Magnetic Reconnection: Theory and Observations III
Presiding: J Birn, Los Alamos National Laboratory; A Klimas, NASA Goddard Space Flight Center
SM33A-01 13:30h
Magnetic Disconnection From the Sun: Observations of a Reconnection Exhaust in the Solar Wind at the Heliospheric Current Sheet
We have recently obtained direct evidence for local, quasi-stationary, magnetic reconnection in the solar wind using plasma and magnetic field data obtained by the Advanced Composition Explorer, ACE. The prime evidence consists of intervals of accelerated or decelerated plasma flow observed within magnetic field reversal regions that we interpret as encounters with (generic) Petschek-type reconnection exhausts that are bounded by Alfven waves. Only 1 out of 42 reconnection exhausts observed by ACE was associated with a crossing of the heliospheric current sheet, HCS, separating open field lines of opposite magnetic polarity. During that singular event, which corresponded to an encounter with the anti-sunward-directed exhaust from a reconnection site that initially lay sunward of the spacecraft, the solar wind electron strahl vanished and the halo electrons interpenetrated from opposite sides of the HCS. These observations demonstrate that magnetic field lines within and immediately surrounding the exhaust were disconnected from the Sun, as expected. More broadly, these observations conclusively demonstrate that strahl dropouts observed in the vicinity of the HCS at least at times are signatures of magnetic disconnection from the Sun.
SM33A-02 13:45h
Fast Magnetotail Reconnection: Challenge to Global MHD Modeling
Representation of fast magnetotail reconnection rates during substorm onset is one of the major challenges to global MHD modeling. Our previous comparative study of collisionless magnetic reconnection in GEM Challenge geometry demonstrated that the reconnection rate is controlled by ion nongyrotropic behavior near the reconnection site and that it can be described in terms of nongyrotropic corrections to the magnetic induction equation. To further test the approach we performed MHD simulations with nongyrotropic corrections of forced reconnection for the Newton Challenge setup. As a next step we employ the global MHD code BATSRUS and test different methods to model fast magnetotail reconnection rates by introducing non-ideal corrections to the induction equation in terms of nongyrotropic corrections, spatially localized resistivity, or current dependent resistivity. The BATSRUS adaptive grid structure allows to perform global simulations with spatial resolution near the reconnection site comparable with spatial resolution of local MHD simulations for the Newton Challenge. We select solar wind conditions which drive the accumulation of magnetic field in the tail lobes and subsequent magnetic reconnection and energy release. Testing the ability of global MHD models to describe magnetotail evolution during substroms is one of the elements of science based validation efforts at the Community Coordinated Modeling Center.
SM33A-03 14:00h
Multiple X line reconnection in the near Earth magnetotail: Cluster multipoint plasma and field observations
Eastwood et al. [2004, manuscript submitted to GRL], have recently reported observations of multiple X line reconnection proceeding in the near Earth (~18Re) magnetotail, leading to the formation and growth of an Earthward moving flux rope. Here we present the associated ion and electron measurements that indicate significant structuring to the magnetic field; in particular, an absence of counterstreaming electrons in the center of the flux rope. The observations, made on October 2 2003, are put into a wider context by examining the surrounding plasma conditions, which indicate that after the event, the plasma sheet was highly dynamic. We also consider how common these observations are in the Cluster dataset, and discuss the implications for previous single spacecraft studies.
SM33A-04 14:15h
Three-Dimensional Modeling of Guide-Field Magnetic Reconnection
The dissipation mechanism of guide field magnetic reconnection remains a subject of intense scientific interest. On one hand, one set of recent studies have shown that particle inertia-based processes, which include thermal and bulk inertial effects, provide the reconnection electric field in the diffusion region. On the other hand, a second set of studies emphasizes the role of wave-particle interactions in providing anomalous resistivity in the diffusion region. In this presentation, we analyze three-dimensional PIC simulations of guide-field magnetic reconnection. Specific emphasis will be on the question whether thermal-inertia processes, mediated by the electron pressure tensor, remain a viable dissipation mechanism in fully three-dimensional systems.
SM33A-05 14:30h
The Scaling of Forced Collisionless Reconnection
We present two-fluid simulations of forced magnetic reconnection in a collisionless two-dimensional slab geometry. In the absence of forcing, our system has Δ' ≤ 0 and as expected exhibits no reconnection. The most common method of driving reconnection in such stable systems is to add a finite amplitude perturbation to the boundaries of the simulation. In contrast, reconnection in our study is driven by a spatially localized forcing function, which is added to the ion momentum equation in the interior of the simulation domain. This function represents a generic external forcing agent that drives plasma and magnetic field toward the reconnection region at a controllable rate. We investigate the behavior of the resulting reconnection as a function of various free parameters in the system, including the temporal and spatial structure of the forcing function, the plasma β , and the presence of an out-of-plane guide magnetic field component. Consistent with previous scaling studies done on systems with relatively large Δ', we find that for sufficiently strong forcing the reconnection process becomes Alfvénic, i.e., the inflow velocity scales roughly like some small fraction of the Alfvén speed based on the reconnecting component of the magnetic field just upstream of the dissipation region. The magnitude of this field and thus the rate of reconnection is ultimately controlled by the behavior of the forcing function.
SM33A-06 14:45h
Reconnection via Thin Current Sheets in Earth's Magnetotail: Theory, Simulations and Cluster Observations
Boundary perturbations of the magnetic field reversal region in plasma result in either the classical X-line reconnection or the formation of tangential discontinuities [Kulsrud and Hahm, 1982]. According to recent Geotail and Cluster observations, the latter scenario is likely to occur in the region earthward of X=-24Re due to the formation of the extended thin current sheet with the thickness comparable to a few thermal ion gyroradii. Within the framework of fluid theory, magnetic reconnection via current sheet is known to be very slow as compared to the X-line case. However, the structure and dynamics of thin current sheets in collisionless plasmas of the geomagnetotail is strongly influenced by the kinetic and nonlocal effects because the ion gyroradius is comparable to the current sheet thickness. Another important factor, which was not considered earlier in the theory, are large-scale flapping motions of the tail current sheet with the amplitude comparable to the current sheet thickness. We present the kinetic theory of thin current sheets, taking explicitly into account the kinetic effects arising from the complicated nonguiding center ion motion near the magnetic field reversal. It is shown, that, in contrast to the fluid theory, the kinetic effects allow fast reconnection even for very long current sheets. The predictions of the theory as well as particle simulations based on the new class of thin current sheets equilibria are compared with Cluster observations of atypical current sheets and their flapping motions as well as with other models predictions. The new theory and simulations are shown to explain the effect of flankward propagation of flapping waves as well as the north-south asymmetry of flapping sheets. Reference: R. M. Kulsrud and T. S. Hahm, Forced magnetic reconnection. Phys. Scripta, v. T2/2, p.525, 1982.