further investigations

4. Plasma sheet Depletion layer LLBL Definition of the LLBL The LLBL is a magnetopause layer with plasma properties between those of magnetosheath and plasma sheet • Depletion layer • LLBL • Plasma sheet The LLBL is on magnetospheric field lines – open or closed Typical crossing of the low-latitude flank magnetopause Two-point observation of the near flank LLBL (~1500 MLT)

5. Flank LLBL observations • Satellites often cross the LLBL in a minute but sometimes they can spend there several hours – does it mean a very thick LLBL? • Plasma parameters inside the LLBL are highly variable - what is the source of these variations? • Sunward flows in the LLBL – where they come from?

6. Dayside LLBL observations • Difficulties with identification of visited regions – two different LLBL modes. • Variations of the LLBL thickness. LLBL LLBL • FTE versus magnetopause surface deformation – are these features created by the same process? What is their mutual relation?

7. Source of LLBL plasma Several sources were suggested • Impulsive penetration – implies negative density gradients inside the LLBL (not observed), cannot explain sunward flows, • Diffusion – too slow even if enhanced by surface deformations • Kelvin-Helmholtz instability – not applicable at the subsolar region • Magnetic reconnection – probably the dominant source on dayside • Combinations of above mechanisms – we will show that none of suggested mechanisms alone is able to explain all LLBL features

8. Questions to be answered LLBL properties • Energization of the magnetosheath plasma • Presence of the plasma sheet-like population • A mixture of both populations • Periodic or quasi-periodic oscillations of parameters • Sunward or antisunward velocities on flanks • Variations of the LLBL thickness • Dependence on the IMF orientation • FTE versus magnetopause surface deformation – are these features created by the same process? What is their mutual relation?

9. LLBL formation - dayside • BZ<0 implies a thin dayside LLBL on open field lines • BZ>0 low outflow speed and re-reconnection rate lead to a thicker LLBL on closed field lines • ? Could these processes produce the flank LLBL? • ? Influence of BY? Northward IMF Southward IMF

10. Flank LLBL formation • Formation of the LLBL at flanks requires significant IMF By • Reconnection proceeds in high latitudes near the cusp. One part of the field line is the cusp (lobe) line; the other part is connected to the conjugated hemisphere and can be considered as a LLBL line. • Hemispheric asymmetry (summer–winter) can result in the dawn–dusk LLBL asymmetry. IMF Bz Nemecek et al. (2003)

11. Sunward flows in the flank LLBL tailward sunward • The flow direction is determined by the location of the spacecraft with respect to the current sheet • Change of the IMF BY sign can lead to change of the flow direction (Safrankova et al., 2007) Nemecek et al. (2002)

12. a b Flow direction and IMF BY a b c • The flow direction is controlled by IMF BY • Counter-streaming populations can be observed during IMF By reversal. • This is not an evidence of the closed field lines, the lines are open. Safrankova et al. (2007)

13. Flank LLBL thickness • The magnetopause follows upstream pressure • LLBL thickness gradually increases – influence of the upstream pressure or an increase of the reconnection rate? • The thickness reaches ~1.2 RE Two distinct pressure jumps in the solar wind push the magnetopause inward, IMF points northward Safrankova et al. (2007)

14. Plasma sheet Inner LLBL Outer LLBL MSH Electron n–T plots • n-T plots are well ordered in spite of large variations of LLBL plasma parameters • Four plasma regimes can be clearly distinguished: • Magnetosheath and PDL • Outer LLBL • Inner LLBL • Plasma sheet

15. LLBL layers for different IMF orientation BZ<0 BZ>0 • The n-T plot for southwardoriented IMF. Note a small number of points between ~1.2 and 2.7 cm-3 that indicates that the outer LLB is missing. • The n-T plot for an interval with northwardpointing IMF. Both LLBL sub-layers are present.

16. Electron distributions inside LLBL layers BZ>0 Inner LLBL – trapped particles on closed lines Plasma sheet Outer LLBL - streaming population suggests open lines Magnetosheath

17. LLBL during upstream changes BZ<0 IMF points southward, upstream density changed from 12 to 8 cm-3. The changes of the upstream density are not reflected in the LLBL – LLBL is disconnected from the magnetosheath.

18. LLBL during upstream changes BZ>0 IMF points generally northward, two sharp jumps of upstream density. The density variations are seen immediately in the LLBL – LLBL is directly coupled to the magnetosheath.

