1. Laboratoire de Physique des Plasmas C ASE STUDY OF AN ATYPICAL MAGNETOPAUSE CROSSING N. Dorville (PHD student : 09/2012-09/2015) (1), G. Belmont (1), L. Rezeau (1), R. Grappin (1), A. Retino (1) (1) Laboratoire de Physique des Plasmas LPP (Ecole Polytechnique, CNRS, UPMC, Université Paris-Sud) Palaiseau, France Abstract The magnetopause boundary implies two kinds of variations: a density/ temperature gradient and a magnetic field rotation. These two kinds are always observed in a close vicinity of each other, if not inseparably mixed. We present a case study from the Cluster data where the two are clearly separated and investigate the natures of both layers. We evidence that the first one is a slow shock while the second is a rotational discontinuity. The interaction between these two kinds of discontinuities is then studied with the help of a 1-D MHD simulation. The comparison with the data is quite positive and leads to think that most of the generic properties of the magnetopause may be interpreted in this sense. Introduction The BV method on the 04/15/2008 crossing Conclusions We have presented a magnetopause crossing case, atypical in the sense that the separation of the compressive and magnetic gradients is clear, enabling to analyze separately the two layers. The density/temperature gradient layer can be identified unambiguously as a slow shock, while the rotation layer has all the properties of a rotational discontinuity, except that it may propagate at 0.7 VA instead of VA, if only protons are involved. A multi-spacecraft analysis suggests that the two structures are interacting. A 1.5-D MHD simulation has been performed to investigate the interaction between a slow shock and a rotational discontinuity when they meet. It shows results comparable with the data: the shock and rotational discontinuity remain attached in a non stationary compound structure and so compatible with the MHD equations. We can conjecture that this kind of interaction between compressional and rotational features is a generic feature of the disturbed magnetopause. 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, R. Grappin, A. Retino Rotational/ Compressional nature of the Magnetopause: application of the BV technique on a magnetopause case study, accepted by JGR, 2014 [4] Dorville N., G.Belmont, L. Rezeau, N. Aunai, A. Retino BV technique for investigating 1-D interfaces, accepted by JGR, 2014 [5] Paschmann, G., and Sonnerup, B. U. Ö. (2008), Proper Frame Determination and Walén Test, in Multi-spacecraft Analysis Methods Revisited, edited by G. Paschmann and P. W. Daly, no. SR-008 in ISSI Scientific Reports, chap. 7, pp. 69-71, ESA Publ. Div., Noordwijk, Netherlands [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 [8] Whang, Y. C. et al., Double discontinuity: A compound structure of slow shock and rotational discontinuity, J. Geophys. Res.,103, 6513, 1998. - Compressive gradients and magnetic field rotation must coexist at the magnetopause boundary. - 1D stationary case: Rankine-Hugoniot jump equations allow only one type of discontinuity, the tangential discontinuity, to ensure simultaneously the two kinds of variations. This implies that the normal magnetic field and flow are null. - However there are numerous observations of rotational discontinuities with non zero Bn at the magnetopause. -It seems that, at least when dynamical processes take place, the two boundaries appear separated from each other - We present here evidence of such a case, using the new BV method [4], and show how these discontinuities can coexist in compound structures with slow temporal evolution Numerical simulation : slow shock + rotational discontinuity On April, 15th, 2008, around 15:22, Cluster C1 encounters the magnetopause at approximately [-10.2, -2.4, 1.6] Re in the GSE frame, as showned by the density gradient, change in plasma composition and magnetic field conditions. The BV method permits to find the normal direction and fit the magnetic field with good accuracy, using FGM [1] data and CIS [6] velocity data. The result is slightly different from a MVABC [7] analysis, as the propagation angle is 75 degrees. Two kinds of layers coexisting… From t=-10s to t=2s, we observe compressional gradients. Here the ion velocity observed cannot be approximated by the velocity of the boundary: we obtain a constant boundary velocity by plotting the mass flux as a function of density in this interval. The normalized gradients in this discontinuity show that it is a slow shock. B and Vn are decreasing as p and n increases, and vn crosses cs (here for the slow mode velocity with the measured Bn) If we look at the second part of the boundary, we observe that B, n, p and T are constant, as the magnetic field is rotating and the normal magnetic field still non zero. This is consistent only with a rotational discontinuity This hypothesis is confirmed by this proxy of a Walén [5] test: V is proportional to VA but the coefficient is less than 1 if we take the proton mass for the ions mean mass. Unfortunately there is no measurement of the plasma composition or electron density at this time. In order to study the co-existence and the interaction of a slow shock and a rotational discontinuity, a 1.5D compressible MHD pseudo-spectral simulation has been run. As the boundary conditions are periodic, we have to initialize gradients at the center with smooth variations to take care of periodicity. A slow shock is first initialized alone. In the initialization, the shock has jumps that respect the exact RH relations and profiles that respect the viscous MHD laws (only approximately for the energy law ).The evolution of the physical quantities in the frame where the shock should be stationary (from the blue curve to the red) is consistent with stationarity, with only a small perturbation propagating to the right. The stationarity is still true at longer times, but the profiles smooth due to dissipation Adding a rotation of the magnetic field and velocity field consistent with a rotational discontinuity, we observe that the slow shock is now moving with the Alfvén wave. The structure is not stationary (we used n and Bz profiles as proxies for the positions of the compressive and rotational parts), and the shock position oscillates inside the structure as far as we can push the simulation. …and interacting The figure shows the respective positions of the two boundaries seen by the four spacecrafts of the mission. The spacecraft potential is used as a proxy for the compressional part. It indeed confirms that both type of gradients are coexisting and interacting.