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1 Planetary and Space Science 9 (11) 9 Contents lists available at ScienceDirect Planetary and Space Science journal homepage: A case study of Kelvin Helmholtz vortices on both flanks of the Earth s magnetotail Masaki N. Nishino a,, Hiroshi Hasegawa a, Masaki Fujimoto a, Yoshifumi Saito a, Toshifumi Mukai b, Iannis Dandouras c,d, Henri Reme c,d, Alessandro Retino e, Rumi Nakamura e, Elizabeth Lucek f, Steven J. Schwartz f a Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, Yoshinodai, Sagamihara, Kanagawa 9-81, Japan b Japan Aerospace Exploration Agency, Marunouchi Kitaguchi Building, 1-6- Marunouchi, Chiyoda-ku, Tokyo 1-86, Japan c University of Toulouse, UPS, CESR, 9 av colonel Roche, F-318 Toulouse cedex 9, France d CNRS, UMR187, F-318 Toulouse, France e Austrian Academy of Sciences, Space Research Institute, Schmiedlstraße 6, 8 Graz, Austria f Space and Atmospheric Physics, Imperial College London, South Kensington, London SW7 AZ, UK article info Article history: Received 3 October 9 Received in revised form 18 February 1 Accepted 9 March 1 Available online 1 March 1 Keywords: Magnetopause Energy transport Kelvin Helmholtz instability abstract Kelvin Helmholtz instability (KHI) is a fundamental fluid dynamical process that develops in a velocity shear layer. It is excited on the tail-flanks of the Earth s magnetosphere where the flowing magnetosheath plasma and the stagnant magnetospheric plasma sit adjacent to each other. This instability is thought to induce vortical structures and play an important role in plasma transport there. While KHI vortices have been detected, the earlier observations were performed only on one flank at a time and questions related to dawn dusk asymmetry were not addressed. Here, we report a case where KHI vortices grow more or less simultaneously and symmetrically on both flanks, despite all the factors that may have broken the symmetry. Yet, energy distributions of ions in and around the vortices show a remarkable dawn dusk asymmetry. Our results thus suggest that although the initiation and development of the KHI depend primarily on the macroscopic properties of the flow, the observed enhancement of ion energy transport around the dawn side vortices may be linked to microphysical processes including wave-particle interactions. Possible coupling between macro- and micro-scales, if it is at work, suggests a role for KHI not only within the Earth s magnetosphere (e.g., magnetopause and geomagnetic tail) but also in other regions where shear flows of magnetized plasma play important roles. & 1 Elsevier Ltd. All rights reserved. 1. Introduction Wavy structures have been observed around the Earth s magnetopause where the high-density magnetosheath flow and the low-density magnetospheric plasma attach to each other (e.g. Kivelson and Chen, 199; De Keyser and Roth, 3). Although these wavy structures around the magnetopause are thought to be generated by several causes such as sudden change in the solar wind (SW) dynamic pressure (Sibeck, 199), a natural candidate for the origin of the surface waves without any SW change is the Kelvin Helmholtz instability (KHI) (Kivelson and Chen, 199), which is expected to grow and generate vortices along a velocity shear layer (Chandrasekhar, 1961). The KHI vortices are thought to play a role in plasma transport across the magnetopause under northward interplanetary magnetic field (IMF) conditions Corresponding author. Tel.: ; fax: address: nishino@stp.isas.jaxa.jp (M.N. Nishino). (Fairfield et al., ; Fujimoto and Terasawa, 199), when the SW entry into the magnetosphere is most enhanced (Terasawa et al., 1997), as well as plasma transport in energy space (Wilber and Winglee, 199). In particular, secondary instabilities excited in the vortices have been proposed as candidates for transport of the SW plasma into the magnetosphere (e.g. Matsumoto and Hoshino, ; Califano et al., 9). Recent spacecraft observations showed that rolled-up vortical structures indeed exist around the KHI-unstable magnetopause under northward IMF conditions (Hasegawa et al., a). However, the growth of the KHI and resultant vortical structures around the magnetopause might involve a dawn dusk asymmetry (Scholer and Treumann, 1997) due to several causes as follows. The growth rate of the KHI is higher on the dusk side than on the dawn side under northward IMF, because it depends on the polarity of field-aligned vorticity when the finite Larmor radius effect is taken into account (Huba, 1996). On the other hand, because the magnetosheath on the dawn side is situated downstream of the quasi-parallel portion of the bow shock under 3-633/$ - see front matter & 1 Elsevier Ltd. All rights reserved. doi:1.116/j.pss

