Deformation of ICME and MC on 1 30 AU Seen by Voyager 2 and WIND

Transcription

1 WDS'10 Proceedings of Contributed Papers, Part II, , ISBN MATFYZPRESS Deformation of ICME and MC on 1 30 AU Seen by Voyager 2 and WIND A. Lynnyk, J. Šafránková, Z. Němeček Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic. J. D. Richardson Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA. Abstract. The Interplanetary Coronal Mass Ejections (ICMEs) which propagate in the solar wind with supersonic velocities create the shock prior to their leading edge. The shock standoff distance depends on the ICME s velocity, form and expansion. The study of the standoff distance can provide us an additional information about the shape of ICMEs that cannot be obtained directly from one-point observation. This study shows that ICME flattens during their propagation. This flattening is increasing with the Alfvén Mach number of the ICME leading edge, the heliocentric distance and, on the other hand, it is decreasing with an increase of the inner ICME magnetic field strength. Introduction The Coronal Mass Ejections (CMEs) are large plasma structures which erupt from the Sun. They can be clearly seen by coronagraphs close to the Sun as they have larger density than the ambient corona with frozen-in magnetic field maintaining the form of the CMEs. They propagate from the Sun into the interplanetary medium (and detected in this medium they are called interplanetary CMEs or ICMEs) usually faster than the ambient solar wind (SW). ICME in the SW can be determined by the one or several of following criteria [Liu et al., 2005; Ebert et al., 2009; Wimmer-Schweingruber et al., 2006; Zurbuchen and Richardson, 2006; Neugebauer and Goldstein, 1997]: at least twice colder protons compare to the ambient SW with the same velocity; higher helium abundance; the presence of suprathermal (<80 ev) counterstreaming electron beams; enhanced ion charge states; low proton β (<0.1); the stronger magnetic field with a smaller variance than in the ambient SW; smooth and large magnetic field rotation, cosmic ray decrease due to magnetic field exclusion. Magnetic clouds (MCs) are a subset of ICMEs with such characteristics as a low proton temperature, low proton β, and a strong magnetic field with a rotation up to 180 degrees satisfied. Typical profiles of the MC parameters are shown in Fig. 1 together with fits of the magnetic field using a force-free model [Lynnyk and Vandas, 2009]. Supersonic propagation and expansion of the ICME create the shock and the sheath in front of it. The example of expanded MC is shown in Fig. 1. One can see the shock prior to the MC boundary and the thick sheath. The gas-dynamic estimation of the shock standoff distance [Russell and Mulligan, 2002] for usual ICME s Mach numbers predicts this distance equals approximately 0.2 of the obstacle radius. However, the observation made by the Pioneer Venus Orbiter spacecraft on August 27 and 28, 1978 [Mulligan and Russell, 2001] shows that this distance approximately equals the half thickness of the MC. As the real radius of curvature is calculated by the radii of curvature in two orthogonal planes, it will be larger than radius of the ICME cross-section (or the half-thicknesses, as it usually considered when derived from one-point observation). Nevertheless, even in a case of a cylindrical ICME with infinite radius of axial bending, this real radius of curvature cannot be two times larger than the ICME halfthickness. Thus, the observed shock standoff distance of the above noticed ICME is about 128

2 Figure 1. An example of the one MC seen by WIND on April 22, From top to bottom: magnetic field magnitude and components (nt), X GSE component of velocity (km/s), the proton density (1/cc) and thermal velocity (km/s) are shown. The fits of the magnetic field [Lynnyk and Vandas, 2009] are shown by the curves, the MC-sheath and MC boundaries are marked by the vertical lines. 2 times larger than it expected according to gas-dynamic estimates. One explanation of this difference is that the radius of curvature of the MC cross-section in the orthohonal plane is larger than the half thickness of the MC. This led to presumption of oblate shapes of MCs. Lynnyk and Vandas [2009] selected 33 MCs with shocks prior to their leading edge from 82 MCs listed by Lepping et al. [2006] and they found that the radius of MC curvature is equal to its radius in the 1/3 of cases and that it is greater than its radius in 3 times in other cases. While the form and inner structure of MCs are broadly studied [Liu et al., 2008], the other kinds of ICMEs are less well described [Démoulin, 2010], mainly because non-mc ICMEs do not show the flux-rope structure and cannot be fitted in the same way as MCs. However, there is a possibility to study the shape of ICMEs using their sheaths. The thickness of the sheath depends on the velocity of the ICME leading edge [Siscoe and Odstrcil, 2008] and its geometry. Thus, we performed the study of ICME- and MC-sheath thickness to estimate a possible deformation of ICMEs/MCs during their propagation from the Sun. 129

