Observation of the Magnetic Reconnection Structure

WDS'12 Proceedings of Contributed Papers, Part II, 163–168, 2012. ISBN 978-80-7378-225-2 © MATFYZPRESS Observation of the Magnetic Reconnection Structure J. Enˇzl,L. Pˇrech, J. Safránková,andˇ Z. Nˇemeˇcek Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. Magnetic reconnection is a phenomenon where the energy stored in the magnetic field dissipates into heating and plasma acceleration. It can occur on boundaries connecting plasma with different magnetic field directions. Magnetic reconnections are common processes in ICME sheaths, where plasma is compressed and plasma with different magnetic field direction encounter each other, or at the boundaries of ICME sheaths and magnetic clouds, where the strongest reconnection with the largest shear angle can be found. In this paper, we trace the magnetic reconnection exhaust which was observed by many spacecraft and obtain its parameters which are dependent on the distance from the X-line. We analyze differences in the structure of the reconnection exhaust, its evolution and direction of propagation. We discuss energetic particle signature of the magnetic reconnection exhaust. Introduction Magnetic field in the solar wind is frozen into the plasma. The process of magnetic reconnection can be most easily understood in a 2D geometry where plasma carries oppositely oriented magnetic field lines (shear angle Θ = 180◦) towards the current sheet. Due to the plasma-kinetic effects non-ideal terms in a generalised Ohm's law eventually start playing a role in some locations in the current sheet and the diffusion region, where the frozen-in condition no more holds, is formed around the X-line. The magnetic field is no longer frozen into the plasma in that region and reconnection of magnetic field lines is possible. The plasma on the reconnected field lines is accelerated into the outflow [Gosling, 2006, 2011; Teh, 2009]. Magnetic field lines reconnect into a topology with less magnetic energy. Released magnetic field energy is dissipated into heating and plasma acceleration. Plasma is accelerated away from the diffusion region to the exhaust with velocities close to the Alfvénspeed [Shay et al., 2001]. Data measured onboard the spacecraft which crosses the exhaust region reveal correlated rotation of the magnetic field, plasma jet and temperature enhancement [Phan et al., 2010; Gosling et al., 2005; Gosling, 2005]. According to the Petschek model of magnetic reconnection (see Fig. 1), the stationary slow shock mode waves should separate the inflow and the outflow regions. Magnetic reconnection often occurs within an ICME [McComas et al., 1994]. A typical region where magnetic reconnection occurs is the boundary between the ICME sheath region and the magnetic cloud or in the tail of the ICME [Farrugia et al., 2001]. We can also find magnetic reconnection in the sheath region, where numbers of current sheaths are frequently present. In this paper, we present multispacecraft observation of 5th April 2010 magnetic reconnection which was registered on the boundary of the ICME sheath and the magnetic cloud. We compute parameters of magnetic reconnection and discuss the differences among measurements. The velocity normal of discontinuity has been obtained by the 4-spacecraft method, see Kivel- son and Russell [1996], and compared with the velocity of the ambient solar wind. On the 10th October 1997 example, we present the reconnection exhaust with clearly visible boundaries and disturbed regions. Structure of the magnetic reconnection exhaust The ICME was observed by the Wind spacecraft on 10th October 1997. Inside the sheath region of the ICME, the reconnection exhaust was found. This reconnection exhaust was 37400 km 163 ENZLˇ ET AL.: OBSERVATION OF THE MAGNETIC RECONNECTION STRUCTURE Figure 1. Petschek model of reconnection. Magnetic reconnection can be explained using a model where plasma with oppositely directed magnetic field lines flows to the current sheath. The grey color denotes the disturbed region which surrounds the exhaust. wide and the shear angle was 149◦. Surrounding plasma was unusually stable enabling us to study the reconnection exhaust structure, which is plotted in Fig. 2. In this plot, we can clearly distinguish the reconnection exhaust, which is bounded by two discontinuities. Moreover, in the surrounding of the exhaust, we can see the region of plasma with disturbed parameters. Inside reconnection exhaust region the magnetic field slowly rotates and plasma beta increases due to a decrease of the magnetic field strength and an increase of the temperature and density. In the anti-sunward side inside the disturbed region, we can see a rotation of the magnetic field, which is perpendicular to the main rotation of reconnection, decrease in the density and increase in the