Small-scale dynamics of dipolarization front resolved by MMS

1. Small-scale dynamics of dipolarization front resolved by MMS Cheng-Ming Liu School of Space and Environment, Beihang University, Beijng, China

2. Introduction DF property [Runov et al., 2009, GRL] [Fu et al., 2012, GRL] • Characterized by sharp Bz increase; • Usually associated with BBF • Typically considered as consequence • of reconnection

3. Introduction DF occurrence rate [Fu et al., 2012, GRL] • Large occurrence rate: 1 event/4 hours, comparable to that of substorm; • Have a maximum occurrence rate at ZGSM=0 RE, R=15 RE.

4. Introduction Electron acceleration [Fu et al., 2011, GRL] • Fermi acceleration dominates in decaying FPR; • Betatron acceleration dominates in growing FPR.

5. Introduction Pitch angle evolution [Liu et al., 2017, GRL]

6. Introduction Ion acceleration [Zhou et al., 2010, JGR] • Ions can be accelerated by reflection ahead of DF;

7. Introduction Wave-particle interactions [Khotyaintsevet al., 2011, PRL] [Yang et al., 2017, JGR]

8. Introduction Energy conversion [Khotyaintsevet al., 2017, GRL] [Angelopoulos et al., 2013, Science]

9. Introduction 3D PIC simulation of DF generated by reconnection [Lapentaet al., 2016, JPCS]

10. Introduction Cluster/THEMIS MMS Higher resolution: 30 (7.5) msfor e moments; Smaller separation: down to electron scale; More information: electron 3D VDF.

11. Electron jet detected by MMS at dipolarization front

12. Electron jet at DF MMS location

13. Electron jet at DF MMS 1-minute overview • Bz increases from 6 to 16 nT at DF; • Bx < 5 nT, MMS were located in the neutral current sheet; • Plasma density represented by spacecraft potential decreases sharply across the DF; • Timing analysis shows that DF velocity is 210.7* [0.68, 0.68, -0.26] km/s.

14. Electron jet at DF Ambient RG dip front Flux pileup region MMS1 10-s overview • Electron jet (up to 2000 km/s) was observed right at the DF; • Ion velocity is roughly stable at 200 km/s • Large Ex and Ey were observed associated withthe jet; • Electrostatic waves (flh-0.1fce, mainly in the perpendicular direction) is observed within the jet;

15. Electron jet at DF Flow property • MVA (on B) was used to obtain local LMN coordinate system; • Electron flow is mainly in the tangential plane to the DF current sheet; • Electron flow in the parallel direction is observed; • E field is dominated by En, and is contributed by Hall term; • No parallel electric field is observe;

16. Electron jet at DF Partitionof energy conversion • Energy conversions calculated by using two methods (obtain the current using particle moments or curlometer) are consistent; • Electrons get energy in the tangential direction; • Ions get energy in the normal direction; • Energy conversion contributed by electron current is about two times stronger than by ion current.

17. conclusions • First observation of electron jet at DF; • the jet appears mainly in the tangential plane to the DF, in association with strong electric fieldand lower hybrid driftwaves; • Energy conversion occurs at the DF is mainly contributed by electron current, rather than the ion current suggested by previous studies.

18. Electron-scale measurements of dipolarization front

19. Electron-scale measurements

20. Electron-scale density gradient [Vapirev et al., 2013, JGR] • First observation of electron-scale density gradient, verifying the results of previous PIC simulation; • Such density gradient is an strong indication of the development of kinetic-scale instabilities at DF.

21. Electron-scale Currents • First observation of electron-scale currents at DF; • Current at DF is mainly along M (tangential) direction; • Current in the normal direction shows finite value, indicting that plasma can flow across the DF; • Strong parallel currents only exist at the DF high-density part;

22. Electron-scale Electric fields • First observation of electron-scale electric fields at DF, indicating the development of LHDI; • No parallel electric fields are observed. [Divin et al., 2015, JGR]

23. Electron-scale energy conversion [Lapenta et al., 2014, POP] Energy conversion at the DF fluctuates withlocalized energy loads and generators, due to the presence of electron-scale currents and electric fields, verifying the results of PIC simulation.

24. conclusions • First observation of electron-scale density gradient, currents, electric fields at DF, suggesting the development of LHDI; • Plasma can flow across the DF at electron-scale; • Parallel currents mainly exist in the DF high-density part; • Electron-scale structure is crucial for energy conversion, leading to fluctuating loads and generators at the DF.

25. Evidence of magnetic nulls around dipolarization fronts

26. PIC Simulation [Lapenta et al., 2018, JPP]

27. PIC Simulation [Lapenta et al., 2015, Nature Physics]

28. MMS Observations

29. MMS Observations

30. MMS Observations

32. conclusions • For the first time, we provide the evidence of radial nulls around DFs; • These radial nulls exist in the upstream region hosting turbulent magnetic fields.

33. Large-amplitude parallel electric fields behind DF (ongoing)

34. Ez~100 mV/m is observed behind DF; • This E structure well corresponds to the plasma boundary;

35. 4MMS measurements of the parallel-E • Timing analysis yields the propagation velocity of this E structure as 1150*[0.03, 0.59, -0.81] km/s (antiparallel to B); • The propagation velocity is close to the local ion thermal velocity (1000 km/s), suggesting this E wave is of Buneman type; • The asymmetric bipolar of the waveform shows that the structures carry a net potential of 100-200 V, indicating possible existence of weak double layers.

36. Low-energy beam • Whistlers are observed near the edges of the E structure; • The low-energy beam may provide the free energy for the E structure.

37. Initial conclusions • For the first time, we observed large-amplitude parallel electric fields behind DF; • These E structures, coexisting with weak double layers, may be generated by Buneman instability; • Evidence of beam-wave interaction may be seen.

38. Thank you