Magnetic Reconnection

Transcription

1 Magnetic Reconnection Prof. Christopher J. Owen UCL/Mullard Space Science Laboratory, Holmbury St. Mary, Dorking, Surrey, RH5 6NT, United Kingdom 9 th September 2015

2 Outline MHD, topology conservation; What is reconnection?; Characteristics of reconnection models; Reconnection in different applications; Observational tests of reconnection processes; Some Cluster results relating to reconnection microphysics; The way forward MMS; Conclusions. 9 th September 2015

3 n t MHD Equations (Recap) n v 0 2 B 1 v v.p E q t t E B v = - = B t o j + P 0* 1 c 2 E t. E. B = 0 9 th September 2015 = q o B. B F 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

4 Ideal MHD Ohms Law Many (most) 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. 9 th September 2015

5 9 th September 2015

6 Magnetic Topology Conserved 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! 9 th September 2015

7 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: 1 ne e E v B j j B.Pe 2 1 ne m ne j t The RHS terms are: 1 ne 1 ne j m e 2 ne j B.P e j t resistive term (like the ordinary Ohms law); Hall term due to Lorentz force; 9 th September 2015 term due to a possible anisotropic electron pressure; term due to contribution of electron inertia to current;

8 Induction Equation Many of you will be familiar with the Induction equation for a plasma which can be derived from an Ohms law with just the resistive term on the RHS: B tt vb 2 B ; 1 o A dimensional analysis on the scales of the terms of the RHS leads to the magnetic Reynolds number for the Convection Term Diffusion Term 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. 9 th September 2015

9 Topology Changer 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.

10 Magnetic Reconnection? 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).

11 Mixing of Plasma Populations

12 SLOW INFLOW FAST OUTFLOW JET? Magnetic Reconnection Key Points 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.

13 What is outflow speed? Can be derived in back-of-the-envelope consideration of energy conversion: Consider elemental volume of inflow plasma, number density n, in field of strength B, which E x B drifts into a current sheet; Consider further that, for simplicity all the associated inflow magnetic energy flux is converted to outflow ion kinetic energy flux during the interaction with the current sheet, ignoring inflow KE: What will be the outflow speed of the particles? 9 th September 2015

14 Answer

15 Timescales Note that the timescale for propagation of the outflow across the system (Tout = L/Vout) is much larger than the diffusion timescale (ΤD = L2/η); Indeed the ratio of these timescales defines the Lundqvist number: S = TD /Tout = LVout/η S is very large in many astrophysical situations: E.g. S ~ 1012 in solar corona hence resistive diffusion can be considered very slow and ineffective. This was a problem for early models of reconnection, despite more sophisticated analysis

16 Early Steady-State Models

17 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!)

18 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.

19 Hall Reconnection Geometry OUTFLOW INFLOW +ΔBY -ΔBY -ΔBY Fast ion flows associated with field reconfiguration Ion and electron decoupling in ion DR Hall electric current +ΔBY HALL CURRENT SYSTEMS Quadrupole out-of-plane magnetic field components Electron diffusion in edr topology change 3D Multi-scale process! electron demagnetized, (electron diffusion region ~25 km, T ~ 0.1s) ion demagnetized (ion diffusion region ~1000 km T ~ 1s) Outflow jets (Global scale ~10s RE, T ~ secs mins)

20 Hall Reconnection Geometry OUTFLOW INFLOW +ΔBY -ΔBY EY -ΔBY EY +ΔBY HALL CURRENT SYSTEMS E vi B j j B ene A cross-tail electric field EY 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:.Pe me dv e ene e dt

21 Ohm s Law Reconnection Electric Field EY EY Z e.g. Hall-MHD simulations by Yin et al., Phys. Plasmas, 2003: Z vi B A) Ideal MHD contribution is significant on the large scale; ene C Pe ene X j B Z C) The contribution from the electron pressure tensor term is very localised. This originates from the derivatives of the off-diagonal terms. B Z Z Z B) Ion decoupling scale (Hall) contribution appears around central sheet; A Sun

22 Ohm s Law Other components of E also appear: EZ Hall Term Contributions to EZ from Hall and electron pressure tensor are spatially more extended; Z - The field from the Hall term points towards the sheet. EZ -.P Term - The field from the.p term points away from the sheet. X

23 Magnetic Reconnection in Action

24 Magnetic Reconnection in the Universe (i) Example: SGR 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)

25 A Model of Solar Flares Solar flares are dramatic events releasing up to 1025 J of stored magnetic energy over a period of hours strong brightening in soft xrays; 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. Tsuneta 1996

26 Yokoyama et al, 2000 Evidence for Reconnection in Flares

27 Evidence for Reconnection in Flares Yokoyama et al, 2000 Plasmoid ejection X-Point Inflow Heated plasma on reconnected loop 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?

28 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;

29 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? BOW SHOCK Magnetopause SOLAR WIND

30 Reconnection in the Solar Wind Reconnection may occur coherently over very large spatial distances and persist for many hours Phan et al., 2006

31 Reconnection in the Solar Wind On the other hand, evidence also exists for reconnection happening in more localised patches in space and/or time:. (this example form Foster et al. 2015) Foster et al., 2015

32 Some tests of reconnection in or around the terrestrial magnetosphere.

33 [Phan et al., 2001] De Hoffman-Teller and Walen Tests On reconnected field lines, MHD and cold plasma theory suggests that as the plasma crosses the current layer, it should be accelerated by speeds ~VA, 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.

