The January 10{11, 1997 magnetic cloud: Multipoint. measurements. J. Safrankova 1,Z.Nemecek 1,L.Prech 1, G. Zastenker 2, K.I.

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1 1 The January 1{11, 1997 magnetic cloud: Multipoint measurements J. Safrankova 1,Z.Nemecek 1,L.Prech 1, G. Zastenker 2, K.I. Paularena 3, N. Nikolaeva 2, M. Nozdrachev 2,A.Skalsky 2, T. Mukai 4 Short title: JANUARY 1997 MAGNETIC CLOUD

2 2 Abstract. The WIND and IMP 8 spacecraft detected a magnetic cloud upstream of Earth's bow shock on January 1{11, 1997 while the INTERBALL{1, MAGION{4, GEOTAIL, and LANL{84 satellites were in the vicinity of the magnetopause and crossed it many times. Based on these multipoint observations we have determined the velocity of the magnetopause motion invoked by the sharp changes of the solar wind dynamic pressure during this event, and also the velocity of propagation of the structures connected with the magnetic cloud through the magnetosheath. Using the determined velocities, we tried to nd the spatial orientations and dimensions of the observed disturbances. The estimated orientation of the interplanetary shock driven by the magnetic cloud is interpreted as a consequence of the local deformation of the cloud surface. The timing of measurements made by the aforementioned satellites suggests that the strong density enhancement behind the magnetic cloud is limited in spatial extend. We attribute this enhancement to the interaction of the magnetic cloud with the ambient solar wind plasma.

3 3 1. Introduction The disturbances of the interplanetary magnetic eld (IMF) and solar wind parameters connected with magnetic clouds provide an excellent opportunity to study the propagation of such disturbances through interplanetary space and to investigate the corresponding response when they hit the magnetosphere. For such a study, wehave used data from ve Earth{orbiting spacecraft in dierent locations of geospace during the January 1{11, 1997 event. Our paper aims at the determination of the position and velocity of the magnetopause under such unusual conditions, the estimation of the velocity of propagation of great disturbances through the magnetosheath, as well as at the assessment of the shape and dimensions of the observed disturbances. 2. Event Overview We begin analysis of the January 1997 coronal mass ejection (CME) event shortly before 1 UT on January 1, when an interplanetary shock was observed by the WIND spacecraft. The shock was followed by a structure that has been identied as a magnetic cloud according to the WIND solar wind plasma and IMF data. During this day the B Z component of the interplanetary magnetic eld turned strongly southward ( 15 nt) at about 43 UT and then rotated slowly back tob Z over a period of some 12 hours, and then continued rotating to a strongly northward orientation during the next 8 hours, reaching a maximum value of B Z 25 nt. While the IMF was northward (January 11, 1 UT), the solar wind ion density became very large (about 15 cm 3 ), although the solar wind velocity remained at rather moderate values (about 45 km/s). The extremely large changes of the dynamic pressure in coincidence with the signicant values of the positive B Z IMF component gave rise to very large momentum ux values incident upon the magnetopause (MP), pushing the MP many times inside the orbits of both INTERBALL{1 and MAGION{4 during this long time interval.

4 4 3. Observations We have concentrated our attention on two subintervals (January 1, -4 UT and January 11, -3 UT) because these intervals are characterized by sharp changes of the solar wind density and the data from many spacecraft are available. Coordinates of the spacecraft which observed the MP crossings (GEOTAIL, INTERBALL{1, MAGION{4, LANL{84) for both time intervals are plotted in Figure 1 Figure 1 together with the position of the magnetopause estimated according to the Shue et al. [1997] model. It should be noted that during the rst time interval (January 1 - thin line and open marks) all depicted satellites were located near the ecliptic plane, but during the second interval (January 11 - thick line and solid marks), the INTERBALL satellites were 5:6 R E north of the ecliptic plane. In general, the model predicted all the observed MP crossings rather well; the slight disagreement between the predictions and observations may be due to the large Y GSE distance of the WIND spacecraft which has been used as the solar wind monitor (X GSE =85R E ;Y GSE = 59:4 R E ;Z GSE = 3:4 R E on January 1 and X GSE =93:5R E ;Y GSE = 57 R E ;Z GSE = 4:5 R E on January 11). We have examined other models in Nikolaeva et al. [1997], but the Shue et al. [1997] model seems to be a good approximation to the MP position for a broad range of coordinates and conditions in geospace. The MAGION{4 and INTERBALL{1 positions cannot be distinguished in Figure 1 due to their short separation (X GSE = 47 km, Y GSE = 2 km and Z GSE = 45 km on January 1; X GSE = 48 km, Y GSE = 64 km and Z GSE = 4 km on January 11); nevertheless, the timing of the observations of these two spacecraft allows us to calculate the velocity of magnetopause motion. The simultaneous measurements of the multiple magnetopause crossings are shown in Figure 2. According to the ion Figure 2 density and magnetic eld data, four MP crossings on January 1 can be identied in the MAGION{4 and INTERBALL{1 observations [Safrankova et al., 1997], the rst of them being attributed to the interplanetary shock registered by WIND at 53:25 UT. Other crossings have no such clear \source" in the solar wind but we suppose that they are connected with smaller uctuations of the solar wind dynamic pressure (the