3 Figure 2. The illustration of possible shapes of a MC together with the sheath thickness (delta), and radius of curvature, Rc. Left: the MC with a circular cross-section; right: MC with an elliptical cross-section elongated perpendicularly to the Sun-MC line. The MC-sheath, delta, MC thickness D2, radius of the MC curvature, Rc observed in the MC leading edge, and radius R of the circular MC are marked. The gray curves indicate such circular MC that has the thickness equal D2. Deformation of Magnetic Clouds In Fig. 2, two possible shapes of MCs are shown. One can see that the MC with an elliptical cross-section elongated perpendicularly to the MC-Sun line (right) has a larger sheath thickness (marked as delta) due to its larger radius of curvature, Rc, in comparison with the circular MC with the same duration of observations (left). To analyze the MC deformation, we estimated the thickness of the MC-sheath using the times of its observation and the average plasma velocity within it and the radius of the MC obtained by fitting of the force-free flux rope model [Lynnyk and Vandas, 2009]. We computed the ratio of the sheath thickness to the MC radius and compare it with the similar ratio calculated using the equation from Russell and Mulligan [2002] delta Rc = Ma 2 1 where delta is the MC-sheath thickness, Rc is a radius of curvature of the obstacle, and Ma is the Alfvén Mach number of the solar wind. For the cylindrical MC it should be the same values, as the MC radius is equal to the radius of curvature in this case. Nevertheless, according to this comparing, the radius of curvature was usualy larger than the radius of the MC. We performed a study of ratios of the MC curvature radius to its radius in order to find the sources that affect this ratio. The results are shown in Fig. 3 where the dependence of Rc/R ratios is plotted as a function of the Alfvén Mach number associated with the MC propagation relative to the solar wind and the MC relative magnetic field strength. This relative magnetic field strenght is the ratio of the axial magnetic field from the fit [Lynnyk and Vandas, 2009] to the magnetic field outside the MC prior to the MC-driven shock. The Rc/R ratios increase with Ma and decrease with an increasing relative MC magnetic field strenght. We expected that a fastly moving MC would be deformed due to the solar wind drag force. On the other hand, if the inner magnetic field is strong, it can help to keep a circular MC shape. 130

4 Figure 3. Dependence of Rc/R ratios on the Alfven Mach number of the MC edge and the inner magnetic field strength. Deformation of the ICME To investigate the deformation of the ICMEs and their evolution, we selected 25 ICMEs listed by Wang and Richardson [2004] that are fulfilling following criteria: the shock prior to the ICME is within two durations of the ICME (a shock observed further than this distance may not be caused by the ICME); the relative velocity of the ICME leading edge to the preceding solar wind velocity is higher than the Alfvén velocity in the solar wind (supersonic ICME); the increase of the magnetic field strength and plasma density are observed in the sheath. According to these criteria, we identified the shock associated with the ICME. Although we do not know the exact geometry of a particular ICME, we assume an elliptical cross-section and thus we can obtain similar information as in the case of the MCs using the same procedure. Even when ICMEs are not identified as MCs, the profiles of the plasma and magnetic field parameters show some of the features that can be used in our study: a discontinuity at the leading edge of the ICME, a shock prior to the ICME and a sheath. An example of such ICME observation is shown in Fig. 4. A comparison of dependences of the Rc/R ratios versus Ma and the relative inner magnetic field for MCs and ICMEs are shown in Figs. 5 and 6. As the relative inner magnetic field for ICMEs we calculate the ratio of the maximal magnetic field observed inside the ICME to the magnetic field measured prior to the ICME-driven shock. For the radius of the MC and ICME, we used thir half-thickness to provide the correct comparison. The differences in the results caused by using this values in the MC case will be discussed later. The Rc/R ratio is greater for ICMEs but ICME and MC ratios increase with Ma and decrease with the relative inner magnetic field. The dependence of the Rc/R ratios on heliocentric distance is shown in Fig. 7. Despite of the wide scatter of the values, we can note that the Rc/R ratio of ICMEs (including MC sub-class) increases with the heliocentric distance. Discussion As it can be seen in Fig. 1, estimates of the relative sheath thickness obtained by comparison of observation times can cause the errors due to the following geometrical features: for non-zero impact parameter (the distance of the spacecraft to the MC axis), the distance between the shock and MC/ICME boundary is larger than for zero impact parameter and the observed thickness of the MC/ICME is smaller. Thus, direct measurements of the relative sheath thickness obtained from observation times of the sheath and MC provide larger values of the relative thickness than the real values. The sources of errors can be avoided by an application of the impact parameter in our calculation. It can be done by fitting of the flux ropes to the magnetic field 131

5 Figure 4. An example of the ICME observed by Voyager 2 on September 20, From top to bottom: magnetic field components (nt), velocity (km/s), the proton density (1/cc) and thermal velocity (km/s) are shown. The vertical lines indicate the ICME-sheath and ICME boundaries. Figure 5. The Rc/R ratios of ICMEs observed by Voyager 2 (left) and that of MCs observed by WIND (right) as a function of the Alfvén Mach number of the ICME/MC leading edge. of the MC and by calculating the MC/ICME boundary normal. Unfortunately, the gaps in the Voyager 2 data make the estimation of the ICME boundary normal very hard, so we did not include the impact factor in our calculation of the ICME- and MC-sheaths to use the similar method for both classes of events. The changing of the Rc/R ratio of MCs versus the impact parameter is shown in Fig. 8. All values of the impact parameter are observed with nearly the same probability and the Rc/R ratio is increasing with the impact parameter by a factor of