2 M.N. Nishino et al. / Planetary and Space Science 9 (11) 9 3 Parker-spiral magnetic field, the magnetosheath on the dawn side may have larger amplitude perturbations, as seeds for the KHI excitation, than on the dusk side (Engebretson et al., 1991; Miura, 199; Collado-Vega et al., 7). Additionally, the IMF draped around the magnetosphere may suppress the development of the KHI on the dusk side (Lee and Olson, 198; Collado-Vega et al., 7). In addition to these local effects, global magnetosphereionosphere coupling also may affect the development of the KHI, because the magnetopause unstable to the KHI and the ionosphere are connected by field lines (Lotko et al., 1987), giving rise to dawn dusk asymmetrical effects (Wei and Lee, 1993). Therefore, a question remains as to whether the KHI vortices develop simultaneously on both sides of the magnetotail. But, since previous observations were performed only on one side at a time, there were no means to see what was occurring on the other side. In this paper, we will present simultaneous measurement of the boundary layers on both sides of the magnetotail, and show that KHI vortices grow more or less simultaneously and symmetrically on both flanks while ion energy distributions are asymmetric.. Symmetrical development of vortices The Cluster 1 (C1) and 3 (C3), Geotail (GT), and Wind spacecraft survey the interplanetary space around 1 AU (astronomical unit), providing simultaneous multi-point measurement of the Earth s magnetosphere and the SW. Such a multi-spacecraft deployment enabled the first simultaneous observation of the boundary layers on both sides of the magnetosphere. The SW data obtained by Wind (Fig. 1) show that the IMF had a northward component around the Earth s magnetosphere for several hours until 9: UT on 6 June 7, which favors the occurrence of the KHI at the magnetopause. (The SW data are time-shifted to the bow shock nose, and shown in the GSM (geocentric solar magnetospheric) coordinates.) Because the IMF had a nonnegligible B X and B Y component during most of the interval of our interest, there might be a dawn dusk asymmetry in the growth of the KHI (Collado-Vega et al., 7; Engebretson et al., 1991; Lee and Olson, 198; Miura, 199). The SW speed and density were in the ranges of 38 km s 1 and.7 6. cm 3, respectively (Fig. 1B and C), and the SW dynamic pressure was in the range of npa (Fig. 1D). Although the dynamic pressure showed modest variations owing to SW density variations, it did not change drastically except for some prompt changes, which suggests that the surface waves around the magnetopause, if they are excited, are probably due to the KHI. Under this prolonged northward IMF condition, C1 and C3 on the dawn side and GT on the dusk side had a chance to stay simultaneously on the low-latitude magnetotail flanks. The boundary layers were observed by Cluster around ( 6, 18, 7) R E (Earth radii; 1 R E ¼6378 km) in the GSM coordinates, and by GT around ( 8,, ) R E ; the two sets of spacecraft were situated on the opposite sides of the magnetosphere, at a similar latitude, and with a similar distance from the magnetopause nose. C1 and C3, separated by km in the direction normal to the average magnetopause surface, were predominantly in the magnetosheath-like region B (nt) - Solar wind data :-1: UT (Time-shifted to the bow shock nose) Bx By Bz N (cm -3 ) 8 6 VSW (km/s) 3 1 PSW (npa) : : : 6: 8: 1: (UT) corresponding interval Fig. 1. Solar wind data for the interval : 1: UT on 6 June 7, observed by the Wind spacecraft far upstream of the Earth s bow shock (X 6R E ) and timeshifted to the bow shock nose. Magnetic field, proton density, flow speed, and dynamic pressure are shown.

