Magnetic reconnection is one of the most important processes in astrophysical, space and laboratory plasmas. Identifying the structure around the point at which the magnetic field lines break and subsequently reform, known as the magnetic null point, is crucial to improving our understanding reconnection. But owing to the inherently three-dimensional nature of this process, magnetic nulls are only detectable through measurements obtained simultaneously from at least four points in space. Using data collected by the four spacecraft of the Cluster constellation as they traversed a diffusion region in the Earth's magnetotail on 15 September, 2001, we report here the first in situ evidence for the structure of an isolated magnetic null. The results indicate that it has a positive-spiral structure whose spatial extent is of the same order as the local ion inertial length scale, suggesting that the Hall effect could play an important role in 3D reconnection dynamics.

Phases of nonlinear double tearing modes are studied numerically. The first two phases lead to the formation and growth of magnetic islands and are followed by a fast reconnection phase to complete the process, driven by a process of neighboring magnetic separatrices merging and magnetic islands coupling. The fast growth can be understood as a result of the island interaction equivalent to a steadily inward flux boundary driven. Resistivity dependences for various phases are studied and shown by scaling analysis for the first time. It is found that after an early Sweet-Parker phase with a eta(1/2)-scale, a slow nonlinear phase in a Rutherford regime with a eta(1)-scale is followed by the fast reconnection phase with a eta(1/5)-scale.

Detection of a separator line that connects magnetic nulls and the determination of the dynamics and plasma environment of such a structure can improve our understanding of the three-dimensional (3D) magnetic reconnection process 1-9 . However, this type of field and particle configuration has not been directly observed in space plasmas. Here we report the identification of a pair of nulls, the null-null line that connects them, and associated fans and spines in the magnetotail of Earth using data from the four Cluster spacecraft . With d i and d e designating the ion and electron inertial lengths, respectively, the separation between the nulls is found to be ~ 0.7±0.3 d i and an associated oscillation is identified as a lower hybrid wave with wavelength ~ d e . This in situ evidence of the full 3D reconnection geometry and associated dynamics provides an important step toward to establishing an observational framework of 3D reconnection.

[1] Flux transfer events (FTEs) are magnetic flux ropes formed at planetary magnetopauses (MPs). Although evidence suggests that FTEs form through time-dependent magnetic reconnection, details of that process and 3D structure of the flux ropes remain largely unclear. This letter presents Double Star/TC-1 data of an FTE occurred on 7 April 2004 which show that the FTE was separated by two X-lines moving south-dawnward. In particular, the electron energy-pitch angle distribution implies that the FTE was composed of flux ropes of all four possible magnetic topologies, indicating that the field lines must have reconnected multiple times. This is an intrinsic property of FTEs formed by 3D multiple X-line reconnection distinguished from quasi 2D FTE models. This knowledge of FTE magnetic topologies helps to improve our understanding of solar wind-magnetosphere coupling at the MP. Citation: Pu,

Pole analysis on unitarized SU (3) × SU (3) one loop χPT amplitudes (3) chiral perturbation theory. By fitting phase shift and inelasticity data, we determine pole positions in different channels (f 0 (980), a 0 (980),f 0 (600), K * 0 (800), K * (892), ρ(770)) and trace their N c trajectories. We stress that a couple channel Breit-Wigner resonance should exhibit two poles on different Riemann sheets that reach the same position on the real axis when N c = ∞. Poles are hence classified using this criteria and we conclude that K * (892) and ρ(770) are unambiguous Breit-Wigner resonances. For scalars the situation is much less clear. We find that f 0 (980) is a molecular state rather than a Breit-Wigner resonance, while a 0 (980), though behaves oddly when varying N c , does maintain a twin pole structure.

[1] On 26 February 2008, the THEMIS satellites observed two substorms that occurred at about 0405 and 0455 UT. Angelopoulos et al. (2008) made a comprehensive study of the second event. In this paper we display detailed features of the two substorms with emphasis on the first. In both substorms, a distinct auroral intensification occurred during the earliest stage of onset, about 1 to 2 min after midtail reconnection began. This initial intensification was weak and localized and thus had the signatures of a pseudobreakup. In both substorms, a second, major intensification occurred next in the substorm onset sequence, followed by rapid and extensive poleward expansion. This second intensification had the features of the major expansion onset and was nearly coincident with observations of earthward flows and magnetic dipolarization in the near-Earth tail. During the growth phase of the two substorms, open magnetic flux accumulated in the polar cap; in the expansion/recovery phase the polar cap open flux was quickly reduced. These observations are in agreement with the assertion that tail reconnection initiates the initial pseudobreakup and the ensuing major expansion and releases and transports energy to eventually cause near-Earth dipolarization and the expansion phase onset of these two substorms.

We perform a comprehensive analysis of the strange-antistrange parton distribution function (PDF) asymmetry in the proton in the framework of chiral effective theory, including the full set of lowest order kaon loop diagrams with off-shell and contact interactions, in addition to the usual on-shell contributions previously discussed in the literature. We identify the presence of δ-function contributions to thes PDF at x = 0, with a corresponding valence-like component of the s-quark PDF at larger x, which allows greater flexibility for the shape of s −s. Expanding the moments of the PDFs in terms of the pseudoscalar kaon mass, we compute the leading nonanalytic behavior of the number and momentum integrals of the s ands distributions, consistent with the chiral symmetry of QCD. We discuss the implications of our results for the understanding of the NuTeV anomaly and for the phenomenology of strange quark PDFs in global QCD analysis.

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