5 5 top panel in Figure 2). Table 1 provides an overview of the magnetopause motion velocities calculated for the crossings in Figure 2. The velocity calculation is based on the assumption that the magnetopause moves along its normal, which is determined by minimum variance analysis of the magnetic eld data. However, it should be noted that these normals are nearly the same as the normals to the model MP surface for all crossings in Table 1. The last row oftable 1 shows the parameters of the MP crossing observed as a result of the sharp enhancement of the ion density observed by WIND on January 11 at 54:3 UT. A comparison of the last two columns of Table 1 leads to the conclusion that the largest changes of solar wind dynamic pressure result in the fastest magnetopause motion. The velocity is surprisingly large; it is larger than the magnetosonic speed and even than the solar wind speed in the last case. However, since a projection of the spacecraft{separation vector onto the normal direction is in the order of thousands of km and the corresponding time delay was determined from high{time resolution ion ux measurements (.2 s for INTERBALL{1 and.25 s for MAGION{4), the error in determination of the magnetopause velocity cannot exceed 25%. Similar results were obtained for other cases (the magnetic cloud on October 18, 1995, etc.) not shown in this study. Thus, we suppose instead that the assumptions of the minimum variance technique do not hold and the direction of the magnetopause normal obtained by this method is not realistic for big pressure jumps.

6 6 Table 1. Velocity of Magnetopause Motion Day in Time t a r ~n b c d v MP p 1= [UT] [s] [km] [km/s] January January January January January a Time delay between MP crossings observed by MAGION{4 and INTERBALL{1. b Projection of the spacecraft separation vector onto the MP normal. c Magnetopause velocity. d Relative solar wind dynamic pressure jump attributed to the particular MP crossing (from the WIND data). Nevertheless, for our further analysis of the event it is important to assume that the high velocity of magnetopause motion and the short distance between the spacecraft and the magnetopause (see Figure 1) allow us to suppose that the time of the magnetopause crossing coincides with the arrival time of the disturbance at the nearest point ofthe magnetopause. The observed time delay between the GEOTAIL and INTERBALL magnetopause crossings can be attributed to the time of propagation of the disturbance from the subsolar point (the GEOTAIL location) to the ank (both INTERBALL satellite locations). To determine the velocity of the disturbance propagation, we should rst estimate the orientation of the discontinuity with respect to the spacecraft separation vector because the satellites are separated by 29R E in the Y GSE direction. At 53:25 UT on January 1 (11:25 UT on the time{shifted data of Figure 2),