3 M.N. Nishino et al. / Planetary and Space Science 9 (11) 9 around the dawn magnetopause, but repeatedly encountered the magnetopause; large fluctuations in the velocity, density, and temperature represent multiple magnetopause crossings by C1 and C3 (Fig. A D). In the magnetosheath (with plasma density of 1 cm 3) tailward flows ð 3 km s 1 Þ were observed, while the low density plasma on the magnetospheric side was stagnant, demonstrating the existence of velocity shear across the magnetopause. Order-of-magnitude variations in the plasma density simultaneously observed by C1 and C3 (between : : and 6: 7:3 UT) suggest the presence of large-amplitude wavy/vortical structures around the magnetopause on the dawn side. At the same time, GT was located on the dusk side the opposite side of the near-earth magnetotail and at almost the same latitude as of Cluster. GT was on the magnetospheric side of the dusk magnetopause for most of the time, but after 9:18 UT it came to the magnetopause boundary layer and then repeatedly encountered the magnetopause (Fig. E H). The encounter corresponded to compression of the magnetosphere by a modest enhancement of the SW dynamic pressure (Fig. 1D). Besides the encounters to the boundary layer after 9:18 UT, GT observed large-amplitude quasi-periodic fluctuations of velocity when in the magnetosphere log(je) 6 Cluster 1 (7:1-7: UT) Magnetospheric plasma P (ppa) Boundary 1 1 Vx (km s-1) Dawn side Cluster 1, 3 Magnetosheath :1 7:1 7: Magnetosphere Time (UT) 6: 7:3 7: 9:3 9:3 9: Boundary 3 6 Geotail (9:1-9: UT) Magnetosheath plasma Magnetosphere P (ppa) X -. Y 19.3 Z : Vx (km s-1) X -6. Y Z -. X -6.3 Y Z -.6 7: Time (UT) log(je) log(e) (ev) Vx (km s-1) N (cm-3) T (MK) Magnetosheath 3 Region GSM (RE) Boundary Dusk side Geotail GSM (RE) GSM (RE) T (MK) N (cm-3) Vx (km s-1) log(e) (ev) Region (Fig. F), which shows that wavy/vortical structures existed around the magnetopause on the dusk side for an extended interval. In the dense region the magnetosheath plasma flowed tailward with a speed of 9 km s 1, while flows were stagnant or slightly sunward on the magnetospheric side, which shows the presence of velocity shear similar in magnitude to that on the dawn side. The magnetospheric plasma temperature on both sides was as low as 1 1 MK, which is much higher than the magnetosheath temperature but is lower than the typical magnetosphere temperature (several tens of MK), and the magnetospheric ion density on both sides ð 1 cm 3 Þ was much higher than its usual value ð :: cm 3 Þ (Nagata et al., 8). Because both Cluster and GT had opportunities to repeat encounters with the boundary layer (Fig. A H), we examine whether the boundary motions detected on both flanks were due to rolled-up vortices generated by the KHI. Variation of the total pressure around the KHI vortices is so sensitive to the development of vortical structures that it can be used as a diagnostic of rolled-up vortices (Hasegawa et al., 9). A low-pressure region emerges in the center of a vortex due to the centrifugal force induced by rotational plasma fluid motions, while a high-pressure :1 9:1 9: 9: Time (UT) : : : 1: Fig.. Plasma and magnetic field data obtained by Cluster1 (C1), Cluster3 (C3), and Geotail (GT). (A H) The magnetopause boundary regions observed simultaneously by Cluster 1 (C1) and 3 (C3) on the dawn side and by Geotail (GT) on the dusk side for the interval 6: 1: UT on 6 June 7. (A D) The omni-directional differential energy flux of ions (JE in ev s 1 cm sr 1 ev 1) from C1, the sunward/anti-sunward flow speed VX in km s 1, the ion density (N in cm 3), and the ion temperature (T in MK) measured by C1 (black line) and C3 (green line) on the dawn side. (E H) Data from GT on the dusk side are shown in a similar format to that for Cluster. (I L) Pressure variations on both sides. Enhancements in the total pressure (indicated by red hatching) occurred at the transitions from slow to fast flow regions inside the vortices. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4 M.N. Nishino et al. / Planetary and Space Science 9 (11) 9 region is formed between two neighboring vortices by plasma compression. A simulation study (Miura, 1997) showed that the amplitude of the pressure variation is about % of the average value in the nonlinearly rolled-up stage, and that the total pressure has a maximum at transition regions between slow and fast flows. These features of pressure variation can be measured by a single spacecraft that is passing by the KHI vortices, and what was observed on the both sides (Fig. I L) were indeed consistent with the simulation results. In addition to the pressure variation features, spatial distribution of plasma density can be used as a measure of the KHI signatures. Although C3 was on the magnetospheric side of C1, the plasma density at C3 sometimes exceeded that at C1 (Fig. 3A); such reversed distribution of the plasma density around the magnetopause is evidence of overturning of KHI waves (Hasegawa et al., a). Low-density plasmas flowing faster than the magnetosheath plasma seen in the scatter plot of V X versus N (Fig. 3B) also are a signature of the rolled-up vortices (Hasegawa et al., 6). Reconstruction of flow lines based on a Grad-Shafranov-like equation for the stream function (Sonnerup et al., 6; Hasegawa et al., 7) can visualize velocity fields in the spatial structures moving past the spacecraft (see Appendix). Although the method assumes time-independent D structure and existence of magnetic field only in the direction perpendicular to the D plane, it has been confirmed by previous studies that the method is still applicable for similar situation around the vortical structure even if there are temporal variations and magnetic field components in the D plane (Hasegawa et al., 7, 9). The two-dimensional map derived from the dawn side data (Fig. B) presents a feature consistent with a rolled-up vortex; a highdensity region of the magnetosheath intrudes into the lowdensity region of the magnetosphere. The rotation of flow direction in the vortex viewed from the north was clockwise, as expected on the dawn side and consistent with earlier observations (Hones et al., 1981). In the map for the dusk side (Fig. C), although intrusion of the magnetosheath plasma is less clear than on the dawn side, counter-clockwise rotations of flow direction around the dense region intruding into the magnetosphere are consistent with KHI vortices expected on the dusk side. These observations and reconstruction results suggest that the KHI develops more or less simultaneously on both sides to form vortical structures at about the same distances from the magnetopause nose when the IMF has a northward component. C1, C3 (dawn) 7:1-7: UT density reversal Density (cm -3 ) :1 7:1 7: 7: 7:3 7:3 7: Time (UT) 1 C1, C3 (dawn) 7:-7: UT GT(dusk) 9:1-9: UT C1 C3 1 Vx (kms -1 ) -1 - Vx (kms -1 ) Density (cm -3 ) Density (cm -3 ) Fig. 3. Observations showing further evidence of rolled-up vortices. (A) Ion density at C3 (green) on more magnetospheric side being occasionally higher than at C1 (black) on more magnetosheath side in the vortical structures (shown by red arrows) is evidence for overturning of KHI waves (Hasegawa et al., a). By considering the travelling time of vortical structures from C1 to C3 locations, C1 and C3 are put onto an equal phase plane perpendicular to the KHI wave vector. The travelling time of the structures along the wave vector is determined to be 9 s, because the two satellites were separated by 16 km along the wave vector direction (parallel to the x axis in the map of flow lines in (B) at 7:3 UT and the propagation speed of the vortices on the dawn side was 16 km s 1. (B) Scatter plot between V X and N from C1 (black) and C3 (green) data obtained on the dawn side. The data samples characterized by magnetospheric density ðno1:cm 3 Þ and the tailward speed higher than in the magnetosheath ðv X o km s 1 Þ represent evidence of rolled-up vortices in the KHI-unstable boundary layer (Hasegawa et al., 6). (C) While the GT data on the dusk side do not include high-density data samples because the satellite did not fully come into the dense magnetosheath region, the plot with low-density and higher-speed samples are consistent with roll-up of KHI vortices. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

5 6 M.N. Nishino et al. / Planetary and Space Science 9 (11) 9 solar wind X dawn Earth plasma sheet in the magnetosphere (view from north) magnetopause Geotail dusk Y Cluster 1, 3 y (km) y (km) Streamline Cluster :3.1-78:.8 UT 1 km s -1-1 N (cm -3 ) x (km) Streamline Geotail :7.97-9:8.876 UT 1 km s N (cm -3 ) x (km) Fig.. Vortical structures due to the KHI on the both flanks, visualized by Grad-Shafranov-like reconstruction technique (Sonnerup et al., 6; Hasegawa et al., 7). (A) A schematic equatorial cut of the Earth s magnetosphere viewed from the north shows locations where KHI vortical structures were observed. (B) A map of streamlines and density reconstructed from C1 data. White arrows show velocity perturbations measured along the spacecraft path, in the frame moving with the vortex, and black curves show flow lines of plasma. The co-moving frame velocity is the center-of-mass velocity based on the measurements on both sides of the