Presentation on theme: "Plans for Magnetic Reconnection Research Masaaki Yamada Ellen Zweibel for Magnetic Reconnection Working group CMSO Planning Meeting at U. Chicago November."— Presentation transcript:
Plans for Magnetic Reconnection Research Masaaki Yamada Ellen Zweibel for Magnetic Reconnection Working group CMSO Planning Meeting at U. Chicago November 18-19, 2003
Contents Current Status and Issues –Experiments (15 min) –Theory and Simulations (15 min) Immediate Plans (20 min) Long-range Plans (5 min) Discussions (30 min)
Two competing models to explain fast reconnection 1-Fluid MHD model + effective resistivity (Effects of waves) 2-Fluids MHD; decoupling of ions and electrons within the ion skin depth, c pi ; -> Including the Hall term, j e xB Dedicated lab experiments [MRX, SSX, TS-3, VTF etc.] Generalized Sweet-Parker modelPetschek-type Model
Sheet thickness agrees well with Harris model is not determined by Classical Sweet-Parker thickness ---- Classical Sweet-Parker width 0. 35 c/ pi Collisional regime
The measured current sheet profiles agree well with Harris theory
Sheet thickness agrees well with Harris model scales with Harris model –Demonstrate the effects of 2-fluids plasmas –Constant normalized drift velocity
Mozer et al., PRL 2002 POLAR satellite A reconnection layer has been documented in the magnetopause
Numerical simulation can assess 2-fluids effects JeJe JiJi ViVi Below c/ pi electron and ion motion decouple electrons frozen-in Observed out-of-plane quadrupole fields Obtained a thin electron current layer of c/ pe Drake et al These results have not been verified in lab experiments
Reconnection speeds up drastically in low collisionality regime What causes the anomalous resistivity? Measured resistivity Trintchouk et al, PoP 2003 Collisionality
Fluctuation Amplitudes Correlate with Resistivity Enhancement
Reconnection in MST Spontaneous and forced reconnection occurs (edge reconnection not linearly driven - measured) Current sheet widths larger than linear MHD prediction (or Sweet-Parker width) Hall effects important (Hall dynamo)
Dynamo in a laboratory plasma Toroidal magnetic flux Large scale magnetic field is generated in continuous and discrete events from small scale fluctuations [From MST data] Helicity ~const.
Effects of reconnection in the lab toroidal magnetic flux heat flux (MW/m 2 ) rotation (km/s) ion temperature (keV) dynamo magnetic reconnection energy transport momentum transport ion heating time (ms)
Relationship between global phenomena and local mechanisms TS-3 experiments indicated that an global driving force (merging speed) determined local reconnection rate. Y. Ono et al., Phys. Plasmas, 5B, 3691 (1993) 1. Role of local reconnection on dynamo [=> MST] 2. Plasma merging [MRX, SSX, and TS-3] Global reconnection local reconnection dynamics
I. Overall goals: 1) Find key relationships between the local physics of the reconnection layer and the dynamics of global plasma reconnection. 2) Study comprehensively the 2 fluids MHD effects through the generalized Ohms law in the neutral sheet and determine the role of turbulence in reconnection process, Identify key 3-D effects3) Identify key 3-D effects on reconnection, whether intrinsic or due to boundary conditions. 4) Evaluate the role of magnetic reconnection in dynamos and, more generally, in magnetic self- organization phenomena.
II Major current issues The most important issue, both in laboratory and astrophysical plasmas, is to characterize the relationship between the macroscopic (global structure) and microscopic (local) scale reconnection physics. Our important key issues are; 1) Reconnection rate; It is widely known that most observations in laboratory and astrophysical plasmas show much faster reconnection rate than the Sweet-Parker rate. It is important to find a new model to explain the observed data. 2) Local reconnection dynamics ; It is necessary to develop a comprehensive understanding of the mechanisms by which large scale systems generate the local reconnection structures, through the formation of current sheets, either arising in situ or forced by boundary conditions. It is also crucial to assess 2 fluids MHD effects through the generalized Ohms law in thin current sheets which have been already demonstrated in laboratories. 3) Large scale effects of reconnection; Reconnection influences the energy balance and dynamics of a global plasma structure. Some depend on the details of the reconnection process itself; possibly others do not. It is important to find the extent to which large scale processes are sensitive to the small scale physics of reconnection, and develop macroscopic diagnostics of the reconnection process. 4) Dynamics of spontaneous and driven reconnection in laboratory and astrophysical contexts. It is important to know when and how reconnection is initiated, the effect of the boundary conditions and rational surfaces.
III. Immediate research objectives 1)* Investigate theoretically, computationally and experimentally the local dynamics in the vicinity of the neutral sheet (reconnection layer), to assess 2-fluids MHD effects, such as Hall and turbulence effects. Potential roles of electron diffusion region will be assessed. Evaluate how these 2-fluids effects can be implemented into the MHD description which applies on large scales. We will attempt to identify criteria for the transition from the one-fluid MHD to the non MHD regime. Possible effects include the relative thicknesses of the Sweet-Parker layer and the ion skin depth, the amplitude of the initial perturbation, and the nature of forcing. We will continue to develop the theory of reconnection in weakly ionized systems. There are preliminary indications that the Hall regime is pushed to larger length scales because ion-neutral friction increases the effective ion inertia. 2)* Explore the relationship between anomalous ion heating and reconnection events in both laboratory and astrophysical plasmas and to investigate why T i is generally higher than T e. Magnetic reconnection and anomalous ion heating are observed to be closely associated in both laboratory and astrophysical plasmas. We will address this problem collaboratively in our center experiments and theoretical/computational studies. The effects of the energetic ions on reconnection will be also studied in linear and non-linear stages. 3*) Investigate how the local reconnection process is related to global reconnection and dynamo activities. Study flux conversion process and the effects of turbulent EMF along the mean magnetic field during magnetic self-organization.
