1. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series

2. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Magnetic Reconnection and its role in the Formation and Dynamics of the Terrestrial Magnetosphere Prof. Christopher J. Owen UCL/Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, United Kingdom c.owen@ucl.ac.uk

3. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Outline MHD, topology conservation; What is reconnection?; Characteristics of reconnection models; Reconnection in different applications; Some observational tests of reconnection processes and/or their consequences; The way forward – MMS; Conclusions. 2nd February 2016 MSSL Lecture Series

4. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP MHD Equations 2nd February 2016 MSSL Lecture Series A set of magneto-fluid equations that are adequate to describe a plasma: On long time-scales; On long spatial scales. * Or terms representing sources of energy

5. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Ideal MHD Ohms Law 2nd February 2016 MSSL Lecture Series Many (most) astrophysical, solar, solar wind, terrestrial and planetary plasmas are sufficiently tenuous that collisions between particles are rare, there are very long mean free paths and classical resistivity is very low: E + v  B = 0 This corresponds to “frozen-in flow” which is a good approximation for large-scale space plasma systems.

6. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series

7. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Magnetic Topology Conserved 2nd February 2016 MSSL Lecture Series Strict ‘frozen-in’ implies no mixing of plasma populations and thus distinct and identifiable (through the plasma on them) tubes of magnetic flux; However, this very nature can lead to a violation of the conditions which created it!

8. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Generalised Ohms Law for a Plasma Potential difference between 2 points can in principle be altered by flow of current and/or other properties of the plasma: The RHS terms are:  j resistive term (like the ordinary Ohms law); “Hall term” due to Lorentz force; term due to a possible anisotropic electron pressure; term due to contribution of electron inertia to current; 2nd February 2016 MSSL Lecture Series

9. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Induction Equation Assuming the resistive term on the RHS of Ohms law dominates the other terms, an induction equation for a plasma can be derived such: A dimensional analysis on the scales of the terms of the RHS leads to the magnetic Reynolds number for the plasma, R M = VL/η and timescale for diffusion Τ D = L 2 /η; Most solar system plasmas have large L, very low η, so large R M, Τ D ; However, if variations over small scale lengths develop, then the diffusion effects can become significant. 2nd February 2016 MSSL Lecture Series Convection TermDiffusion Term

10. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Topology Changer 2nd February 2016 MSSL Lecture Series Treumann and Baumjohann, 2014 If sharp gradients (small L) are allowed to form then diffusion processes can become important. This diffusion allows the field to evolve on the small scale, but this can lead to changes in the more global topology of the field.

11. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series ? On small scale-lengths (i.e. at sharp gradients), a diffusion region (physics unknown) can form where the magnetic field can diffuse through the plasma (i.e. a breakdown of the frozen-in principle). Magnetic Reconnection

12. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Mixing of Plasma Populations

13. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Application of strict frozen-in flow implied that magnetic field and plasma from different sources could not mix; However, the frozen-in flow approximation is not always valid where gradients are sharp; Reconnection allows: 1.Magnetic field regions that were previously independent to interact; 2.The plasma populating the magnetic fields to intermix; 3.Plasma is accelerated into jets as magnetic energy is released. Magnetic Reconnection – Key Points SLOW INFLOW FAST OUTFLOW JET ~V A ?

14. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Timescales 2nd February 2016 MSSL Lecture Series

15. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Early Steady-State Models 2nd February 2016 MSSL Lecture Series

16. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP But does η ≠ 0 arise? True Ohmic resistivity (due to collisions of the charge carriers retarding flow) in the plasma remains low; Anomalous resistivity can, in principle, arise through a number of ways of scattering particles and retarding the flow of current, e.g.: –Wave-particle interactions in the diffusion region; –Turbulent/stochastic motions in the diffusion region; –Effects of various possible instabilities in the diffusion region; Identification of specific mechanisms (which may anyway be different in different contexts) has been hampered by lack of sufficiently detailed observations and simulations; –(Indeed in early simulations reconnection often occurred due to ‘numerical resistivity’ inherent within the codes!) 2nd February 2016 MSSL Lecture Series

17. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP ‘Hall Reconnection’ Of course, each of the other terms in the generalised Ohms law potentially provides a means to support the occurrence of magnetic reconnection; Over the last ~10 years consideration of reconnection in the background of ‘Hall MHD’ has received significant attention; Hall term arises due to differences in the behaviour of ions and electrons – i.e. this is a two-fluid effect: –Key issue – electrons remain frozen to field for ‘longer’ than the ions; –Thus diffusion region develops two-scale structure – the ion diffusion region is larger than the electron diffusion region; –Quadrupolar out-of-plane magnetic fields develop; –Net effect – inclusion of Hall term increases reconnection rate. 2nd February 2016 MSSL Lecture Series

18. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series ‘Hall’ Reconnection Geometry Fast ion flows associated with field reconfiguration Ion and electron decoupling in ion DR  Hall electric current Quadrupole out-of-plane magnetic field components Electron diffusion in e - DR – topology change  3D Multi-scale process! INFLOW OUTFLOW -ΔBY-ΔBY +ΔBY+ΔBY -ΔBY-ΔBY +ΔBY+ΔBY HALL CURRENT SYSTEMS ion demagnetized (ion diffusion region ~1000 km T ~ 1s) electron demagnetized, (electron diffusion region ~25 km, T ~ 0.1s ) Outflow jets (Global scale ~10s R E, T ~ secs → mins)

19. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series ‘Hall’ Reconnection Geometry INFLOW OUTFLOW HALL CURRENT SYSTEMS -ΔBY-ΔBY +ΔBY+ΔBY -ΔBY-ΔBY +ΔBY+ΔBY An electric field E Y drives reconnection (it can be shown to be equivalent to the reconnection rate). How/where is this electric field supported? Consider the generalised Ohms Law: E Y

20. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016MSSL Lecture Series e.g. Hall-MHD simulations by Yin et al., Phys. Plasmas, 2003: A) Ideal MHD contribution is significant on the large scale; B) Ion decoupling scale (Hall) contribution appears around central sheet; C) The contribution from the electron pressure tensor term is very localised. This originates from the derivatives of the off-diagonal terms. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Ohm’s Law – Reconnection Electric Field E Y Z Z Z EY EY Sun Z Z Z X A B C

21. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Magnetic Reconnection in Action 2nd February 2016 MSSL Lecture Series

22. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016MSSL Lecture Series Magnetic Reconnection in the Universe (i)Example: SGR1806-20 Magnetar produced an intense gamma-ray flare on December 27, 2004: Loss of equilibrium through magnetic diffusion and/or reconnection drives small cracks in the neutron star crust  frequent short γ-ray bursts; Global rearrangements of the magnetic field in the interior and magnetosphere (reconnection?) of the star  giant flares. (Movie from NASA Web)

23. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP A Model of Solar Flares Solar flares are dramatic events releasing up to 10 25 J of stored magnetic energy over a period of hours – strong brightening in soft x- rays; Flares generate plasma heating and fast particle beams - signatures across the EM spectrum from gamma rays to radio; Primary energy release process believe to be magnetic reconnection. 2nd February 2016MSSL Lecture Series Tsuneta 1996

24. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Yokoyama et al, 2000 Evidence for Reconnection in Flares

25. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Heated plasma on reconnected loop Plasmoid ejection Inflow X-Point Yokoyama et al, 2000 CME ejection and coronal loop formation interpreted as a result of magnetic reconnection – the breaking and reconfiguring of the solar magnetic field. These are interpretations of remotely sensed data – but can we directly examine the physics of this process? Evidence for Reconnection in Flares

26. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series The ‘In situ’ Advantage? How can we get to the real physics underpinning reconnection processes and test otherwise poorly constrained hypotheses? The only hope to realistically probe reconnection physics within a reasonable timescale is through relevant in-situ measurements in regions in which it occurs in and around our own magnetosphere;

27. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series The Magnetospheric Plasma Laboratory – The in situ Advantage The terrestrial magnetosphere is a readily accessible ‘laboratory’ Reconnection occurs in a number of contexts: Although the parameter regimes may differ significantly, unique in situ measurements of processes in and around the terrestrial magnetosphere have potential value to the broader understanding of reconnection? SOLAR WIND BOW SHOCK Magnetopause

28. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Reconnection in the Solar Wind 2nd February 2016 MSSL Lecture Series Phan et al., 2006 Reconnection may occur coherently over very large spatial distances and persist for many hours

29. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Reconnection in the Solar Wind 2nd February 2016 MSSL Lecture Series Foster et al., 2015 On the other hand, evidence also exists for reconnection happening in more localised patches in space and/or time:. (this example from Alice Fosters work)

30. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 28th August 2013 STFC Summer School Formation of the ‘Open’ Magnetosphere Solar Wind Bow Shock Magnetosheath Magnetopause Interplanetary Magnetic Field  E SW     J TAIL J MP

31. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Some tests of reconnection in or around the terrestrial magnetosphere.

32. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series De Hoffman-Teller and Walen Tests [Phan et al., 2001] On reconnected field lines, MHD and cold plasma theory suggests that as the plasma crosses the current layer, it should be accelerated by speeds ~V A, the external Alfvèn speed (± the external flow speed). Strong correlations from these analyses can provide an operational test of whether an observed plasma flow may be a result of reconnection.

33. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Particle Distributions in the Dayside Boundary Layer - Theory Magnetospheric populations of ionospheric (I) and ring current (R) origin are partially reflected and accelerated at the MP (I R, R R ). The external magnetosheath population (M T ) is accelerated and transmitted through the MP. Only those magnetosheath particles with speeds greater than V F (=V DHT ) can move into the boundary layer. The resulting distribution of sheath particles has a ‘D’-shape [Cowley, 1982]. (I) VFVF (R) (I R ) (R R ) (M T ) He + O+O+ V L (km s -1 ) Log f (m -6 s 3 )

34. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Particle Distributions in the Dayside Boundary Layer - Observation Smith and Rodgers [1992] Magnetosheath Boundary Layer

35. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Effects of IMF B Y Gosling et al., [1990]

36. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Asymmetric Plasma Entry to Magnetotail 2nd February 2016 MSSL Lecture Series

37. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 30 th March 2009 Reconnection Workshop Magnetotail Twisting i) Southward B Z  Cowley [1982] Owen et al. [1995] 

38. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 30 th March 2009 Reconnection Workshop Magnetotail Twisting ii) N B Z

39. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series ‘Hall’ Reconnection Geometry Fast ion flows associated with field reconfiguration Ion and electron decoupling  Hall electric current Quadrupole out-of-plane magnetic field components Electron diffusion  3D Multi-scale process! INFLOW OUTFLOW -ΔBY-ΔBY +ΔBY+ΔBY -ΔBY-ΔBY +ΔBY+ΔBY HALL CURRENT SYSTEMS ion demagnetized (ion diffusion region ~1000 km T ~ 1s) electron demagnetized, (electron diffusion region ~25 km, T ~ 0.1s ) Outflow jets (Global scale ~10s R E, T ~ secs → mins)

40. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Some Observational Evidence for Hall Reconnection ▲Runov et al., [2003] demonstrated the existence of quadrupolar B Y structure during multiple Cluster crossings of the current sheet; ► Alexeev et al., [2005] determined parallel currents carried by electrons and showed average electron J ||e reverses at B’ X ~ 15-18 nT, with a corresponding enhanced +ve B’ Y. B’ X (nT) B’ Y (nT) J || (nA m -2 ) Current Sheet Lobe Earthward Flow Region, North Lobe Quadrant

41. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series ….and many more… Flux Transfer Events; Magnetospheric substorms; Twisting of the magnetosphere by magnetic torques applied by reconnected field lines; Asymmetric plasma entry into magnetosphere; Reconnection in Kelvin-Helmholtz vortices; Velocity-dispersed plasma entry into the boundary layers and magnetic cusps; Etc. Manifestations of time dependent reconnection modulated by the interplanetary magnetic field.

42. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series The Way Forward Although Cluster (and other multi-point missions) have provided some real constraints for hypothesis testing, it is unable, for example, to resolve the controlling microphysics of the electron diffusion region: –Particle instruments generally restricted to timescales associated with spacecraft spin rates for full 3D measurements This is insufficient to determine, for example: –the roles played by electron pressure gradient and inertial effects (c.f. generalised Ohms law); –the role of turbulent dissipation in driving magnetic reconnection in the electron diffusion region; –the rate of magnetic reconnection and the parameters that control it. –the role played by ion inertial effects in the physics of magnetic reconnection.

43. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 43 100,000 km 500 km 100 km Unstable, thin current sheets have thickness < 1000 km “Electron diffusion region” thickness is of order 10 km Current sheet motion is typically 10 to 100 km/s Time resolution needed to adequately sample electron diffusion region ~30 ms Important Scale Sizes 2nd February 2016 MSSL Lecture Series

44. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 44 Magnetospheric Multiscale Mission The MMS Mission science addresses magnetic reconnection using the Earth’s magnetosphere as a laboratory; Small spacecraft separations, very fast plasma measurements used to target the time and space scales of the electron diffusion region; Launch occurred on March 12, 2015. http://mms.space.swri.edu 2nd February 2016 MSSL Lecture Series

45. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Fast Plasma Instrument (FPI) First “Video” Plasma Analyzer 45 DIS DES Dual Electron Sensor: DES Dual Ion Sensor: DIS DES DIS Objective: Image full sky at 32 energies: electrons in 30 ms, ions in 150 ms Design Concept: Four ion and four electron dual deflecting-aperture top hat sensors for field of view and aperture DES Cutaway DIS DES

46. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP Some things not discussed…. ….include: A plethora of simulation results testing / predicting the action of various physical processes in driving magnetic reconnection; Reconnection in more complex 3D magnetic field geometries (mostly relevant to the solar photosphere/corona application); –Reconnection at nulls, spines, fans, separators, quasi-separatrix layers, etc. and more…. 2nd February 2016 MSSL Lecture Series Priest & Forbes, 2000

47. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP 2nd February 2016 MSSL Lecture Series Conclusion: Some Key Questions on Magnetic Reconnection How is reconnection initiated? –What triggers reconnection within a current sheet? –How are thin current sheets formed? –What is the role of external driving in reconnection onset? What parameters control the spatial/temporal characteristics of reconnection site? –How is a reconnection neutral line (or multiple X-line) structured ? –Why (when?) is reconnection bursty/steady/intermittent ? –How significant are the effects of a guide field, velocity shear and density gradients? –What are the effects of different particle populations on the reconnection process? What are the consequences of reconnection? –How are ions and electrons energized as a consequence of reconnection ? –How are Alfvén waves and other plasma waves generated near the reconnection site?; how these waves interact with ambient plasma? –How does the reconnection jet interact with Earth dipole field?

48. UCL DEPARTMENT OF SPACE & CLIMATE PHYSICS SPACE PLASMA PHYSICS GROUP END Thank you for listening. 2nd February 2016 MSSL Lecture Series