7 7 WIND registered a density jump from 6.4 to 15.5 cm 3 accompanied by avelocity change from 385 to 45 km/s. This jump resulted in the magnetopause crossings registered by GEOTAIL at 16:2 UT (8.2, -6.2, 1)R E and by INTERBALL{1 at 115:15 UT (-18.7, 21, 4)R E. The velocity of propagation of this jump obtained from the continuity equation is 42 km/s; i.e., only slightly higher than the solar wind speed. Taking into account the time of propagation of the disturbance from the WIND to the GEOTAIL locations, and assuming propagation along the X GSE axis with a velocity of 42 km/s, we can transform the GEOTAIL coordinates into the WIND frame. The result is shown as G1in Figure 3. In our calculations, we have neglected a possible Figure 3 deceleration of the shock in the subsolar magnetosheath because in this location the magnetosheath is thin and the corresponding time delay would be small. A similar procedure (again neglecting the magnetosheath thickness) leads to the INTERBALL{1 coordinates denoted as IB 1 in Figure 3. The resultant edge of the disturbance as observed at GEOTAIL and INTERBALL{1 is plotted as a dotted line. As such a sharply{curved surface of the interplanetary shock can hardly be realistic, we suppose instead that the disturbance proceeds from the bow shock toward INTERBALL{1 with a velocity dierent from that in the solar wind. If the distances from the bow shock are 2 R E for INTERBALL{1, and 4 R E for GEOTAIL, and the disturbance propagates with the magnetosheath velocity as measured by INTERBALL{1 ( 25 km/s), we obtain the positions denoted as G2and IB 2 in Figure 3 and thus a locally{planar surface of the interplanetary shock. This estimation does not account for the spatial changes of the magnetosheath velocity which follow from Spreiter et al. [1966] but corresponding errors would be within the range of experimental uncertainties because the WIND and GEOTAIL data with 1 min time resolution have been used. Although a general expectation of the magnetic clouds radially expanding from the Sun leads to the assumption of an edge nearly perpendicular to the Sun{Earth line, in our case the determined edge is declined by 31 o from the expected direction. As the disturbance is an interplanetary shock, we can determine its orientation from the magnetic eld measurements and the coplanarity theorem. Taking 5 min averages in front of and behind the shock, we obtain a 34 o angle between the shock normal and

8 8 the X GSE axis. This value is in very good agreement with the value determined from the geometrical estimation above. Table 2. Multipoint Observations of the Density Enhancement on January 11, 1997 Spacecraft GSE Coordinates a Leading Trailing X; Y; Z[R E ] edge [UT] edge [UT] WIND 93.5,-57, :3 159:3 IMP 8 b -6.5,31.2, :2 24:34 GEOTAIL -9.7,15.6, :3 28: INTERBALL -14.,14.5, :15 28:43 a Coordinates refer to the middle of the intervals. b Maximum width of the density enhancement (see caption in Figure 4). During the passage of the back edge of the magnetic cloud (January 11, 2 UT) the situation was more complicated. Figure 4 presents proton densities as observed Figure 4 by the WIND, INTERBALL{1 and IMP 8 spacecraft. IMP 8 and WIND were in the solar wind, GEOTAIL (not shown in the gure) was in the magnetosheath, and INTERBALL{1 crossed the magnetopause. The solar wind velocity, as observed by WIND, was nearly constant (41 km/s), the magnetosheath velocity as measured by the INTERBALL{1 was 29 km/s, and there was no change of the IMF at WIND in this time interval. The spacecraft coordinates and the times of observation of the edges of the density enhancement are given in Table 2. We can expect that the density enhancement propagates from WIND to IMP 8 with the solar wind velocity because there is no dierence in the velocity inside and outside the structure. The timing of observations allows determination of the angles between the edges of the disturbance and the X GSE axis (78 o for the leading edge and 48 o for the trailing one). The same result is obtained if we use the WIND and GEOTAIL

9 9 or INTERBALL{1 observations (Table 1) and the assumption that the disturbance proceeds through the magnetosheath with the velocity of the magnetosheath plasma ( 29 km/s). The fact that the edges are strongly divergent suggests a spatial limitation of the structure in the direction perpendicular to the ion ow. A simple extrapolation of the edges leads to a spatial extent of about 1 R E along the +Y GSE axis. The extent along the X GSE direction calculated from the enhancement duration varies from 24 R E at Y GSE = 59:4 R E (WIND position) to 15 R E at Y GSE =31R E (IMP 8 position). This implies that the enhancement could be shaped like awedge pointed in the +Y direction or like a bubble. On the other hand, the SOHO spacecraft, which was located Y GSE 95 R E, registered a smaller enhancement of the proton density during this event [Ipavich et al., 1997]. This suggests spatial limitation of the structure in the Y GSE direction and a closed shape for this structure. The spatial limitation of the structure indicates that it could not have its origin on the Sun because it is not bounded by magnetic barriers and thus it should disappear due to the internal thermal pressure. A possible explanation for its origin is the interaction of the magnetic cloud with the fast stream which, according to the WIND measurements, follows the magnetic cloud. 4. Summary and Conclusions Multipoint observations that are now available can provide qualitatively new information about the local geometry of magnetic clouds and on their interaction with the Earth's magnetosphere. The results of our study show that: Sharp and large changes of the solar wind dynamic pressure result in fast magnetopause motion along its normal. The multispacecraft observations of the interplanetary shock driven by the January 1, 1997 magnetic cloud can be explained if the angle between the Sun{Earth line and the projection of the shock normal onto the ecliptic plane is about 31 o. This value is consistent with the direction of the shock normal computed from the coplanarity theorem (34 o ). The observed declination of the shock normal from the Sun{Earth line is much higher than that determined for the October 18{19, 1995