III-A. Specific experimental work plans 1)* We will investigate quantitative relationship between the observed enhanced resistivity and all fluctuations of broad spectrum. Our proposed research includes experimental study of the role of magnetic fluctuations in observed in MRX and SSX where reconnection layer is well identified, and also in MST and SSPX where reconnection layer exists in multiple places. 2) An attempt will be made to identify a dominant reconnection layer in MST and SSPX. In these laboratory plasmas, reconnection can occur simultaneously at a number of rational surfaces. In solar flares, energy appears to be released within a large volume. A turbulent medium may have many reconnection sites in close proximity. We will investigate the effects of interaction between multiple reconnection sites on the reconnection rate. 3)** We will investigate how the local reconnection process is related to global dynamo activities or flux conversion process in MST. More specifically, it includes accurate measurements of time evolution of magnetic and flow topology around reconnection sites. This task requires support from theory and simulation; linear, quasi- linear, and possibly nonlinear theories of dominant instabilities (e.g. tearing modes) in realistic 3-D geometry. New theories and simulations using 2-fluid models are likely required to explain experimental data. 4) Joint development of diagnostics tools will be made in the area of spectroscopic measurements (IDSP), and local measurements of magnetic field fluctuations among all four devices. 5)* We attempt to find a scaling of reconnection rate with respect to local plasma parameters in MRX, SSX, MST and SSPX.
III-B. Specific theoretical work plans 1)* Develop asymptotic formulae for the scaling of the reconnection rate in systems in which the reconnection layer is many orders of magnitude less than the size of the system. These formulae can be tested, but not replaced by, numerical simulations. The appropriate model may be a two stage scenario which addresses the development of small scales followed by rapid dissipation. 2)* Reconnection is often posited as an energy source in astrophysical settings, but there are few quantitative models of the energetic effects of reconnection which could be compared with observations. We will compute radiative signatures of plasma heating, particle acceleration, and generation of bulk flows by reconnection in particular systems. 3)* Computations of large scale dynamics cannot simultaneously include the reconnection scale. We will use detailed models of the reconnection region itself, and the coupling between the large and small scales, to develop a parameterization of reconnection which can be used in large scale computations. Optimally, this parameterization would give a good approximation to the energetic and dynamical effects of reconnection. 4)* MHD analysis of reconnection will continue, especially with regard to the effects of turbulence and 3D geometry. The generalized Sweet-Parker model and the Kulsrud model based on 1-fluid MHD will be tested by FLASH code as well as in MRX and SSX.
III-C. Specific computational work plans We anticipate the need for simulations, combined with theory, to complement many of the experimental initiatives. Not all computational capabilities can be realized within a single code, so we expect to use multiple codes, with overlapping regimes of validity. This will permit internal benchmarking studies. Important features will be: 1)Implicit time stepping so that both fast dynamical phenomena and slow resistive phenomena can be studied in the same problem. NIMROD and DEBS are implicit. FLASH is being made implicit. The IRC (anachronistic term for the code developed at U. Iowa) is implicit in 2D and explicit in 3D. 2)Ability to treat time dependent problems in 3 space dimensions, for a greater degree of physical realism in both lab and astrophysical plasmas. NIMROD, DEBS, FLASH, and IRC all have this capability. 3)Flexibility in the boundary conditions, including periodic, line tied, and open, to suit different physical problems. For example, reconnection problems in stellar coronae require line field boundary conditions. 4)Physics beyond MHD. Hall effects and possibly anisotropic electron pressure must be included to model laboratory measurements of the reconnection layer. In astrophysical problems (and lab applications where energetic ions are present) species such as dust grains and cosmic rays should be treated as particles even when the background plasma can be modeled as two fluids. 5)Collaborations involving codes written outside the Center. The IRC is a particularly attractive possibility, as it has a detailed treatment of non-MHD physics, was written with space plasma applications in mind, and there is some overlap of personnel. Once the codes are in place we will use them to model layer physics, including the electron pressure term, and to develop a simple parameterization of layer physics that can be used for larger scale computations in MHD codes.
IV. Long range plans 1)*Inject 1MW NBI in MST and MRX to investigate the role of magnetic reconnection in dynamo phenomena. Within the next two years, the effects of injected hot ions on dynamo and reconnection phenomena will be studied in MST. After that the 20kV, the NBI beam will be injected into MRX to study dynamo effects of injected high-energy ions as well as the effects of hot ions in the reconnection region. These two studies will be carried out collaboratively. 2)*Investigate the effects of turbulence and magnetic chaos on reconnection. We will investigate the effect of a variety of perturbations on reconnection in laboratory plasmas and develop a theory for the interaction between multiple reconnection sites. 3)*Evaluate the roles of magnetic reconnection in other self- organization phenomena, particularly in dynamos and ion heating. In addition we expect strong connection with magnetic chaos and momentum transport phenomena.
System L * (10cm)B * (100G)T * (10ev) MRX11112.04 Astrophysics0.10.0014 Solar Flares11026.53 Magnetosphere12320 Tokamak1010011000893.6, which comes from, where, and is the Lundguist number. Comparison the Sweet-Parker and Ion Inertia Lengths
How can we apply lab results to astrophysics? 1. Can we find a Taylor state in space plasmas? Do we see magnetic helicity conservation? -- There is no defined boundary in most space plasmas 2. Can we identify a reconnection layer? (-> May be Yes) Observed in the magnetosphere Solanskis data in the Sun