4,129 research outputs found

    The interaction between transpolar arcs and cusp spots

    Get PDF
    Transpolar arcs and cusp spots are both auroral phenomena which occur when the interplanetary magnetic field is northward. Transpolar arcs are associated with magnetic reconnection in the magnetotail, which closes magnetic flux and results in a "wedge" of closed flux which remains trapped, embedded in the magnetotail lobe. The cusp spot is an indicator of lobe reconnection at the high-latitude magnetopause; in its simplest case, lobe reconnection redistributes open flux without resulting in any net change in the open flux content of the magnetosphere. We present observations of the two phenomena interacting--i.e., a transpolar arc intersecting a cusp spot during part of its lifetime. The significance of this observation is that lobe reconnection can have the effect of opening closed magnetotail flux. We argue that such events should not be rare

    Temperature Evolution of the Quantum Gap in CsNiCl3

    Full text link
    Neutron scattering measurements on the one-dimensional gapped S=1 antiferromagnet, CsNiCl3, have shown that the excitation corresponding to the Haldane mass gap Delta at low temperatures persists as a resonant feature to high temperatures. We find that the strong upward renormalisation of the gap excitation, by a factor of three between 5 and 70K, is more than enough to overcome its decreasing lifetime. We find that the gap lifetime is substantially shorter than that predicted by the scaling theory of Damle and Sachdev in its low temperature range of validity. The upward gap renormalisation agrees with the non-linear sigma model at low temperatures and even up to T of order 2Delta provided an upper mass cutoff is included.Comment: Latex, 3 figures, accepted by Pysical Review

    Magnetic excitations of spin and orbital moments in cobalt oxide

    Full text link
    Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN~290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hund's rule model the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom whose bandwidth is controlled by interionic superexchange. The different ordering domains give rise to several magnetic peaks at each wavevector transfer.Comment: Accepted for publication in Canadian Journal of Physic

    Model of two-fluid reconnection

    Full text link
    A theoretical model of quasi-stationary, two-dimensional magnetic reconnection is presented in the framework of incompressible two-fluid magnetohydrodynamics (MHD). The results are compared with recent numerical simulations and experiment.Comment: 4 pages, 1 figure, accepted to Physical Review Letter

    Two and Three Dimensional Incommensurate Modulation in Optimally-Doped Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta}

    Full text link
    X-ray scattering measurements on optimally-doped single crystal samples of the high temperature superconductor Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} reveal the presence of three distinct incommensurate charge modulations, each involving a roughly fivefold increase in the unit cell dimension along the {\bf b}-direction. The strongest scattering comes from the well known (H, K±\pm 0.21, L) modulation and its harmonics. However, we also observe broad diffraction which peak up at the L values complementary to those which characterize the known modulated structure. These diffraction features correspond to correlation lengths of roughly a unit cell dimension, ξc\xi_c∼\sim20 A˚\AA in the {\bf c} direction, and of ξb\xi_b∼\sim 185 A˚\AA parallel to the incommensurate wavevector. We interpret these features as arising from three dimensional incommensurate domains and the interfaces between them, respectively. In addition we investigate the recently discovered incommensuate modulations which peak up at (1/2, K±\pm 0.21, L) and related wavevectors. Here we explicitly study the L-dependence of this scattering and see that these charge modulations are two dimensional in nature with weak correlations on the scale of a bilayer thickness, and that they correspond to short range, isotropic correlation lengths within the basal plane. We relate these new incommensurate modulations to the electronic nanostructure observed in Bi2_2Sr2_2CaCu2_2O8+δ_{8+\delta} using STM topography.Comment: 8 pages, 8 figure

    Gyrokinetic simulation of entropy cascade in two-dimensional electrostatic turbulence

    Full text link
    Two-dimensional electrostatic turbulence in magnetized weakly-collisional plasmas exhibits a cascade of entropy in phase space [Phys. Rev. Lett. 103, 015003 (2009)]. At scales smaller than the gyroradius, this cascade is characterized by the dimensionless ratio D of the collision time to the eddy turnover time measured at the scale of the thermal Larmor radius. When D >> 1, a broad spectrum of fluctuations at sub-Larmor scales is found in both position and velocity space. The distribution function develops structure as a function of v_{perp}, the velocity coordinate perpendicular to the local magnetic field. The cascade shows a local-scale nonlinear interaction in both position and velocity spaces, and Kolmogorov's scaling theory can be extended into phase space.Comment: 8 pages, 10 figures, Conference paper presented at 2009 Asia-Pacific Plasma Theory Conference. Ver.2 includes corrected typos & updated reference

    AX J0049.4-7323 - a close look at a neutron star interacting with a circumstellar disk

    Full text link
    Detailed evidence on the system AX J0049.4-7323 is presented here to show how the passage of the neutron star in the binary system disrupts the circumstellar disk of the mass donor Be star. A similar effect is noted in three other Be/X-ray binary systems. Together the observational data should provide valuable tools for modelling these complex interactions.Comment: 4 pages, accepted for publication in MNRA
    • …
    corecore