Magnetic Reconnection in ICME Sheath

Transcription

1 WDS'11 Proceedings of Contributed Papers, Part II, 14 18, ISBN MATFYZPRESS Magnetic Reconnection in ICME Sheath J. Enzl, L. Prech, K. Grygorov, A. Lynnyk Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. Abstract. Magnetic reconnection is a phenomenon where the energy stored in magnetic field dissipates into heating and particle acceleration. It can occur on boundaries connecting plasma with different magnetic field topology. We can frequently find magnetic reconnection in ICME sheaths, where plasma is compressed and different plasma topologies encounter each other. In these regions we can recognize magnetic reconnection by number of plasma signatures such as temperature enhancement, or changes in magnetic field and bulk flow direction. A brief introduction into Interplanetary Coronal Mass Ejection (ICME), magnetic reconnection and its properties are presented and short statistics on shear angle dependence are provided. We have found dependence of temperature enhancement inside reconnection exhaust on shear angle. We have also obtained dependence of reconnection outflow speed on shear angle. Introduction Interplanetary Coronal Mass Ejections (ICMEs) are one of the most studied solar wind phenomena. Their source is in the solar corona, above active regions, where coronal mass ejections (CMEs) originates [Forbes, 2000]. The ejection of CME can be seen in Fig. 1. After ejection the CME propagates into interplanetary space where we refer to it as ICME. As the CME propagate into interplanetary space it stretches and changes its shape. There is usually strong magnetic field which dominates over other forces and determines ICMEs behavior. Typical ICMEs propagation velocity is much higher than the ambient solar wind velocity. As a consequence, the fast forward shock (FFS) forms on the leading edge. After FFS a sheath region follows. Plasma in sheath region is compressed, piled up and flows around the ICME [Kaymaz and Siscoe, 2006; Siscoe and Odstrcil, 2008]. Typical signatures of this region are higher proton temperature and jumps in magnetic field and velocity directions. Reconnections of magnetic field lines are frequently observed in this region [Gosling, 2011, 2006; McComas et al., 1994]. The next region contains ejected mass, which has different parameters such as composition or temperature. This region can be identified by numerous of signatures [Zurbuchen and Richardson, 2006; Russell and Shinde, 2005], for instance, enhanced alpha to proton ratio [Borrini et al., 1982] or decreased proton temperature. In a lot of cases we observe magnetic clouds (MCs) within the region with ejected mass [Lepping et al., 2006]. Magnetic clouds can be described as a magnetic flux rope with its tips connected to the Sun surface [Lepping et al., 1990]. The flux rope can be identified by magnetic field variance decrease, slow magnetic field rotation and low plasma beta parameter. In this paper we search 29 ICME events which were measured by the Wind spacecraft during a period of reconnection events were found inside the ICME sheath regions. We try to discuss dependence of reconnection parameters on shear angle. Figure 1. An example of rising CME, side-view (SOHO spacecraft by the EIT instrument). 14