magnetopause, and is antiparallel to the x axis in the map. The y axis is defined by the cross product of the mean magnetic field orientation for the reconstructed interval and the x axis, and the z axis completes the right-handed orthogonal system; x ¼ð:936; :3; :138Þ; y ¼ð :39; :91; :16Þ, and z ¼ð :; :; :967Þ in GSM (see also Appendix). (C) The map reconstructed from GT data in a similar format to that for C1 but on a larger spatial scale. The reconstruction axes are: x ¼ð:938; :37; :Þ; y ¼ð:3; :98; :17Þ, and z ¼ð:; :136; :989Þ in GSM. Table 1 Symmetric spatial scales of the KHI vortices on both sides. Region Vorticity Speed (km s 1 ) Period (s) Wavelength (km) Width (km) Aspect ratio Dawn CW : Dusk CCW The center-of mass speed of the vortical structure in the rest frame of satellite is taken as the phase speed of the KHI waves. The aspect ratio in the last column is the ratio of the width to the wavelength of each KHI vortex. We check the stability of the KHI at the boundary layer. The condition for the linear KHI growth is represented as sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi jðv 1 V Þ ^kj r 1 þr jðb m r 1 r 1 B Þ ^kj ð1þ where subscripts 1 and denote either side of the velocity shear layer, and r, V, B, and ^k represent mass density, velocity, magnetic field, and a unit vector along the wave vector, respectively (e.g. Ogilvie and Fitzenreiter, 1989; Gratton et al., ). The direction of the center-of-mass velocity ( x direction, see caption for Fig. ) is chosen as the wave direction ^k, and data obtained on either side of the boundary layer are used for the calculation. On the dawn side, the growth condition was satisfied, although both hand sides of (1) were at times comparable to each other. On the dusk side, the growth condition was only intermittently satisfied, depending on the data in the dense region used for the calculation. This is probably because GT did not fully come into the denser region of the magnetosheath. The calculations suggest that the boundary layer on both sides was more or less KHI-unstable, although the observed KHI waves must have been initiated at a region upstream of the satellites because they were propagating tailward along the magnetopause. Spatial scales of the KHI vortical structures on both sides are compared with each other (Table 1). The wavelength of the KHI vortical structures can be estimated from the observations; the period of multiple boundary crossings was about 13 s on both sides, and the center-of-mass speed of the vortical structures in the rest frame of spacecraft, taken as the phase speed of the KHI waves (Chandrasekhar, 1961), was 16 km s 1 onthedawnsideand 18 km s 1 on the dusk side. (The phase speed of the KHI waves is equal to the center-of-mass speed only for a zero-thickness surface (Chandrasekhar, 1961), while it is between the average speed jðv 1 þv Þ=j and the center-of-mass speed jðr 1 V 1 þr V Þ=ðr 1 þr Þj in the cases of the real magnetopause (Hasegawa et al., 9). However, the center-of mass speed is adopted for rough estimation in the current study.) Therefore, the wavelength was km on the dawn side and km on the dusk side, which means that the KHI vortical structures had an approximately equal wavelength on both sides. The widths of the vortical structures also were about the same; the boundary layer on the dawn side was observed simultaneously by C1 and C3, which were separated by km in the direction perpendicular to the average (unperturbed) magnetopause surface. This fact indicates that the vortex width on the dawn side was more than km. On the dusk side, the vortex width roughly estimated from the Grad-Shafranov-like map (Fig. C) is about 1 km, which is nearly equal to that on the dawn side. Similar aspect ratios (the width to wavelength ratio) on both sides also support that the vortical structures were indeed rolled-up not only at dawn but also at dusk. These results suggest that the rolled-up KHI vortices on both sides had similar spatial scales and grew roughly symmetrically. 3. Asymmetrical ion energy distribution The rolled-up KHI vortices are expected to play an important role in transporting magnetosheath and magnetospheric plasma in both energy space and real space (Wilber and Winglee, 199). For