19. Dayside LLBL observations • Themis spacecraft close to the local noon • Themis B as a magnetosheath monitor • LLBL observed simultaneously by five spacecraft

20. Dayside LLBL thickness • LLBL is not observed at the beginning and at the end of the interval • IMF points northward at the middle of the interval and southward at its edges • The LLBL can be as thick as 1.2 REon the dayside • The LLBL is thin or not present during intervals of southward IMF Tkachenko et al. (2009)

21. 3 2 1 Dayside LLBL profile • Ions bring similar information as electrons • All spacecraft form a line in the n-T plot regardless of their positions – continuous LLBL profile • Changes of the IMF direction lead to plasma penetration into the outer LLBL innermost intermediate outermost

22. Reconstruction of the LLBL profile • n-T plots suggest that the density or temperature scale can be converted into a spatial scale • Themis spacing allows us to do it for a case of a thick LLBL • As could be expected, the density gradient is much steeper in the inner LLBL Tkachenko et al. (2010)

23. Dayside LLBL: N–T plots BZ<0 BZ>0 transition • All spacecraft show essentially the same profiles • Problems with inter-calibration and density/temperature determination • Similar shapes of n-T plots as for the flanks • Outer LLBL is not present for BZ<0

24. Themis C: particle flows PS LLBL magnetosheath LLBL p/a high p/a low • Streaming population in the outer LLBL (low energies) • Trapped population in the inner LLBL • Counter-streaming population in the outer LLBL • Outer LLBL on open or closed field lines • Inner LLBL on closed field lines BZ>0

25. Magnetopause transients 1, 3, 4 2, 5 • Significant changes of the magnetosheath magnetic field direction are accompanied with magnetopause transient events • Some of them are seen as magnetopause deformations due to enhanced magnetosheath pressure • Others can be attributed to the LLBL thickening following the BZ northward turn Tkachenko et al. (2011)

26. FTE or surface deformation ? • TH-B in the magnetosheath, TH-C in the magnetosphere • Typical FTE features – bipolar BN signature, |B| enhancement • Changes of the plasma velocity in the magnetosheath inconsistent with a simple FTE scenario • Structure moves northward • The most probable scenario – a magnetopause deformation induced by the magnetosheath density depletion associated with the BZ rotation (not observed in the solar wind) Tkachenko et al. (JGR, submitted)

27. Conclusions What we know: • LLBL is a permanent feature of the whole dayside and low-latitude flank magnetopause. • LLBL exhibits a layered structure being divided into inner and outer sub-layers. • Both sub-layers are observed during northward pointing IMF, whereas the outer sub-layer is missing during southward IMF intervals. This is true for dayside as well as for flanks. • Outer LLBL lies on open/closed field lines, inner LLBL is on closed field lines. • High-latitude reconnection couples the LLBL with the magnetosheath during northward IMF. • Patchy reconnection and pressure pulses modulate LLBL parameters, diffusion is probably responsible for the smooth LLBL cross-section.

28. Implications for the further research • Reconnection is generally consistent with LLBL properties but is it the only source of the LLBL plasma? • Relation between magnetosheath and LLBL parameters • Heating of plasma on closed lines • Sources of periodic oscillations of the LLBL thickness/location: • Sources of negative density gradients • variations of the magnetosheath pressure • variations of the magnetosheath magnetic field direction • supersonic energetic plasma jets • K-H instability can contribute to local entry along flanks • variations of the magnetosheath pressure • intrinsic instability of the formation process - FTE • (K-H instability can be ruled out near the subsolar point)

29. Call for papers 39th COSPAR Scientific Assembly Mysore, India 14-22 July 2012 D3.5 Towards Understanding of Anomalous Dynamics of Magnetospheric Boundaries

30. Summary of flank observations • LLBL is always present at magnetospheric flanks • Its properties are consistent with reconnection near the cusps influenced by IMF BY • Flank LLBL generally consists of two layers with different plasma parameters • Outer layer contains magnetosheath plasma mixed with plasma-sheet plasma and lies on open or closed field lines • Inner layer is on closed field lines, the plasma parameters gradually evolve to plasma-sheet values • Pure northward IMF can create the LLBL on dayside and magnetosheath flow and magnetospheric convection bring it toward the flanks • Pure southward IMF cannot produce flank LLBL, it persists on closed field lines from periods of different IMF orientations • Major source of the LLBL plasma is probably reconnection around cusps, diffusion and interchange instabilities can contribute mainly on dayside, K-H instability with reconnection in rolled-up vortices can contribute along flanks.

31. Dayside LLBL summary • LLBL is present at dayside regardless of the IMF orientation. • Dayside LLBL generally consists of two layers with different plasma parameters. • Outer layer contains magnetosheath plasma mixed with plasma sheet plasma and lies on both open and closed field linesbut it is not observed for southward IMF. The reason is probably localized reconnection and if the spacecraft does not encounter FTE it crosses closed field lines only. • Inner layer is on closed field lines, the plasma parameters gradually evolve to plasma sheet values. This layer is created by double reconnection during intervals of northward IMF and persists during other IMF orientations. • Observations reveal frequent transients for all IMF orientations. They are a part of process creating a regular LLBL. • Major source of the LLBL plasma is probably continuous or patchy reconnection, the reconnection rate can be significantly enhanced by pressure pulses or IMF rotations.

32. Density gradient inside the LLBL innermost intermediate outermost • Typically, the density gradient is positive, i.e., the outermost spacecraft observes largest density • An opposite gradient can be sometimes observed during sharp changes of the magnetopause location • The opposite gradients can be connected with distortion of the magnetopause and thus apparent magnetosheath LLBL