2 Figure 2. Example of spacecraft trajectory through the magnetic reconnection exhaust. Magnetic field rotation can be observed as the spacecraft passes through the reconnection exhaust. The solar wind is accelerated away from the reconnection zone [Gosling et al., 2005]. Magnetic reconnection The magnetic field in the solar wind is frozen into the plasma. Magnetic reconnection [Gosling, 2005] can be explained using a model where plasma with oppositely directed magnetic field lines (shear angle Θ = 180 ) are carried into a current sheath. In the current sheath, magnetic field lines are no longer frozen into the plasma and can reconnect (Fig. 2). Magnetic field lines reconnect into topology with less magnetic energy. Released magnetic field energy is dissipated into heating and particle acceleration. Plasma is accelerated away from the reconnection region to the exhaust region. The plasma in the exhaust region is accelerated close to the Alfven speed V a and heated [Shay et al., 2001]. Data measured onboard the spacecraft which crosses the exhaust region reveal correlated rotation of the magnetic field, temporary velocity change, and temperature enhancement [Gosling, 2011; Gosling et al., 2005; Phan et al., 2010]. Magnetic reconnection in ICMEs Magnetic reconnection often occurs within the ICME [McComas et al., 1994]. Typical region where magnetic reconnection occurs is the boundary between the sheath region and the magnetic cloud or in the tail of ICMEs [Farrugia et al., 2001]. We can also find magnetic reconnection in the sheath region, where a number of current sheaths are frequently present. Not all discontinuities in the magnetic field are connected with magnetic reconnection because certain conditions are needed to allow reconnection. Some of these conditions are still unknown. Magnetic reconnection is likely to occur when β 2 and the difference in plasma β values on the two sides of the current sheath is low [Phan et al., 2010]. Flow into the reconnection site is either driven by compression or occurs spontaneously and is then sustained by pressure gradient forces associated with the pressure drop produced by the exhaust outflows [Gosling, 2011]. We derived a simple reconnection index that marks regions where correlation in change of the magnetic field and velocity occurs. Irec = db.dv (1) When we obtain correlation and anti-correlation immediately after each other, it is possible that it corresponds to the reconnection outflow region. Enhancement in the proton density, temperature and counter-streaming protons inside the reconnection outflow can be often found and used to verify the finding [Gosling, 2011; Gosling et al., 2005]. Statistic results Data from the WIND spacecraft during the period of were used to track magnetic reconnection in the ICME sheath. We use the 3DP instrument plasma and magnetic field data from the MFI instrument. We have processed 29 ICME events; 61 reconnection events inside the ICME sheaths were found. A small scale statistics was computed in order to provide basic characteristics of the magnetic reconnection events inside the ICME sheath. Average values from 61 reconnection events were computed firstly. Due to the small number of events, the error in estimation of average values is quite large. In the table below, these parameters are presented: shear angle, average magnetic field measured outside 15

3 Figure 3. Examples of reconnection measured by WIND. Rotation of the magnetic field in X and Z axes on :24 12:27 UT; magnetic field rotation in all axes on :16 5:18 UT can be seen in correlation with a double change in the solar wind speed. the exhaust, velocity rotation magnitude, which was obtained as a maximum velocity deviation inside the exhaust from the average speed, temperature enhancement inside the reconnection exhaust, density enhancement inside the exhaust and plasma β. Θ[ ] 89 ± 42 B[nT] 14 ± 6 V rot [km/s] 33 ± 25 T enhance [%] 23 ± 27 N enhance [%] 17 ± 21 β 0.04 ± 0.04 The average values for T enhance and N enhance can hardly be calculated, because a large number of reconnections occurs on the boundary between plasma with different temperatures and densities as it can be seen in Fig. 3. Therefore, the enhancement is difficult to find. Despite this, we can find a dependence of T enhance and N enhance on the shear angle, Θ (see Fig. 4). Other statistic results are focused on comparison of the Alfven speed V a and V rot which should represent the magnitude of plasma acceleration inside the exhaust. Two Alfven speeds were computed, V a average was computed from average values of the plasma parameters outside the exhaust, second V a exhaust was computed from the plasma parameters measured inside the exhaust. Fig. 5 shows that V rot almost never exceeds the average Alfven speed, but it exceeds the exhaust Alfven speed in several cases. This could happen because of measurement errors inside the exhaust, where parameters are not stable. Discussion and conclusion On the basis of our small-scale statistics we can conclude that there is a dependence of T enhance on the shear angle. Reason for this should be in the shear angle of reconnection, which is in basic theory considered as 180 [Kivelson and Russell, 1996]. For reconnection which occurs at the shear angle < 180, the reconnected magnetic field line releases less energy and we measure lower T enhance. V rot which should represent the plasma outflow speed from the reconnection zone and which can be expected V a is almost in all cases smaller (see Fig. 6) in comparison with the average Alfven speed V a average and even in comparison with the Alfven speed computed inside the exhaust V a exhaust. 16