magnetic field strength. The explanation of the existence of the disturbed region is unknown. Multipoint Measurements On 5th April 2010, an ICME was observed by a variety of spacecraft, the example of measurements from the Wind spacecraft is in Fig. 3. The magnetic cloud was preceded by a wide turbulence sheath. On the boundary of the ICME sheath and magnetic cloud, the magnetic reconnection exhaust was found. A detail measurement of this reconnection exhaust was obtained by the following spacecraft: Wind (Fig. 4), Themis B (Fig. 5), Cluster 1 (Fig. 6) and Themis C (Fig. 7). From their measurements it is evident that they all observed the same reconnection exhaust and that all spacecraft were on the same side from the X-line, because all spacecraft detected the accelerated flow of the reconnection exhaust in the same direction. The direction of the outflow was oriented mainly along the +YGSE axis. The reconnection exhaust in figures is highlighted by dashed lines. We can recognize reconnection exhaust signatures such as the decreased magnetic field, plasma jet, field rotation, enhanced temperature and enhanced density. The Themis C spacecraft was the closest spacecraft to the X-line and from its measurements the slow shock structure can be recognised on the boundaries of the exhaust. Table 1 summarizes the parameters of the reconnection exhaust. Our presumption is that the discontinuity can be handled as planar. Using this presumption, we calculated by the 4- spacecraft method the velocity v and the normal direction of propagation ~n. Reconnection can be transform to the coordinates which are stationary in the discontinuity frame and then the speed of reconnection exhaust outflow vrec can be estimated. Exhaust width was estimated from the mutual movement of the discontinuity and spacecraft and the duration of the reconnection exhaust. We also calculated the mean velocity of the solar wind in the vicinity of reconnection on each spacecraft. In Table 1, the spacecraft are sorted by their YGSE coordinate which is correlated to the X-line distance. In Table 1 we can see the trends in the acceleration speed vrec and in the exhaust width. 164 ENZLˇ ET AL.: OBSERVATION OF THE MAGNETIC RECONNECTION STRUCTURE Figure 2. Example of the magnetic reconnection exhaust. The boundaries of the exhaust consist of the directional discontinuity and standing slow shock mode wave. The disturbed region which surrounds the reconnection exhaust is highlighted between the dashed and doted lines. Table 1. Summary of measured parameters for each spacecraft which are sorted according to their distance from the X-line. Presented parameters: Components of the solar wind velocity normal; V : for each single spacecraft as the velocity of the solar wind in the vicinity of the exhaust, and for 4-spacecraft as the 4-spacecraft exhaust velocity magnitude; the velocity inside the exhaust, ~vrec in the solar wind frame; the exhaust width; and the shear angle. nx ny nz v vrecx vrecy vrecz vrec vrec=valf Exhaust width Shear angle [km/s] [km/s] [km/s] [km/s] [km/s] [km] [◦] 4-spacecraft -0.99 0.12 0.0 717.5 Wind -0.99 0.05 0.05 750.3 6.0 58.7 10.0 59.8 0.42 30700 105 Themis B -0.99 0.06 0.03 743.3 0.6 73.7 -1.6 73.8 0.26 108 Cluster 1 -0.998 0.06 0.02 721.5 11.8 79.2 23.0 83.3 0.46 30200 103 Themis C -0.999 0.01 0.02 679.9 43.4 76.1 6.7 87.9 0.39 29900 82 The dependence of vrec on the distance from the X-line is shown in Figs. 8 and 9. Unfortu- nately, the absolute distance from the X-line can not be estimated because no spacecraft is on the other side of the X-line. The nonlinear dependence of the acceleration speed on the X-line distance was measured. This suggests that some dissipative mechanism such as wave generation was decelerating the jet. Discussion and conclusion From the spatial distribution of the spacecraft we determined the minimal length of de- scribed reconnection exhaust as 130 RE. Reconnection was observed in the time range of 50 minutes. Acceleration of plasma inside the exhaust is dependent on the distance from the X-line. The exhaust speed seems to decrease with the X-line distance due to some dissipation 165 ENZLˇ ET AL.: OBSERVATION OF THE MAGNETIC RECONNECTION STRUCTURE Figure 3. Observation of the ICME by the Wind spacecraft. The fast forward shock, sheath region and MC can be found in this plot. The position of the fast forward shock is highlighted by a doted line. The position of magnetic reconnection on the boundary between the sheath region and magnetic cloud is highlighted by a dashed line. process. We do not see the dependence of the ratio vrec=valf on the distance from the spacecraft. The mean value of this ratio was ≈ 0:4 . The exhaust width was on the scale of measurements constant or slowly increasing as it can be see in Table 1.

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