Abstract. We surveyed fast current sheet crossings (flapping motions) over the distance range 10-30 R E in the magnetotail covered by the Geotail spacecraft. Since the local tilts of these dynamic sheets are large and variable in these events, we compare three different methods of evaluating current sheet normals using 4-s/c Cluster data and define the success criteria for the single-spacecraft-based method (MVA) to obtain the reliable results. Then, after identifying more than ∼ 1100 fast CS crossings over a 3-year period of Geotail observations in 1997-1999, we address their parameters, spatial distribution and activity dependence. We confirm that over the entire distance covered and LT bins, fast crossings have considerable tilts in the YZ plane (from estimated MVA normals) which show a preferential appearance of one (YZ kinklike) mode that is responsible for these severe current sheet perturbations. Their occurrence is highly inhomogeneous; it sharply increases with radial distance and has a peak in the tail center (with some duskward shift), resembling the occurrence of the BBFs, although there is no one-to-one local correspondence between these two phenomena. The crossing durations typically spread around 1 min and decrease significantly where the high-speed flows are registered. Based on an AE index superposed epoch study, the flapping motions prefer to appear during the substorm expansion phase, although a considerable number of events without any electrojet and auroral activity were also observed. We also present statistical distributions of other parameters and briefly discuss what could be possible mechanisms to generate the flapping motions.

We investigate the scaling properties a model of passive vector turbulence with pressure and in the presence of a large-scale anisotropy. The leading scaling exponents of the structure functions are proven to be anomalous. The anisotropic exponents are organized in hierarchical families growing without bound with the degree of anisotropy. Nonlocality produces poles in the inertial-range dynamics corresponding to the dimensional scaling solution. The increase with the Péclet number of hyperskewness and higher odd-dimensional ratios signals the persistence of anisotropy effects also in the inertial range. PACS number(s): 47.10.+g, 05.10.Cc, In the last five years, much progress has been achieved in understanding of the physical origin of intermittency and anomalous scaling in fluid turbulence. The study of the passive advection of a scalar quantity (temperature, concentration of an impurity etc) by a random Gaussian field, white in time and self-similar in space, the rapid-change model [1], has played a crucial role in these developments. There, it was for the first time possible to demonstrate the emergence of anomalous scaling and to relate it to the existence of statistical conservation laws of the dynamics [2,3]. Such conservation laws appear as scaling zero modes of the hierarchy of linear operators governing the inertialrange dynamics of the correlation functions. The dominance of such zero modes has been confirmed in numerical experiments [4] and demonstrated analytically by means of the renormalization group and operator product expansion [5,6]. The importance of these results is seen from the fact that statistical invariants of dynamics are features of the passive advection by more general classes of turbulent velocity fields [7]. A review and more references can be found in Ref. [8].The first-principle model of fluid turbulence is the Navier-Stokes equation, a nonlinear integro-differential equation for the solenoidal vector velocity field. Whether the relation between statistical invariants and anomalous scaling can be generalized to the nonlinear dynamics of the advecting velocity field remains an open and challenging issue. Setting aside the problem of the nonlinearity of interactions, the scope of the present Letter is to investigate how nonlocality affects scaling.In the Navier-Stokes equation nonlocality arises through the pressure term fixed by the incompressibility condition for the velocity field. Here, we consider the passive advection of an incompressible vector field u according to the most general dynamics consistent with the Galilean invariance:where P is the pressure and κ is the diffusivity coefficient. The advection field v is specified by the Kraichnan ensemble. It models a homogeneous and isotropic nonintermittent velocity field and, due to Gaussianity, is determined completely by the second-order structure functionwith r ≡ x − y andr = r/|r|. Here 0 < ξ < 2 is a kind of Hölder exponent which measures the "roughness" of the velocity field. In the renormalization group approach, it play...

No abstract

No abstract