SM51D-01 08:00h
Impulsive Reconnection Dynamics: From the Laboratory to the Local Cosmos
Reconnection in nature is rarely quasi-steady. Most often, it is impulsive or bursty. Impulsive reconnection is characterized not only by a fast growth rate, but by a rapid change in the time-derivative of the growth rate. Recent developments in collisionless reconnection theory, based on the generalized Ohm's law, hold the promise of providing solutions to outstanding problems involving impulsive reconnection phenomena spanning laboratory, magnetospheric, and solar physics. We illustrate the application of the theory to magnetopsheric substorms, solar flares, and sawtooth crashes in toroidal plasmas. Although reconnection is spontaneous in some of these cases and forced in others, all of these cases can be studied from a common perspective. We present analytical as well as high-resolution simulation results based on Hall MHD (or two-fluid) equations, and address the important issue of the scaling of the (time-dependent) reconnection rate with respect to resistivity and/or particle inertia.
SM51D-02 08:15h
The Preferential Occurrence of Electrostatic Structures in Collisionless Reconnection
Recently, very strong electrostatic structures with Debye scale at the dayside magnetopause have been frequently observed by the Polar satellite. These structures were commonly found on the magnetospheric side of the Earth's magnetopause. In our early full particle simulation starting from a symmetric initial conditions, we demonstrated that these strong electrostatic layers with Debye scale are associated with the magnetic reconnection and assist the formation of Petschek-type slow shock just inside of the reconnection separatrices. In this presentation, we carry out a full particle simulation with an asymmetric initial condition. The results indicate that the electrostatic layer is found only in the low density side or the magnetospheric side, which is consistent with the observations. Comparing different initial conditions, we try to identify the key parameters that control the occurrence of the Debye length scaled electrostatic layer.
SM51D-03 08:30h
Non-Equilibrium and Current Sheet Formation in Astronomical Magnetic Fields
For over three decades, E. N. Parker [Astrophys. J., 174, 499 (1972)] has argued that current sheets, or tangential discontinuities, of the magnetic field generally exist in magnetic equilibrium in astronomical magnetic fields, such as in stellar corona, and contribute to heating these plasmas to million degree temperatures, thus producing X-ray emission. We have recently presented a theorem [Phys. Plasmas, 5, 4028 (1998)] showing that the Parker model of coronal heating is rigorously viable. Our theorem proves that there can be at most one smooth magnetostatic equilibrium for a given smooth footpoint mapping between boundaries with line-tied magnetic fields. It follows that if such a static equilibrium is driven unstable by footpoint motions, there is no other smooth equilibrium for the plasma to relax to, leading to magnetic non-equilibrium and the formation of current sheets. New theoretical and numerical supports of this result will be presented. Possible topologies of such current sheets based on analytical considerations and numerical simulations will be discussed. Comparison will be made with methods of determining the locations of the current sheets (and thus possible sites of magnetic reconnection) based on theories of quasi-separatrix layers (QSLs).
SM51D-04 08:45h
Electron Energization During Magnetic Reconnection
The energization of electrons during magnetic reconnection is explored. The production of energetic electrons has been documented in observations of solar flares, reconnection in the Earth's magnetosphere and in laboratory experiments yet the understanding of these widespread observations remains poor. The parallel electric field that develops during reconnection controls the acceleration of electrons in situations with an ambient guide field. The structure of this parallel electric field differs greatly from earlier ideas: during reconnection a deep cavity in the electron and ion density forms that extends through the x-line along one of the magnetic separatrices. This density cavity enables the parallel reconnection electric field to remain finite over an extended region, forming an acceleration cavity that controls the strength of the current layer that defines the dissipation region. The acceleration of electrons in a single pass through this cavity, however, does not explain the observed powerlaw spectra of energetic electrons seen in some data. In particle simulations a distinct high energy tail, extending well above the rest energy, forms on the electron energy distribution. These very energetic electrons are found to arise from multiple encounters with acceleration cavities. The simulations provide evidence that reconnection with a guide field is dominated by the formation of many islands and that electron energization results from multiple accelerations. In this picture the surprising amount of energy going into electrons in comparison with ions is first because of the significant length of the acceleration cavities (large numbers of electrons enter the cavities) and because of their high mobility -- they can rapidly interact with many cavities to reach high energy.