6 Figure 6. The Rc/R ratios of ICMEs observed by Voyager 2 (left) and that of MCs observed by WIND (right) as a function of the inner magnetic field strength. Figure 7. The dependence of the Rc/R ratio of MCs (crosses) and ICMEs (stars) on the heliocentric distance, D. The real values of the Rc/R ratio are 1-3 times lower than those in Figures 5-7. However, the results in Fig. 3 are obtained with a right estimation of the MC radius. Conclusion We presented an analysis of MC and ICME deformations based on the study of their sheath thickness. We calculated the Rc/R ratio of ICMEs and MCs and found that this ratio increases with the Alfvén Mach number of the ICME leading edge. This fact can be explained by a deformation of ICMEs and MCs during their propagation with the supersonic speed through the solar wind. Furthermore, this dependence is stronger for ICMEs seen by Voyager 2 in the range of 1 to 30 AU due to a further deformation of the ICME during its propagation and due to weakening of the inner magnetic structure (ICMEs seen by Voyager 2 do not have so distinct magnetic structure as magnetic clouds), consistently with decreasing of the Rc/R ratio with an increasing inner magnetic field. Acknowledgments. The authors thank L. F. Burlaga for providing Voyager 2 magnetic field data used in this work. The present work was partly supported by the Czech Grant Agency under Contract 205/07/0694 and partly by the Research Plan MSM that is financed by the Ministry of Education of the Czech Republic. A. Lynnyk thanks also to the Grant Agency of Charles University for a support (GAUK ). J. D. Richardson was supported by the NASA Voyager project and grant NNX08AC04G. 133

7 Figure 8. The ratios of the MC deformation as seen by WIND versus the impact parameter (in percents of radius) obtained from the fit. References Démoulin, P. (2010), Interaction of ICMEs with the Solar Wind, American Institute of Physics Conference Series, 1216, , doi: / Ebert, R. W., D. J. McComas, H. A. Elliott, R. J. Forsyth, and J. T. Gosling (2009), Bulk properties of the slow and fast solar wind and interplanetary coronal mass ejections measured by Ulysses: Three polar orbits of observations, J. Geophys. Res., 114, , doi: /2008ja Lepping, R. P., D. B. Berdichevsky, C. Wu, A. Szabo, T. Narock, F. Mariani, A. J. Lazarus, and A. J. Quivers (2006), A summary of WIND magnetic clouds for years : model-fitted parameters, associated errors and classifications, Ann. Geophys., 24, Liu, Y., J. D. Richardson, and J. W. Belcher (2005), A statistical study of the properties of interplanetary coronal mass ejections from 0.3 to 5.4 AU, Planet. Space Sci., 53, 3 17, doi: /j.pss Liu, Y., J. G. Luhmann, K. E. J. Huttunen, R. P. Lin, S. D. Bale, C. T. Russell, and A. B. Galvin (2008), Reconstruction of the 2007 May 22 Magnetic Cloud: How Much Can We Trust the Flux-Rope Geometry of CMEs?, Astrophys. J. Let., 677, L133 L136, doi: / Lynnyk, A., and M. Vandas (2009), Fitting of expanding magnetic clouds: A statistical study, Planet. Space Sci., 57, , doi: /j.pss Mulligan, T., and C. T. Russell (2001), Multispacecraft modeling of the flux rope structure of interplanetary coronal mass ejections: Cylindrically symmetric versus nonsymmetric topologies, J. Geophys. Res., 106, 10,581 10,596, doi: /2000ja Neugebauer, M., and R. Goldstein (1997), Particle and field signatures of coronal mass ejections in the solar wind, Geophys. Monogr. Ser., 99, Russell, C. T., and T. Mulligan (2002), On the magnetosheath thicknesses of interplanetary coronal mass ejections, Planet. Space Sci., 50, Siscoe, G., and D. Odstrcil (2008), Ways in which ICME sheaths differ from magnetosheaths, J. Geophys. Res., 113, A00B07, doi: /2008ja Wang, C., and J. D. Richardson (2004), Interplanetary coronal mass ejections observed by Voyager 2 between 1 and 30 AU, J. Geophys. Res., 109, A6104, doi: /2004ja Wimmer-Schweingruber, R. F., et al. (2006), Understanding Interplanetary Coronal Mass Ejection Signatures. Report of Working Group B, Space Sci. Rev., 123, , doi: /s x. Zurbuchen, T. H., and I. G. Richardson (2006), In-Situ Solar Wind and Magnetic Field Signatures of Interplanetary Coronal Mass Ejections, Space Sci. Rev., 123, 31 43, doi: /s