[1] Using magnetic field data from the four Cluster spacecraft between February and May 2001, a detailed statistical analysis of interplanetary discontinuities is accomplished. In order to find the surface normals, we apply three different methods:(1) minimum variance analysis (MVA), (2) cross-product method, and (3) triangulation, the latter being the only one that needs information from all four spacecraft. Whereas the cross-product normals are always almost the same at the position of the four spacecraft, strong deviations between the MVA normals are observed. The crossproduct normals also agree fairly well with the triangulation normals, which mostly point in a direction approximately perpendicular to the magnetic field. The scatter of the MVA normals around the triangulation normal can be reduced by triggering the parameters controlling the MVA accuracy, i.e., l 2 /l 3 L , the lower limit of the MVA eigenvalue ratio, and w L , the lower limit of the spreading angle. Our analysis allows to compare the impact of these two parameters on the MVA normals. The error analysis shows that both parameters have strong impact on the MVA accuracy; however, l 2 /l 3 L is more important. In order to ensure reliable normal estimates, MVA should only be applied if w L > 60°. The MVA error analysis also enables us to provide a new criterion to distinguish rotational discontinuities (RDs) from tangential discontinuities. However, the triangulation results show that there are no clearly identified RDs in our data set. We will discuss implications for the theory of solar wind discontinuity generation processes. A practical consequence is that the cross-product method presumably yields better normal estimates than MVA when B n is small. The colinerarity of the cross-product normals at the positions of the four spacecraft demonstrate that the discontinuities are one-dimensional structures on the Cluster separation scale.

[1] The first close Titan encounters TA, TB, and T3 of the Cassini mission at almost the same Saturnian local time $1030 and in the same spatial region downstream of Titan have enabled us to study the formation of the tail of its induced magnetosphere. The study is based on magnetic field and electron plasma observations as well as threedimensional modeling. Our most important findings are the following: (1) No crossings of a bow shock of Titan were observed, and all encounters occurred at high plasma b > 1 for transsonic and trans-Alfvénic Mach numbers. (2) The magnetic draping signature of the induced magnetosphere often shows a sharp outer boundary called the draping boundary (DB) in the near-tail region. (3) The DB is often occurring as a discontinuity in magnetic field spatial derivatives, and therefore the DB is a discontinuity in the spatial distribution of plasma currents. (4) Perpendicular to the incident flow direction the DB shows an approximately elliptic cross section elongated along the incident magnetic field direction and a displacement toward the Sun. (5) We argue that the DB in the magnetic tail region corresponds to the boundary of a structure which is analogous to an Alfvén wing at very small b and in our case of larger b contains Alfvénic and slow mode features. It forms a tail like a delta wing in aerodynamics. (6) For the two less disturbed flybys, TA and T3, a polarity reversal layer has been observed with thicknesses of $320 km and $230 km, respectively.

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Abstract. We examine the Alfvénicity of a set of 188 solar wind directional discontinuities (DDs) identified in the Cluster data from 2003 by Knetter (2005), with the objective of separating rotational discontinuities (RDs) from tangential ones (TDs). The DDs occurred over the full range of solar wind velocities and magnetic shear angles. By performing the Walén test in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for which a good HT frame was found had plasma flow speeds exceeding 80% of the Alfvén speed at an average angular deviation of 7.7°; 33 cases had speeds exceeding 90% of the Alfvén speed at an average angle of 6.4°. We show that the angular deviation between flow velocity (in the HT frame) and the Alfvén velocity can be obtained from a reduced form of the Walén correlation coefficient. The corresponding results from the Walén test expressed in terms of jumps in flow speed and corresponding jumps in Alfvén speed are similar: 66 of the same 127 cases had velocity jumps exceeding 80% with average angular deviation of 5.8°, and 22 exceeding 90% of the jump in Alfvén speed, with average angular deviation 6.2°. We conclude that a substantial fraction of the 127 events can be identified as RDs. We present further evidence for coupling across the DDs by showing that, for most of the 127 crossings, the HT frame velocities, evaluated separately on the two sides of the DD, are nearly the same – a result required for RDs but not for TDs. We also show that the degree of Alfvénicity is nearly the same for the DDs and fluctuations in which the DDs are embedded. Whatever process causes deviations from ideal Alfvénicity appears to operate equally for the DDs as for the surrounding fluctuations. Finally, our study has established a unique relation between the strahl electron pitch angle and the sign of the Walén slope, implying antisunward propagation in the plasma frame for all 127 cases.

PACS. 05.40.-a -Fluctuation phenomena, random processes, noise, and Brownian motion. PACS. 05.50+q -Lattice theory and statistics (Ising, Potts, etc.). PACS. 75.50.Lk -Spin glasses and other random magnets.Abstract. -We discuss with the aid of random walk arguments and exact numerical computations the magnetization properties of one-dimensional random field chains. The ground state structure is explained in terms of absorbing and non-absorbing random walk excursions. At low temperatures, the magnetization profiles follow those of the ground states except at regions where a local random field fluctuation makes thermal excitations feasible. This follows also from the non-absorbing random walks, and implies that the magnetization length scale is a product of these two scales. It is not simply given by the Imry-Ma-like ground state domain size nor by the scale of the thermal excitations.In statistical mechanics of random systems the search for universality can be interpreted geometrically. That is, if the introduction of disorder into a system is relevant, the real-space properties of the physical states can be understood in terms of scaling exponents. These describe the fluctuations of a domain wall, or the behavior of a spin-spin correlation function. The central ingredient is that the configurational energy is coupled to geometric fluctuations. Consider a domain wall in a magnet. If the spatial fluctuations are described by a roughness exponent ζ, then there is an associated exponent θ describing the free or ground state energy fluctuations. Assuming that the 'zero temperature fixed point' scenario is true or that the entropy is irrelevant at low enough temperatures, this is all what is needed to describe the physics. The system evolves via Arrhenius-like dynamics so that the cost of moving in the energy landscape is given by the usual exponential factor exp(∆Eβ), where β = 1/T and T is the temperature, and ∆E ∼ l ζ relates the cost to the scale length of the perturbation l. Consider now a random magnet. It has a ground state (GS) which is described exactly by the positions and arrangement of the domain walls. Examples abound in particular in Ising systems, where non-trivial GSs exist for spin glasses and random field systems [1]. In this work we investigate with random walk arguments and exact numerical computations how the aforementioned picture applies in the case of one-dimensional random field chains. We find that for arbitrary field distributions [2] the GS structure can be understood via the c EDP Sciences