1. Laboratoire de Physique des Plasmas BV proper: time description of the tangential B hodogram and use of the velocity data BV initialization : Shape of the tangential B hodogram BV TECHNIQUE FOR INVESTIGATING 1D INTERFACES N. Dorville (PHD student : 09/2012-09/2015) (1), G. Belmont (1), L. Rezeau (1), N. Aunai (2), A. Retino (1) (1) Laboratoire de Physique des Plasmas LPP (Ecole Polytechnique, CNRS, UPMC, Paris Sud) Palaiseau, France (2) Institut de Recherche en Astrophysique et Planétologie (IRAP) Abstract To investigate the structure of the magnetopause boundary with satellite data, it is crucial to be able to determine its normal direction and to convert temporal variations into spatial profiles. We propose a new single-spacecraft method, the BV method, to reach these two objectives using a combination of magnetic field and particle velocity data. The method is tested on simulation and Cluster data, and a short overview of the possible products is given. Introduction and purpose of the BV method Conclusions We presented here the BV method, that allows to combine magnetic field and ion velocity data to obtain a normal direction and a spatial coordinate across the magnetopause boundary (and the tihckness) Using it, we are able to draw spatial profiles of plasma and field quantities inside the boundary, usable to understand better the physics of the layer. The method has been tested with success on generated data and simulation. It obtained good results on real Cluster data benchmark cases, and permits to investigate precisely the structure of the layer on a 04/15/08 atypical case study. Bibliography: [1] Balogh, A., Dunlop, M. W., Cowley, S. W. H., Southwood, D. J., Thomlinson, J. G., and the Cluster magnetometer team (1997), The Cluster magnetic field investigation, Space Sci. Rev., 79, 65 [2] Belmont, G., Grappin, R., Mottez, F., Pantellini, F.,Pelletier, G. (2013), Collisionless plasmas in astrophysics, Wiley-VCH [3] Dorville N., G.Belmont, L. Rezeau, N. Aunai, A. Retino BV technique for investigating 1-D interfaces, accepted by JGR, 2014 [4] Dorville N., G.Belmont, L. Rezeau, R. Grappin, A. Retino Rotational/ Compressional nature of the Magnetopause: application of the BV technique on a magnetopause case study, accepted by JGR, 2014 [5] Haaland, S., Sonnerup, B., Dunlop, M., Balogh, A., Georgescu, E., Hasegawa, H., Klecker, B., Paschmann, G.,Puhl-Quinn, P., Rème, H., Vaith, H., and Vaivads, A. (2004a), Four-spacecraft determination of magnetopause orientation, motion and thickness: comparison with results from single-spacecraft methods, Ann. Geophys., 22, 1347–1365 [6] Rème, H., Cottin, F., Cros, A., et al. (1997), The Cluster Ion Spectrometry (CIS) Experiment, Space Sciences Review 79, 303-350 [7] Sonnerup, B. U. Ö., and Scheible, M. (1998), Minimum and Maximum Variance Analysis, in Analysis Methods for Multispacecraft Data, edited by G. Paschmann and P. W. Daly, no. SR-001 in ISSI Scientific Reports, chap. 8, pp. 187-196, ESA Publ. Div., Noordwijk, Netherlands Magnetopause: boundary between the dense and cold shocked solar wind (magnetosheath) and the hot and tenuous magnetospheric plasma. compressive gradients and magnetic field rotation must coexist in the same vicinity. Understanding the magnetopause nature needs two informations: - The direction of the normal direction to the boundary with respect to the magnetic field - An approximate coordinate along its normal Several methods have been developed for both of these purposes (see [5], [7]), especially single-spacecraft variance methods and multi- spacecraft timing methods, and the Transition Parameter based on electron density and temperature The Cluster mission (ESA): investigates the structure of the boundary using 4 spacecrafts, with measurements of fields, waves and particles. The purpose of the BV method [3] is to combine the FGM [1] magnetic field measurement and the CIS [6] ion velocity in order to determine a normal direction with improved accuracy, and a transition parameter with better time resolution, closer to a real spatial coordinate, with respect to other methods Examples of spatial profiles accessible with the BV method Magnetic field hodogram in the GSE frame for a 03/03/08 magnetopause crossing, when Cluster C3 encounters the boundary around 23h15:15 at [13.6, 3.7, -0.7] Re. GSE is not linked to the boundary position and orientation. We will use MVABC method [7] to obtain a first approximated frame. In this frame, the data looks C-shaped (see [4]). -> idea of an elliptical shape fit B y = B y0 B x = B x0 cos  B z = B z0 sin  Model for the magnetic field for the BV method in the discontinuity frame, with the normal along the y axis: Second phase: selection of representative points (yellow) and initialization of the ellipse (red) and frame with only FGM data. We use IDL pre- programmed Powell algorithm to find the parameters of the fit. assumptions : -  position y) linear (verified on simulation data) - normal velocity in the boundary frame negligible with respect to the velocity of the boundary Direct products of the method: -The normal direction -A spatial coordinate (and the thickness of the boundary) -The normal component of magnetic field -A fit for magnetic field -An approximation of the boundary’s velocity Particle data spatial profiles for the 03/03/08 magnetopause crossing: there is a density jump but no temperature jump. All these profiles present clear differences depending on the cases (see other poster and [4]) Electric field for the same crossing. The maximum variance is on normal field (blue), consistently with theory, as the tangential fields are nearly constant, proof that the normal direction should be good Validation tests Test of initialization on generated data Test of elliptical shape on Cluster and simulation data Test of the linear shape of alpha on simulation data Comparison with other methods on several Cluster datasets and on generated data Test on a benchmark case [5] : Comparison of the BV result (red) with other single and multi-spacecraft methods (center= mean of MVABCs, polar position on the plane perpendicular to this normal) 1 54 7 2 3 7 Validity conditions and limits 6 The validity range is different from the other single or multi spacecraft methods Weak conditions: - One needs at list a few velocity data points inside the boundary - One needs to be able to discriminate the couple (Bm, Vm) from (Bn, Vn), that is in most cases a weaker condition than for variance analysis methods Constraining hypotheses - There is no hypothesis on the shape of the velocity profile (constant/ constant acceleration etc…) but the flow in the boundary frame has to be constant or negligible - The shape of the tangential hodogram  (y) has of course an influence on the result