6 M.N. Nishino et al. / Planetary and Space Science 9 (11) 9 7 this reason, plasma properties around the vortices on both sides are worth being compared with each other. Especially, because energyversus-time spectrograms of ions in the boundary region (indicated by green bars in Fig. A and E) show a dawn dusk asymmetry, we examine ion energy distributions there in detail (Fig. ). On the dawn side, changes of the energy distribution from the dense region to the tenuous magnetospheric-side region seem to show a gradual heating (i.e. transport in energy space) of ions in and around the vortices (Fig. A). A low-energy component ðo1 kevþ was enhanced in the denser magnetosheath-like regions (black and blue curves), while a high-energy one ( several kev) was dominant in the tenuous magnetosphere-like regions (a red curve). There is only a single peak in each energy spectrum, and the energy at the peak gradually increases as the satellite moves from denser to tenuous regions, which suggests that ion transport in energy space was active in and around the vortices. Since the observed plasmas were likely on closed field lines (Fig. 6), the data suggest that the plasma of magnetosheath origin, which had been captured on the magnetosphere (i.e. transported in real space), was transported in energy space. However, concerning ion transport in real space, we do not find evidence for the capture of the magnetosheath ions onto the closed field lines via the KHI vortices. On the dusk side, on the other hand, ion energy distributions in the stagnant boundary region near the KHI vortices were characterized by a superposition of high-energy (hot) ions and low-energy (cold) ions with peak energies of and.8 kev, respectively (Fig. B). Since the high-energy component is of magnetospheric origin while the low-energy one came from the magnetosheath, the above feature shows that spatial mixing (i.e. ion transport in real space) was enhanced but ion transport in energy space was poor on the dusk side. (Although electron data from GT on the dusk side is not shown because the data are still under calibration, the lower density at the GT location suggests that the observed plasma were on closed field lines of more magnetospheric region.) However, it is still unknown whether the spatial mixing of ions really occurs inside the KHI vortices on the dusk side. In the current analysis, it cannot be judged whether on both sides ion transport in real space is due to KHI vortices. Although the secondary instabilities induced by the KHI are candidates for mechanism of ion transport in real space, we do not find direct evidence for transport of ions from the magnetosheath to the region on closed field lines around the vortices. (For the ion transport in real space, see also Discussion.) In short, the observation shows dawn dusk asymmetry of ion transport in energy space around rolled-up KHI vortices; ion energy transport occurs on the dawn side, while it seems to be suppressed on the dusk side. The dawn dusk asymmetry in the ion energy distributions is also seen in the plasma sheet under the northward IMF condition (Fujimoto et al., ; Hasegawa et al., b; Wing et al., ), which suggests that some mechanism taking place around the vortices has significant effects on the magnetospheric plasma.. Discussion We have shown that the KHI vortices grow roughly symmetrically on both sides of the magnetotail, while ion energy distributions are asymmetric. The dawn dusk asymmetry of the ion energy distribution in and around the vortices suggests that 1 7 Dawn side (C1) 1 7 Dusk side (GT) MSH-like MSph-like MSH-like MSph-like differential energy flux (ev s -1 cm - sr -1 ev -1 ) differential energy flux (ev s -1 cm - sr -1 ev -1 ) energy (kev) energy (kev) 7:7: 1. /cc 1.8 7:8: 1.1 /cc.1 7:8:1.6 /cc 7.7 7:8:3. /cc 1 7:8:31.8 /cc 11 9:18:7 1.3 /cc 3.9 MK 9:13:8 1. /cc 6.9MK 9:16:.73 /cc 9.MK 8:7:.6 /cc 1MK Fig.. Dawn dusk asymmetry in ion energy distribution function around the KHI vortices. (A) Time series of differential energy fluxes from C1 on the dawn side. Each flux has a single peak in the energy space, and the energy at the peak gradually becomes higher as the satellite comes from denser magnetosheath-like region to tenuous magnetosphere-like region. (B) On the other hand, a typical distribution function in the dusk boundary layer (green line) is a superposition of a low-energy magnetosheath component ðo1 kevþ and high-energy magnetospheric one ( several kev), which shows a remarkable fact that ion transport occurs in the real space but is inefficient in the energy space. These observations show dawn dusk asymmetry of ion transport in energy space around the KHI vortices. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