34 Particle Distributions in the Dayside Boundary Layer - Theory VF Log f (m-6 s3) (I) (IR) (MT) He+ (R) O+ (RR) VL (km s-1) Magnetospheric populations of ionospheric (I) and ring current (R) origin are partially reflected and accelerated at the MP (IR, RR). The external magnetosheath population (MT) is accelerated and transmitted through the MP. Only those magnetosheath particles with speeds greater than VF (=VDHT) can move into the boundary layer. The resulting distribution of sheath particles has a D -shape [Cowley, 1982].

35 Particle Distributions in the Dayside Boundary Layer - Observation Magnetosheath Boundary Layer Smith and Rodgers [1992]

36 Velocity Filter Effects B VEXB 2VP VEXB VF VP NL Gosling et al. [1990]

37 Cusp Velocity Dispersion for Southward IMF

38 Gosling et al., [1990] Effects of IMF BY

39 Asymmetric Plasma Entry to Magnetotail

40 Cowley [1982] Magnetotail Twisting i) Southward BZ Owen et al. [1995] 30th March 2009 Reconnection Workshop

41 Magnetotail Twisting ii) N BZ 30th March 2009 Reconnection Workshop

42 .and many more Manifestations of time dependent reconnection modulated by the interplanetary magnetic field. 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.

43 The Cluster Contribution CLUSTER is an ESA mission consisting of 4 spacecraft orbiting in formation with separations varying between a few hundred and ~10000 km Multi-point measurements allow some assessment of the action of gradients, motion of waves and boundaries, and variations across different regions; In situ measurements by Cluster reveal that large-scale dynamics are controlled by processes such as magnetic reconnection; We have been able to directly test some aspects of theoretical models.

44 Hall Reconnection Geometry OUTFLOW INFLOW +ΔBY -ΔBY -ΔBY Fast ion flows associated with field reconfiguration Ion and electron decoupling Hall electric current +ΔBY HALL CURRENT SYSTEMS Quadrupole out-of-plane magnetic field components Electron diffusion 3D Multi-scale process! electron demagnetized, (electron diffusion region ~25 km, T ~ 0.1s) ion demagnetized (ion diffusion region ~1000 km T ~ 1s) Outflow jets (Global scale ~10s RE, T ~ secs mins)

45 J (na m-2) Current Sheet Lobe Some Observational Evidence for Hall Reconnection Runov et al., [2003] demonstrated the existence of quadrupolar BY structure during multiple Cluster crossings of the current sheet; B Y (nt) Earthward Flow Region, North Lobe Quadrant Alexeev et al., [2005] determined parallel currents carried by electrons and showed average electron J e reverses at B X ~ nt, with a corresponding enhanced +ve B Y. B X (nt)

46 Empirical Evaluation of Terms in Generalised Ohms Law Note both j B B o B and P e are derived from gradient terms, and thus both need multi-point (i.e. Cluster) measurements; Henderson et al. (2006) attempted to measure and compare these two terms; Looked at small Cluster separations in the magnetotail current sheet: ~ 200 km in Calculated for the first time the spatial derivatives of the full tensor of the plasma distribution; required non-trivial inter-spacecraft calibration efforts

47 2. Analysis techniques Henderson et al., (2006) Observations taken close to an active X-line on 17th Aug 2003; No encounter with the diffusion region, but observed an anti-correlation in.pe and Hall terms in the component normal to the neutral sheet.

48 JxB/ne (-.P/ne) JxB/ne (-.P/ne) x -5.3

49 Result consistent with the simulations shown earlier, but what are the wider implications? the result is repeatable in other events, although the ratio of the two terms varies; Is this a function of plasma conditions/reconnection rates at the X-point itself (electron scale)? Here the electrons play a crucial role, but the relevant variations occur on time and spatial scales (0.1sec, 10 km) that can not be fully resolved by Cluster. E.g. Electron scale current layers shown here (~20km, 510 le,re) seen for only a fraction of a spin (4 secs, minimum time needed to resolve full particle distribution) Alternately this may be a function of the large (MHD) scale boundary conditions (lobe field strength, plasma sheet density/ temperature)? Andre et al., secs

50 Energetic Particle Acceleration Electron acceleration within the reconnection region: Cold electrons are accelerated to kev energies by very small timescale (millisecond) interactions with electric field solitary waves [Cattell et al., 2005]. Further acceleration to 100 s kev involves nonadiabatic processes in the outflow region [Imada et al., 2007].

51 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.

52 Important Scale Sizes 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 Required time resolution for electron diffusion region is ~30 ms 52

53 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,

54 Fast Plasma Instrument (FPI) First Video Plasma Analyzer Objective: Image full sky at 32 energies: electrons in 30 ms, ions in 150 ms DIS DES DIS DES Design Concept: Four ion and four electron dual DES deflecting-aperture top hat DIS sensors for field of view and aperture DIS DES Dual Electron Sensor: DES DES Cutaway Dual Ion Sensor: DIS 54

55 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. Priest & Forbes, 2000

56 Conclusion: Some Key Questions on Magnetic Reconnection How is reconnection initiated? What parameters control the spatial/temporal characteristics of reconnection site? What triggers reconnection within a current sheet? How are thin current sheets formed? What is the role of external driving in reconnection onset? 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?

57 END Thank you for listening.