SH42B-01 INVITED
Magnetic Reconnection in the Solar Wind: Recent Results
Magnetic reconnection occurs frequently at thin current sheets in the solar wind and produces exhausts of jetting plasma bounded by back-to-back rotational discontinuities. The exhausts are identified as roughly Alfvenic accelerated plasma confined to field reversal regions that almost always take the form of bifurcated (double-step) current sheets. The exhausts are embedded within the solar wind flow and are convected past a spacecraft on time scales ranging from seconds up to several hours, with time scales less than 100 s being dominant. Near solar minimum reconnection exhausts are swept past Earth at a rate of 40 to 70 events/month, with numerous events being associated with local magnetic field shear angles considerably less than 90 degrees. The exhausts are observed predominantly in the low-speed solar wind or in association with interplanetary coronal mass ejections in plasma predominantly having low proton beta; however, reconnection also occurs in the turbulent, high-speed wind from coronal holes, but less frequently. Multi-spacecraft observations demonstrate that the exhausts often result from prolonged (hours) reconnection at extended (up to at least 4.3 x 106 km) and continuous X-lines.
SH42B-02 INVITED
Trapped electron model for in situ observations of magnetotail reconnection.
Numerical simulations of magnetic reconnection [1] show that if the plasma resistivity is uniform then reconnection will be slow and occur in the familiar Sweet-Parker geometry. On the other hand, if some mechanism exists that allows for a locally enhanced resistivity then reconnection can proceed at a much faster Alfvenic rate in geometries similar to those predicted by the Petschek reconnection model. Recently, direct in situ observations by the Wind satellite of the electron distribution function during reconnection in the magnetotail [2] have revealed that electrons in the inner reconnection region follow electrostatically trapped trajectories [3]. Perhaps the most important effect of this trapping is that it implies a nearly symmetric bounce motion of the electrons along the magnetic field; a symmetry which is also reflected in their distributions function f(v\parallel,v\perp) ~ f (-v\parallel,v\perp). For reconnection geometries including a guide magnetic field it follows that the current along the reconnection X-line must be limited, j\parallel=\int v\parallel f d3v~0. Thus, the kinetic behavior of the trapped electrons causes a locally enhanced resistivity, which in a fluid description would appear as a finite \nabla· P-term in the generalized Ohm's law. In turn this localized effect yields the fast reconnection geometry for which the rate is likely to be controlled by two-fluid dynamics [4] externally to the diffusion region of trapped electrons. Magnetic reconnection including electron trapping is studied experimentally at the Versatile Toroidal Facility (VTF) at MIT. We find that electron trapping is so efficient in limiting the plasma current that in a driven scenario reconnection proceeds at the rate imposed externally. In the talk I will first discuss the evidence in the Wind data for electrostatic electron trapping. Then I will present a theory for why the electrostatic trapping potential develops and finally I will provide direct experimental observations from VTF of how trapped electrons yield a large effective and localized resistivity. [1] D Biskamp and E Schwarz, (2001) Phys. Plasmas 8, 4729. [1ex] [2] M Oieroset, RP Lin, TD Phan, DE Larson, and SD Bale, (2002) Phys. Rev. Lett. 89, 195001. [1ex] [3] J Egedal, M Oieroset, W Fox and RP Lin, (2005) Phys. Rev. Lett. 94, 025006. [1ex] [4] Y. Ren et al., (2005) Phys.\ Rev.\ Lett. 95, 055003.[1ex] Work supported by DOE Junior Faculty Award DE-FG02-06ER54878.