4 Figure 4. Enhancements of the temperature T and the plasma density N inside the exhaust as a function of the shear angle Θ. Figure 5. Alfven speed as a function of the velocity rotation magnitude. Dots correspond to the Alfven speed computed from average values outside the exhaust, circles correspond to the Alfven speed computed from values measured inside the reconnection exhaust. Figure 6. The velocity rotation magnitude V rot to the Alfven speed V a ratio as a function of the shear angle Θ. The Alfven speed was computed from average values outside the exhaust and the values inside the exhaust. The dependence of V rot on the shear angle was also found (see Fig. 6). Values of V rot /V a are systematically lower than expected values even for the shear angle close to 180. We assume this to be an effect of the low number of events or the effect of the distance between the reconnection zone and the measurement point, which is for all measurements unknown. 17

5 Acknowledgment. We thank to WIND and SOHO working team and CDAWeb data center for providing data and images. The present work was supported by the Czech Grant Agency under contract 205/07/0694. References Borrini, G., Gosling, J. T., Bame, S. J., and Feldman, W. C., Helium abundance enhancements in the solar wind, J. Geophys. Res., 87, , DOI: /JA087iA09p07370, Farrugia, C. J., Vasquez, B., Richardson, I. G., Torbert, R. B., Burlaga, L. F., Biernat, H. K., Mühlbachler, S., Ogilvie, K. W., Lepping, R. P., Scudder, J. D., Berdichevsky, D. E., Semenov, V. S., Kubyshkin, I. V., Phan, T.-D., and Lin, R. P., A reconnection layer associated with a magnetic cloud, Adv. Space Res., 28, , Forbes, J. T., A review on the genesis of coronal mass ejections, J. Geophys. Res., 105, 23,153 23,165, Gosling, J. T., Magnetic reconnection in the solar wind: A brief overview, Proceedings of the Solar Wind 11 / SOHO 16, Connecting Sun and Heliosphere, Gosling, J. T., Petschek-type magnetic reconnection exhausts in the solar wind well inside 1 au: Helios, J. Geophys. Res., 111, Gosling, J. T., Magnetic reconnection in the solar wind, Space Sci. Rev., DOI: /s , Gosling, J. T., Skoug, R. M., McComas, D. J., and Smith, C. W., Direct evidence for magnetic reconnection in the solar wind near 1AU, J. Geophys. Res., 110, DOI: /2004JA010809, Kaymaz, Z. and Siscoe, G., Field-line draping around ICMEs, Solar Phys., 239, , DOI: /s x, Kivelson, M. G. and Russell, C. T., Introduction to space physics, Cambridge University Press, ISBN , Lepping, R. P., Jones, J. A., and Burlaga, L. F., Magnetic field structure of interplanetary magnetic clouds at 1 AU, J. Geophys. Res., 95, 11,957 11,965, Lepping, R. P., Berdichevsky, D. B., Wu, C.-C., Szabo, A., Narock, T., Mariani, F., Lazarus, A. J., and Quivers, A. J., A summary of WIND magnetic clouds for years : model-fitted parameters, associated errors and classfications, Ann. Geophys., 24, , DOI: /angeo , McComas, D. J., Gosling, J. T., Hammond, C. M., Moldwin, M. B., Phillips, J. L., and Forsyth, R. J., Magnetic reconnection ahead of a coronal mass ejection, Geophys. Res. Lett., 21, , Phan, T. D., Gosling, J. T., Paschmann, G., Pasma, C., Drake, J. F., ieroset, M. O., Larson, D., Lin, R. P., and Davis, M. S., The dependence of magnetic reconnection on plasma beta and magnetic shear: evidence from solar wind observations, ApJL, 719, , DOI: / /719/2/L199, Russell, C. T. and Shinde, A. A., On defining interplanetary coronal mass ejections from fluid parameters, Solar Phys, 229, , Shay, M. A., Drake, J. F., Rogers, B. N., and Denton, R. E., Alfvenic collisionless magnetic reconnection and the hall terme, J. Geophys. Res., 106, , Siscoe, G. and Odstrcil, D., Ways in which ICME sheaths differ from magnetosheaths, J. Geophys. Res., 113, DOI: /2008JA013142, Zurbuchen, T. H. and Richardson, I. G., In-situ solar wind and magnetic field signatures of interplanetary coronal mass ejections, Space Sci. Rev., 123, 31 43, DOI: /s ,