10 1 magnetic cloud [Lepping et al., 1997]. The density enhancement behind the magnetic cloud (January 11, 1997) exhibits a dierent orientation at the leading and trailing edges. This indicates that the observed disturbance is probably spatially limited. Disturbance dimensions on the order of 2 R E across the Sun{Earth line could explain the fact that the density enhancement was smaller at the position of the SOHO spacecraft. The timing of the IMP 8, GEOTAIL and INTERBALL{1 observations is consistent with the expectation that the interplanetary shock onjanuary 1, 1997 as well as the density enhancement on January 11, 1997 proceeds through the magnetosheath with the velocity of the magnetosheath plasma. This result is conrmed by the analysis made in Nikolaeva et al. [1997], where the LANL{84 geosynchronous satellite data are included. Our interpretation of the event geometry is based on the assumption that the structures in the solar wind are locally planar at 1 AU on the scale of 1 R E. The location of the spacecraft allows us to investigate the geometry of the event in the ecliptic plane only, but we hope that in the future we will be able to carry out a similar tracing of other events to resolve a full 3D structure. Acknowledgments. The authors thank A. Lazarus for the WIND plasma data, and R. Lepping for the WIND magnetic eld. The present work was supported by the Czech Grant Agency under Contracts 25/96/1575, and 22/97/1122 and by the Charles University Grant Agency under Conctract No 18. G. Zastenker's work was supported by RFBR Grant The analysis of the IMP 8 and WIND data were supported by NASA contracts NAG5-584 and NAG5-2838, respectively.

11 11 References Formisano, V., Orientation and shape of the Earth's bow shock in three dimensions, Planet. Space Sci., 27, 1151, Ipavich, F. M., A. B. Galvin, P. Bochsler, H. Gruenwaldt, M. Hilchenbach, D. Hovestadt, F. Gliem, Solar wind measurements from SOHO during the CME events of early 1997, Spring Meeting of AGU, EOS Suppl., S288, 1997 Lepping, R. P., L. F. Burlaga, A. Szabo, K. W. Ogilvie, W. H. Mish, D. Vassiliadis, A. J. Lazarus, J. T. Steinberg, C. J. Farrugia, L. Janoo, and F. Mariani, The Wind magnetic cloud and events of October 18-2, 1995: Interplanetary properties and as triggers for geomagnetic activity, J. Geophys. Res., 12, 1449, Nikolaeva, N. S., G. N. Zastenker, M. N. Nozdrachev, A. A. Skalsky, N. A. Eismont, J. Safrankova, Z. Nemecek, O. Santolik, J. Steinberg, A. Lazarus, A. Szabo, R. Lepping. J.-H. Shue, J. Borovski, M. Thomsen, L. Frank, Analysis of position and motion of the magnetopause during the magnetic cloud of January 1-11, 1997, Kosmich. Issled., 1997, in press. Safrankova J., Zastenker G., Nemecek Z., Fedorov A., Simersky M., Prech L., Small scale observation of the magnetopause motion: Preliminary results of the INTERBALL project, Annales Geophysicae, 15, No.5, 562, Spreiter, J. R., A. L. Summers, and A. Y. Alksne, Hydromagnetic ow around the magnetosphere, Planet. Space Sci., 14, 223, Shue, J.-H., J. K. Chao, H. C. Fu, C. T. Russell, P. Song, K. K. Khurana, and H. J. Singer, A new functional form to study the solar wind control of the magnetopause size and shape, J. Geophys. Res., 12, 9497, J. Safrankova, Z. Nemecek, L. Prech, Faculty of Mathematics and Physics, Charles University, V Holesovickach 2, 18 Prague, Czech Republic ( safr@ aurora.troja.m.cuni.cz) G. Zastenker, N. Nikolaeva, M. Nozdrachov, A. Skalsky, Space Research Institute, Russian Academy of Science, 84/32 Profsoyuznaya, Moscow, Russia ( gzastenk@afed.iki.rssi.ru)

12 12 K.I. Paularana, Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 2139 ( T. Mukai, Institute of Space and Astronautical Science, Yoshinodai 3-1-1, Sagamihara, Kanagawa 229, Japan ( Received 1 Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic. 2 Space Research Institute, Russian Academy of Science, Moscow. 3 Center for Space Research, Massachusetts Institute of Technology, Cambridge. 4 Institute of Space and Astronautical Science, Sagamihara, Japan. To appear in the Geophysical Research Letters, 1997.