SH42B-03
Laboratory Observation of Large-Amplitude Electrostatic Fluctuations Driven by Magnetic Reconnection
We report recent observations of nonlinear electrostatic fluctuations excited during spontaneous reconnection events on the VTF experiment~[1]. Electrostatic fluctuations are observed by small, high-bandwidth, impedance- matched Langmuir probes. Among a large number of wave phenomena, we observe narrow, large-amplitude, positive potential spikes, with fluctuations in probe current on the order of the ion saturation current drawn by the probe, implying ñ/n ~ 1, or e\tilde{φ} / k Te ~ 1. The spikes are observed in conjunction with large inductive electric fields (the "reconnection electric field"). With arrays of such probes we have observed the speed and shape of these propagating structures. The spike drift speed, which is parallel to the electron drift, is roughly 5 106~m/s, or 2vte (vte = \sqrt{2kTe/me}). Based on the drift speed and the time to cross a single probe tip, we infer that the parallel size is 1-2~mm (50-100~λD, or 5-10~ρe). Observations with probes spaced perpendicular to the magnetic field shows that the perpendicular size is also 1-2~mm. Finally, we will discuss our interpretation of the structures and their generation mechanism, with insights from a newly installed electron energy analyzer. [1] J. Egedal, W. Fox, N. Katz, M. Porkolab, K. Reim, and E. Zhang. (2007). PRL 98, 015003. This was supported by DOE contracts DE-FC02-04ER54786 and DE-FG02-06ER54878, and NSF/DOE PHY-0613734
SH42B-04
Large-scale Reconnection at the Dayside Magnetopause: Results from Global Simulations
Measurements from multi-spacecraft mission offer a unique opportunity to investigate the large-scale topology and dynamics of magnetic reconnection at the dayside magnetosphere. Recent comparisons of 3D global magnetohydrodynamic (MHD) simulations with observations for the April 6, 2004 conjunction event between the Double Star (TC1) and Cluster spacecraft indicate that antiparallel merging at high latitudes and component merging in the subsolar region can occur simultaneously. To assess the relative contribution of each mechanism in the transfer of solar wind energy and mass to the magnetosphere, we use large-scale kinetic (LSK) calculations to compute a large sample of ion trajectories in the time-dependent MHD electric and magnetic fields. We determine the locations of the dayside magnetopause where ions gain significant amount of energy and compare them with the results of the global MHD simulations to reconstruct the topology of the merging region.
SH42B-05
Magnetopause Reconnection in the Lyon-Fedder-Mobarry Code
Magnetic reconnection is an important process for the transfer of solar wind plasma to the magnetosphere, and one which has been simulated in many three dimensional numerical models. This talk will present the results of a study of reconnection at the Earth's dayside magnetopause using the Lyon-Fedder-Mobarry (LFM) global magnetospheric simulation code, a code based on the equations of magnetohydrodynamics where the frozen-in condition is broken via numerical resistivity. In this study we were able to identify regions of magnetic reconnection at the magnetopause and categorize them as component or anti-parallel in nature for several IMF clock angles. We will also present results summarizing the time dependence of these regions and how magnetic reconnection depends on the spatial resolution of the simulation.
SH42B-06
Evidence for Collisionless Magnetic Reconnection at Mars
Magnetic reconnection is a fundamental plasma process that enables the rapid conversion of magnetic to particle energy and is important in astrophysics as well as solar, space and planetary physics. Using data from the Mars Global Surveyor (MGS) spacecraft in combination with simulations of reconnection, we present the first direct evidence of collisionless magnetic reconnection at Mars. The evidence indicates that the spacecraft passed through the diffusion region where reconnection is initiated and observed the magnetic field signatures of differential electron and ion motion that uniquely indicate the reconnection process. These are the first such in- situ reconnection observations at an astronomical body other than the Earth. Reconnection may be the source of Mars" recently discovered auroral activity and the changing boundaries of the closed regions of crustal magnetic field.
SH42B-07
Dissipation in Relativistic Pair-Plasma Reconnection
We present an investigation of the relativistic dissipation in magnetic reconnection. The investigated system consists of an electron-positron plasma. A relativistic generalization of Ohm's law is derived. We analyze a set of numerical simulations, composed of runs with and without guide magnetic field, and of runs with different species temperatures. The calculations indicate that the thermal inertia-based dissipation process survives in relativistic plasmas. For anti-parallel reconnection, it is found that the pressure tensor divergence remains the sole contributor to the reconnection electric field, whereas relativistic guide field reconnection exhibits a similarly important role of the bulk inertia terms.