7 8 M.N. Nishino et al. / Planetary and Space Science 9 (11) 9 Fig. 6. Evidence of closed magnetic field configuration at the C1 location. Well-balanced fluxes of parallel and anti-parallel electrons obtained by C1 on the magnetosphereside region in the vortical structure is evidence of closed magnetic field configuration at the satellite location (Traver et al., 1991). The data shown here were obtained at 7:8: UT, when C1 detected gradual heating of ions inside the vortical structure (Fig. ). (A) Two-dimensional representation of the electron phase space density, in which the vertical (horizontal) axis shows electron speed in the parallel (perpendicular) direction to the local magnetic field, shows elongations of the contours (greencolored region) in the direction parallel to the magnetic field. The red-colored region around the origin is due to photoelectrons present around the satellite. (B) The onedimensional cuts in the directions parallel (red line) and perpendicular (black dashed line) to the magnetic field. An enhancement of parallel electron flux is evident in the range of a few to ten thousand km s 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) ion energy transport around the KHI vortices depends on the polarity of field-aligned vorticity. While the observed quasisymmetrical growth of the KHI vortices is of a macroscopic scale, the asymmetric ion energy transport in and around the vortices may be associated with microscopic ion processes. However, relation between the ion energy transport and the KHI vortices remained unsolved. Candidates for the ion transport in energy space activated on the dawn side are interactions between ions and waves such as kinetic Alfvén waves (Johnson and Cheng, 1997; Johnson et al., 1), which might be excited more strongly on the dawn side. A strong candidate for ion transport mechanism in real space is double-lobe magnetic reconnection on the dayside which occurs under the northward IMF condition (Lavraud et al., 6; Øieroset et al., 8). In this case, KHI vortices do not play a role in ion transport from the magnetosheath into the regions on closed field lines. Another candidate for ion transport mechanism in real space is magnetic reconnection induced by the KHI vortices (Otto and Fairfield, ; Nykyri and Otto, 1; Nakamura et al., 6). Occurrence of magnetic reconnection in the vortices may be asymmetric on both sides, because reconnection can be induced by Alfvénic fluctuations in the magnetosheath (Belmont and Razeau, 1) that can be asymmetric. If magnetic reconnection in the vortices occurs more frequently on the dawn side than on the dusk side, ion energy transport enhanced on the dawn side may also be related to this mechanism. Future studies of microscopic physics including wave-particle interactions as well as coupling between macroscopic and microscopic physics will provide a clue to understand exactly what occurs in the KHI vortices and gives birth to the asymmetry in the ion transport. Acknowledgments The authors thank T. Nagai for providing magnetic field data obtained by the Geotail spacecraft. The authors are grateful to H.U. Eichelberger for generating magnetic field data from Cluster/ FGM. The time-shifted SW data were generated by J.H. King and N. Papitashvilli, and provided via the GSFC/SPDF OMNIWeb interface at Appendix A. Methods The D maps of streamlines shown in Fig. were generated by a data analysis technique (Sonnerup et al., 6; Hasegawa et al., 7), which recovers the velocity field and other plasma parameters in a rectangular domain surrounding the path of an observing spacecraft from integration of a Grad-Shafranov-like (GS-like) equation for the compressible stream function by using measured plasma and magnetic field data as spatial initial values. The basic assumptions underlying the technique are that the structures moving past the spacecraft are D and time-independent when seen in their proper moving frame of reference; the GS-like equation describes the balance between the forces from the total pressure gradient and centrifugal forces exerted by plasmas flowing along curved streamlines, in a plane perpendicular to the axis along which no or only a weak spatial gradient exists. These assumptions are never precisely satisfied in any real applications, but experiments using synthetic data from timedependent D numerical simulations indicate that the reconstruction nevertheless can recover qualitatively correct streamline features such as the aspect ratio of a KHI vortex (Hasegawa et al., 7). The estimation of the co-moving frame velocity is based on the linear theory of the KHI (Chandrasekhar, 1961), and assumes that the computed center-of-mass velocity, taken as the phase or moving velocity of the KHI wave, is parallel to the KHI wave vector. The orientations of the reconstruction axes, x, y, and z, were determined in a similar way to an earlier study (Hasegawa et al., 7).

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