13 13 Figure 1. Positions of the GEOTAIL and INTERBALL{1 satellites during the January 1{11, 1997 magnetic cloud. The magnetopause shapes and positions are computed according to the Shue et al. [1997] model (thin line: January 1, 12 UT, p SW = 4.4 npa, B Z = 6 nt; heavy line: January 11, 122 UT, p SW = 31.9 npa, B Z = 13 nt). Open marks show the positions of all satellites on January 1, lled marks are for January 11. Figure 2. A comparison of the ion density as measured by dierent satellites on January 1, From top to bottom: the WIND solar wind dynamic pressure, the WIND, MAGION{4 and INTERBALL{1 ion density, the INTERBALL magnetic eld modulus. The WIND data are time{shifted by 17 minutes. Figure 3. Illustration of the calculated interplanetary shock geometry using the Formisano et al. [1979] bow shock model and the Shue et al. [1997] magnetopause model. Figure 4. Proton density prole at dierent locations in space. The dashed line in the IMP 8 data indicates an estimation of the maximum density based on examination of the plasma spectra and one point which could be properly analyzed.

14 8 2 INTERBALL-1 1:12 11:2 4:2 GEOTAIL 2:1 1:22 INTERBALL-1 3: :1 YGSM, Re GEOTAIL 7: 5:1 LANL-84 1:52 3:13 1: XGSM, Re Figure 1. Positions of the GEOTAIL and INTER- BALL{1 satellites during the January 1{11, 1997 magnetic cloud. The magnetopause shapes and positions are computed according to the Shue et al. [1997] model (thin line: January 1, 12 UT, psw = 4.4 npa, BZ = 6 nt; heavy line: January 11, 122 UT, psw = 31.9 npa, BZ = 13 nt). Open marks show the positions of all satellites on January 1, lled marks are for January 11. Figure 1. Positions of the GEOTAIL and INTERBALL{1 satellites during the January 1{11, 1997 magnetic cloud. The magnetopause shapes and positions are computed according to the Shue et al. [1997] model (thin line: January 1, 12 UT, psw = 4.4 npa, BZ = 6 nt; heavy line: January 11, 122 UT, psw = 31.9 npa, BZ = 13 nt). Open marks show the positions of all satellites on January 1, lled marks are for January 11. WIND, MAGION-4 & INTERBALL-1 1-Jan p WI [npa] n WI [cm -3 ] 15 1 n M4 [cm -3 ] n IB [cm -3 ] B IB [nt] :: 1:: 2:: 3:: 4:: UT (WIND +17min) Figure 2. A comparison of the ion density as measured by dierent satellites on January 1, From top to bottom: the WIND solar wind dynamic pressure, the WIND, MAGION{4 and INTERBALL{1 ion density, the INTER- BALL magnetic eld modulus. The WIND data are time{ shifted by 17 minutes. Figure 2. A comparison of the ion density as measured by dierent satellites on January 1, From top to bottom: the WIND solar wind dynamic pressure, the WIND, MAGION{4 and INTERBALL{1 ion density, the INTERBALL magnetic eld modulus. The WIND data are time{shifted by 17 minutes.

15 Y GSE [R E ] X GSE [R E ] Figure 3. Illustration of the calculated interplanetary shock geometry using the Formisano et al. [1979] bow shock model and the Shue et al. [1997] magnetopause model. Figure 3. Illustration of the calculated interplanetary shock geometry using the Formisano et al. [1979] bow shock model and the Shue et al. [1997] magnetopause model. 2 WIND, IMP-8 & INTERBALL-1 11-Jan n WI [cm -3 ] n I8 [cm -3 ] n IB [cm -3 ] 1 5 :: 1:: 2:: 3:: UT Figure 4. Proton density prole at dierent locations in space. The dashed line in the IMP 8 data indicates an estimation of the maximum density based on examination of the plasma spectra and one point which could be properly analyzed. Figure 4. Proton density prole at dierent locations in space. The dashed line in the IMP 8 data indicates an estimation of the maximum density based on examination of the plasma spectra and one point which could be properly analyzed.