[1] With 11 years of Geotail measurements we analyzed sources of plasma sheet B y and constructed a model, depending on IMF B y , coordinates X, Y, and geodipole tilt angle. B y dependence on dipole tilt due to the neutral sheet warping and hinging has an odd (antisymmetric) profile with respect to Y. In addition, a new, even with respect to Y, B y component was discovered, which is positively correlated with dipole tilt with the maximal amplitude ±1-2 nT. In the postmidnight sector the dipole tilt effects in B y almost cancel each other, while at the premidnight sector they are summed up and are comparable with the IMF penetration. Such season-dependent net B y creates a principal azimuthal asymmetry of the magnetotail and is consistent with some polar convection and aurora observations. Plasma sheet B y is often substantially larger than the statistically expected value. This effect can be understood as "amplification" due to internal plasma sheet dynamics. As a result an asymmetric tail of the B y distribution forms, causing certain overestimation of the regression coefficients in statistical models.

Abstract. Cluster observations in the magnetotail revealed an abundance of strongly inclined current sheets. We determine the magnetic configuration of a particular subset of such phenomena: a series of sheet crossings, having significantly differing inclinations and occurring during quiet conditions. These wave-like variations appear to propagate azimuthally and their magnetic amplitude and magnetic gradient (current density) inside the sheet are proportional to their steepness (degree of inlcination). In spite of significant normal direction changes between neighboring crossings up to 150 • , the magnetic field direction inside the neutral sheet remains almost constant. The wavelengths and spatial amplitudes are of the order of 2-5 R E . These observations are interpreted as crossings of a quasi-periodic dynamical structure produced by almost vertical slippage motion of the neighboring magnetic flux tubes in the high-β plasma sheet, rather than large-scale flapping of a stationary structure.

Abstract. In this paper, we consider the mechanism of ion acceleration by dipolarization fronts in the Earth's magnetotail. The statistics of dipolarization front observations by Interball-tail have been collected from 1995 to 1998 (51 events). We demonstrate that near dipolarization fronts bursts of energetic ions are often observed with an average energy of about 100-200 keV. We develop the analytical model of the ion resonance interaction with dipolarization fronts to describe the observed acceleration. We compare the model and the observations to estimate the width of fronts along the dawn-dusk direction, R y . The mean value is R y ∼ 6 R E .

Abstract. Kinetic structure of embedded thin horizontal current sheets is investigated. Current density estimated by curlometer technique is in general agreement with a sum of electron and proton currents. Embedding of observed thin current sheets in the much wider plasma sheet is apparent in the current density profiles. Ion velocity distributions consist of two parts: the cold non-drifting core likely belongs to the plasma sheet background, while the hotter asymmetric "wings" carry the main portion of the current. Oxygen ions (if present) and higher-energy tails of distribution function can contribute up to 30% of the total current. We compared current density profiles across sheets with three typical current sheet models. Models which allow embedding, describe observed structures equally well at the level of experimental accuracy.

[1] We study a statistics of @B z /@x in a thin stretching current sheet (substorm growth phases) observed by Cluster between 8 and 18 R E downtail. After 2005, spacecraft separation allowed to measure directly this derivative of B z along the tail axis. The near-tail events (within 14 R E ) exhibited a straight decrease of an initially large positive @B z /@x to 1-2 nT/R E . In the more stretched middle tail, usually the small |@B z /@x| < 0.5 -1 nT/R E had no clear trend and fluctuated around zero with time scales 5-15 min. In general, negative @B z /@x were ubiquitous. At some onsets, larger negative @B z /@x < -1 nT/R E were associated with transient dipolarizations, propagating earthward. There was no clear association of local plasma sheet activity onset with any value of @B z /@x. We discuss relation of observations and recent modeling results.Citation: Petrukovich, A. A., A. V. Artemyev, R. Nakamura, E. V. Panov, and W. Baumjohann (2013), Cluster observations of @B z /@x during growth phase magnetotail stretching intervals,