Pages 725-728

BOW SHOCK EXPANSION CAUSED BY THE MAGNETIC CLOUD PASSAGE ON OCT. 18, 1995

K.Tsubouchi1, T.Terasawa1, N.Shimada1, T.Mukai2, Y.Saito2, T.Yamamoto2, A.Nishida2, S.Machida3, S.Kokubun4, H.Matsumoto5, H.Kojima5, A.J.Lazarus6, and R.P.Lepping7

1 Dept. of Earth and Planetary Physics, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, JAPAN,   E-mail: tsubo@grl.s.u-tokyo.ac.jp
2 ISAS, Sagamihara, Kanagawa 229, JAPAN
3 Dept. of Earth and Planetary Science, Kyoto University, Sakyo-ku, Kyoto 606, JAPAN
4 STEL, Nagoya University, Toyokawa, Aichi 442, JAPAN
5RASC, Kyoto University, Uji, Kyoto 611, JAPAN
6 Center for Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
7 Laboratory for Extraterrestrial Physics, NASA/GSFC, Greenbelt, MD 20771, USA

ABSTRACT

During Oct. 18-20, 1995, the WIND and GEOTAIL spacecraft recorded passage of a large magnetic cloud. Soon after entering into the cloud, plasma/field/wave measurements on GEOTAIL showed that the earth's bow shock appeared about 10 Re upstream from the nominal position ((9, -23, -2) Re in GSE). This shock expansion were apparently triggered by the arrival of the magnetic cloud: A high Alfvén velocity within the magnetic cloud, which was due to the combination of high magnetic field intensity and low plasma density, made the magnetosonic Mach number of the bow shock small, and in turn resulted in the rapid outward expansion of the bow shock. We discuss the transient phenomena caused by this rapid expansion and sequential bow shock crossings associated with the solar wind condition inside this magnetic cloud. INTRODUCTION The shape and size of the bow shock has been investigated statistically (e.g., Fairfield, 1971), and compared with a theoretical prediction (e.g., Spreiter et al., 1966). These studies proved that the shape and size of the bow shock is governed by the upstream solar wind condition, specifically the dynamic pressure and the Mach number (sound, Alfvén, or magnetosonic one). In the present paper, we have particular interest in a dynamic variation of a bow shock in response to the solar wind disturbance. A magnetic cloud released by the sun, which is defined by relatively strong magnetic fields with a smooth rotation of the magnetic field direction and a low proton  and temperature (Burlaga et al., 1981), is a good example of the disturbance in the solar wind. Lepping et al. (1996) recently reported the interaction of the magnetic cloud and the bow shock on Feb. 8, 1995 by using WIND and IMP-8 data. They showed that the bow shock is expanding outward during the passage of the cloud because of the low Alfven Mach number inside the cloud. Examples that the low Mach number solar wind makes the distant appearance of the bow shock have also been examined by Russell and Zhang (1992) and Farris and Russell (1994).

In our present study, we examine another magnetic cloud - bow shock interaction phenomenon during Oct. 18 - 19, 1995, which was chosen as a part of the first IACG campaign period. We utilize WIND plasma and magnetic field and GEOTAIL magnetic field data. We describe the bow shock motion and its shape during the cloud passage. Our results complement those of Lepping et al. in that we obtain information on the site where the cloud - bow shock interaction started: GEOTAIL path covered the dawn side to the near subsolar region, while IMP-8 path was around the dusk side of the Earth (Lepping et al. 1996). Lepping et al. found the "isotropic" expansion of the bow shock, while our result shows another type of expansion, the bow shock blunting. Time resolutions of WIND and GEOTAIL data used in this study are 90 seconds and 3 seconds, respectively. Coordinate system used is GSE (Geocentric Solar Ecliptic) throughout the paper.

OBSERVATION

Plasma density and magnetic field data observed from WIND spacecraft during Oct. 18 through 19 are shown in the upper panel of Figure 1. The magnetic cloud passage can be definitely seen from 19:01 UT on Oct. 18 to the end of Oct. 19 (the trailing boundary of the magnetic cloud is not so distinct though): At 19:01 UT on Oct. 18, when WIND was located at (175.2, -3.7, -13.3) Re, the orientation of the magnetic field abruptly changed southward and started to rotate towards north gradually. During the passage of this structure, the magnetic field strength was relatively large and nearly constant with quite low density inside (Burlaga et al., 1981). The interplanetary shock at 11:25 UT on Oct. 18 (Terasawa et al., 1996) is driven by this magnetic cloud (Gosling, 1993; Gosling et al., 1994).

Fig. 1. (Upper): ion density (cm-3 (; the magnitude (|B|, nT), the elevation (,  deg.), and the azimuth (, deg.) of the magnetic field from WIND observation during Oct. 18 to 19. Two vertical solid lines represent the extent of the magnetic cloud. (Lower): the magnetic field data (magnitude, elevation, and azimuth) obtained by GEOTAIL, simultaneously with the period in the upper panel. I, IIa and IIb marked above the figure represent the set of the bow shock crossing refered in the text.