SM51D-05 09:00h
Kinetic simulations of magnetic reconnection in plasma with different beta values
We present kinetic simulations of collisionless magnetic reconnection in Harris current sheets. We simulate plasmas with different beta values by varying the guide fields. For all values of $\beta >m_e/m_i$, fast reconnection is made possible by the separation of the electron and ion dynamics in the reconnection region. The primary mechanism that relaxes the frozen-in conditions is given by the non-gyrotropic electron pressure terms for all guide fields considered. The reconnection rate is then enhanced by the Whistler dynamics in high beta plasmas and by the Kinetic Alfven Waves dynamics in lower beta plasmas. The guide field diminishes the reconnection rate and decreases the reconnection saturation level. The ion and electron flow pattern, acceleration, and heating are strongly influenced by the guide field. The simulations are performed by using two Particle-in-Cell codes: CELESTE3D, an implicit PIC code that runs on a single processor machine, and NPIC, a massive parallel explicit code that is able to resolve all the plasma time and length scales. The results of the two codes are compared in detail.
SM51D-06 09:15h
The Magnetic Reconnection Diffusion Region from Four-Spacecraft Observations
Magnetic reconnection leads to energy conversion in large volumes in space but is initiated in small diffusion regions. Due to the small sizes of the diffusion regions, their crossings by spacecraft are rare. We report on Cluster observations of diffusion region encounters. These four-spacecraft observations allow us to reliably distinguish spatial from temporal features. Examples show that diffusion regions are stable on ion time and length scales in agreement with numerical simulations. The electric field normal to the current sheet is balanced by the Hall term in the Generalized Ohm's Law, thus establishing that Hall physics is dominating the diffusion region. The reconnection rate is fast, about 0.1. Strong parallel currents flow along the separatrices, and are correlated with Langmuir or upper hybrid waves.
http://link.aps.org/abstract/PRL/v93/e105001
SM51D-07 09:30h
Laboratory Studies of the Physics of Two-Fluids MHD Physics for Magnetic Reconnection
Magnetic reconnection is one of the most important self-organization processes in plasmas. The recent advances in laboratory plasma physics, along with the surge of space physics data from satellites, have made cross-cutting research very useful for obtaining new physics understanding of its key processes. Also the recent rapid advance of numerical simulation has played an important role to bridge laboratory data with space-astrophysical observations [1]. Reconnection plays a central role in the interaction between the solar wind and the earth's dipole field. Recently the two-fluids MHD physics issues for reconnection dynamics have been intensively investigated in both 2-D and 3-D geometries. The results from dedicated laboratory experiments on magnetic reconnection [2] depict many striking commonalities with the observations in the dayside and tail-side magnetosphere sheaths [3,4]. This paper provides a brief review of the most recent results from MRX [Magnetic Reconnection Experiment] [5,6] which address two-fluid physics issues for magnetic reconnection as well as comparison of the MRX data with the recent space observations. In collaboration with H. Ji, S. Gerhardt, R. Kulsrud, A. Kuritsyn, Y. Ren Work supported by DOE, NASA, and NSF 1. M.A. Shay and J.F. Drake, Geophys. Res. Lett. 25, 3759 (1998). 2. M. Yamada, Earth Planets Space v.53, 539 (2001) 3. F. Mozer et al., Phys. Rev. Lett 89, 15002-1 (2002); 4. T. D. Phan et al., Nature, 404:848, (2000). 5. M. Yamada et al., Phys. Plasmas 7, 1781 (2000) 6. H. Ji et al., Phys. Rev. Letts. V.92, 115001 (2004)
http://mrx.pppl.gov/
SM51D-08 09:45h
Rotational Stress as a Factor in Magnetotail Reconnection
Efforts to understand the dawn-dusk asymmetries of the Jovian magnetosphere lead to new ways of thinking about the process of reconnection in the magnetotail. Outflowing plasma, accelerated by the centrifugal stresses on the rotating flux tube, strongly depletes the inner portion of the flux tube, which can then readily reconnect. The reconnection produces a zone of depleted closed flux tubes at the outer boundary of the plasma disk. The irreversible outflow and associated loss of heavy ions maintains a quasi-steady state in the Jovian magnetosphere.