Simultaneous GEOTAIL data of the magnetic field are represented in the lower panel of Figure 1. The arrival time of the magnetic cloud at the GEOTAIL site ((9.3, -22.8, -1.6) Re) was 19:45 UT on Oct. 18; about 45 minutes after the WIND observation, indicating ~410km/s of the solar wind velocity which is well consistent with the observation. Variation of the field directions is very well associated with the characteristic of the magnetic cloud found in the preceding WIND observation. |B| profile in the lower panel of Figure 1, on the other hand, shows that GEOTAIL made a lot of bow shock crossings throughout the cloud passage on its site. First inbound bow shock crossing was occurred at 19:50 UT on Oct. 18, which was just 5 minutes after the cloud arrival. This bow shock position was ~10 Re upstream from the nominal position (e.g. the model by Fairfield (1971)). In Figure 2, several physical parameters obtained from WIND results are plotted. As a typical feature of a magnetic cloud, the plasma  and the Alfvén Mach number MA suddenly drop, showing the dominance of the magnetic pressure inside the cloud (Burlaga et al., 1981). The statistical study by Peredo et al. (1995) showed that the shock flanks widen outward in response to decreasing MA. Indeed, the GEOTAIL shock crossing occured 2.1 Re inside the bow shock surface for MA = 2.01 expected from the model by Peredo et al. (1995).

Fig. 2. Physical parameters calculated from WIND data from Oct. 18 to 19; plasma  , Alfven Mach number MA, dynamic pressure mnV2 (nPa), and ion thermal pressure in addition to the total (thermal + magnetic) pressure (nPa). Two vertical solid lines represent the extent of the magnetic cloud.

From sequential multiple bow shock crossings, we can analyze the motion of the bow shock interacting with the magnetic cloud. We divide these shock crossing into two sets: Set I consists of the first seven examples (19:50 - 22:10 UT on Oct. 18) and set II (on Oct. 19) is further divided into two subsets, set IIa (seven examples during 7:55 - 10:52 UT) and set IIb (eleven examples during 13:27 - 16:30 UT). In Figure 3, we show the projection of the shock normal vectors on the GSE x-y plane (3(a) for the set I shock normals and 3(b) for the set II). The solid curve represents the nominal bow shock shape by Fairfield (1971). ave denotes the azimuthal projection angle between the normal vector of the observed shock and of the expected nominal shock on that location ("isotropically" expanded Fairfield model as Lepping et al. (1996) referred), which is averaged in each set of the shock crossings (*-marked shocks, 21:19 and 22:02 UT on Oct. 18, are excluded for the averaging because of their irregularity). In most of the shock normal configurations presented here, the azimuthal component is dominant in comparison with the elevation one, so that the value of ave indicates how the shock deviates from the ordinary shape. Large ave} (29.6 ° ) in set I suggests that the bow shock is bluntly expanded, while in set IIa and IIb the shock returns to its ordinary shape and size (Figure 3(b), ave=6.5 ° 2.9 ° . The preceding solar wind condition is well matched with these bow shock crossings: During the period of set I and IIa, the WIND observation shows the local peaks of the solar wind dynamic pressure mnv2 , while set IIb interval corresponds with the gradual increase of mnv2 into the background solar wind level (Figure 2). It is likely that these variations of the solar wind dynamic pressure directly drive the back- and forward motion of the bow shock.

Fig. 3. The projection of the bow shock normal vectors on the GSE x-y plane. The footpoint of each arrow represents the GEOTAIL observing site ((a) for the set I shock crossing, (b) for the set II). Direction of each arrow depicts in- or outbound motion of the bow shock. Dotted curve is scaled to fit for the observed shock site from the nominal shock model by Fairfield (1971) (solid curve). Gray curve in Figure 3(a) and 3(b) illustrates the shock shape expected from the observation.

DISCUSSION A magnetic cloud, which was observed first by WIND about 180 Re distant from the Earth and 45 minutes later by GEOTAIL 10 Re upstream from the nominal bow shock position on Oct. 18, 1995, has an ordinary structure of the magnetic field lines: helical rotation and relatively large magnitude. During the passage of this cloud for nearly 30 hours, GEOTAIL crossed the Earth's bow shock many times as it traced the orbit from the dawn side upstream into the near-subsolar point. We have reported the distant appearance of the bow shock, compared with the nominal position, from GEOTAIL observation after entering into this magnetic cloud. Similar results, the bow shock excursion driven by the magnetic cloud, was recently reported by Lepping et al. (1996) for the event Feb. 8, 1995. However, there is one distinct difference between these studies. Lepping et al. showed that the shock normals were approximately constant throughout the cloud passage (referring to `isotropic expansion of BS'). On the other hand, in our results, the time sequence of the shock normal vectors pointed more anti-sunward than the expected one in the early stage of the cloud passage; the shock shape becomes blunter as was expected in the condition of decreasing MA} (Cairns et al., 1995; Peredo et al., 1995). The difference of the bow shock expansion sense refered above seems to be attributed to the difference of the observing positions between IMP-8 and GEOTAIL; the orbit of IMP-8 was nearly along the shock normal at the dusk side (Figure 1 of Lepping et al. (1996)), while GEOTAIL skimmed the shock surface approximately from dawn side to the subsolar point.

Further collaborative works will be expected to provide us more detailed mechanism on the bow shock motion interacted with the inner structure of the magnetic cloud, which is intrinsically important to comprehend the formation of lower Mach number bow shocks.

ACKNOWLEDGEMENT

The authors are grateful to members of each of the GEOTAIL MGF, LEP, PWI and the WIND MFI, SWE teams. One of the authors (K. T.) was supported by JSPS Research Fellowships for Young Scientsts.

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