The far magnetotail response to an interplanetary shock arrival

Author's Accepted Manuscript

The far magnetotail response to an interplanetary shock arrival K. Grygorov, L. Přech, J. Šafránková, Z. Němeček, O. Goncharov

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S0032-0633(14)00222-0 http://dx.doi.org/10.1016/j.pss.2014.07.016 PSS3792

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Planetary and Space Science

Received date: 25 February 2014 Revised date: 25 June 2014 Accepted date: 28 July 2014 Cite this article as: K. Grygorov, L. Přech, J. Šafránková, Z. Němeček, O. Goncharov, The far magnetotail response to an interplanetary shock arrival, Planetary and Space Science, http://dx.doi.org/10.1016/j.pss.2014.07.016 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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The far magnetotail response to an interplanetary shock arrival ˇ ankov´ K. Grygorov, L. Pˇrech, J. Safr´ a , Z. Nˇemeˇcek, O. Goncharov Charles University in Prague, Faculty of Mathematics and Physics, V Holeˇ soviˇ ck´ ach 2, 180 00 Prague 8, Czech Republic.

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Abstract We present a study of the impact of the December 7, 2003 fast forward interplanetary (IP) shock on the distant tail of the Earth’s magnetosphere. Using the data from the several spacecraft located in the solar wind/magnetosheath upstream the Earth, we monitor a propagation of the IP shock from the L1 point to the magnetosphere. A behavior of the far magnetotail is inferred from the Wind observations at XGSM ≈ −230 RE . Shortly after the shock arrival, Wind crossed consequentially southern and northern lobes and observed a flux rope and the tailward fast plasma flow (≈ 780 km/s) within the plasmasheet. Moreover, a change of the solar wind VZ component across the shock creates a huge kink of the tail magnetosphere that propagates down the tail with the IP shock.

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Keywords: interplanetary shocks, substorms, reconnection, far magnetotail, flux ropes, plasmoid PACS: 94.30.C-, 94.30.Lr, 95.05.Sd, 94.30.cl 1. Introduction Interplanetary (IP) shocks are a frequent and important phenomenon in the solar wind. Fast forward IP shocks and the enhanced plasma densities downstream of them (Kennel et al., 1985) compress the magnetosphere when they impact it (Tsurutani et al., 1988) and this compression causes an intensification and inward motion of the Chapman-Ferraro magnetopause currents resulting in a sudden positive variation of the horizontal component of the low-latitude geomagnetic field. These ground-based features are called Sudden Impulses (SIs) (Siscoe et al., 1968; Smith et al., 1986). The shocks have various sources like coronal mass ejections (CMEs) or corotating interaction regions (CIRs) and other transients in the solar corona. Due to the large scale of these events, IP shocks are usually considered as planar structures (Russell et al., 2000). They are characterized by abrupt changes of plasma parameters and the interplanetary magnetic field (IMF) strength and direction. According to changes of these parameters (Burlaga, 1971), shocks can ˇ ankov´ Email address: [email protected] (J. Safr´ a) Preprint submitted to Planetary and Space Science

August 6, 2014

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be classified as fast/slow or forward/reverse. The angle between the shock normal and the magnetic field direction divides IP shocks into quasi-perpendicular or quasi-parallel. However, being caused by various sources, IP shocks differ by physical properties, consequently, they induce a variety of geomagnetic effects on the Earth. These effects have been extensively studied by many authors (e.g., Gosling et al., 1991; Jurac et al., 2002; Echer et al., 2006, 2011; Alves et al., 2011) and it was found that more than 21% of the shocks were followed by strong (DST < -100 nT) geomagnetic storms. The lobe magnetic field magnitude exhibits a step-like increase due to the IP shock passage through the tail (e.g., Kawano et al., 1992; Tsurutani and Zhou, 2003; Kim et al., 2004). Based on OGO observations in the near-tail region (X ≈ −20 RE ), Sugiura et al. (1968) suggested that the impact of an IP shock on the magnetopause nose would launch fast mode waves that would propagate down the tail much faster than the IP shock in the ambient solar wind. Collier et al. (1998) analyzed IMP 8 measurements in the magnetosphere complemented with the Wind upstream monitoring of two IP shocks. The authors concluded that the observations are consistent with a simple model of a stepwise compression caused by the increase of the ambient pressure behind the shock. They have shown that the magnetotail maintains an approximate MHD equilibrium even as it responds rapidly to interplanetary pressure discontinuities due to fast mode speed in the lobes being much larger than the IP shock speed in the solar wind. Huttunen et al. (2005) interpreted the Cluster data and they have shown that the time delay between the IP shock arrival to the magnetopause nose and sudden increase of the lobe magnetic field is consistent with the IP shock propagation speed rather than with the fast mode speed in lobes. An increase of the solar wind dynamic pressure can significantly (by ≈ 30% at X = −15 RE ) reduce dimensions of the lobe (Zhou and Tsurutani, 2002) and can lead to the increase of the number density and temperature in the Earth’s magnetotail (Borovsky et al., 1998). Zhou et al. (2013) have shown a tail current sheet thickness decrease by ∼ 80% caused by two factors: compressional waves generated around the whole magnetosphere and an increase of the solar wind dynamic pressure after the IP shock passage. Substorms are transient magnetospheric processes where the solar windmagnetosphere interaction energy is dissipated mainly at the nightside auroral ionosphere (Akasofu, 1981). The features associated with IP shock triggering of substorms discuss many papers (e.g., Baker et al., 1987; Kokubun et al., 1977, 1996; Zhou and Tsurutani, 2002; Tsurutani and Zhou, 2003; Echer et al., 2011; Liou et al., 2003). It has been found that the southward IMF and magnetic reconnection faciliate the energy transfer necessary for substorm trigger (Akasofu, 1981). Nevertheless, sudden solar wind dynamic pressure pulses such as fast forward IP shocks have also been noted to trigger intense substorms that occur minutes after shock impingement on the magnetosphere. In this case, the preconditioning of the magnetosphere is an important factor (Alves et al., 2011; Echer et al., 2011). One of the most typical signatures of the substorm is the observations of magnetic field line loops referred as plasmoids (Schindler, 1974; Moldwin and 2

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Hughes, 1994). They are probably formed by reconnection of the magnetic field on opposite sides of the current sheet and they can move in both tailward and earthward directions. A plasmoid formation and propagation scenario in the Earth magnetotail were proposed by (e.g., Hones, 1976, 1979; Baker et al., 1987, 1996). Plasmoid events are embedded in high-speed plasma flows in the central plasmasheet (Moldwin and Hughes, 1992; Mukai, 1996). According to Geotail plasma flow measurements (Fairfield et al., 1998), their velocities can exceed 2000 km/s with peaks lasting about 10 s (Baumjohann et al., 1990). Also, the three-dimensional helical magnetic flux ropes (with a rotation from the axial magnetic field in the center to the azimuthal direction at the edge) can be produced by multiple X-line reconnection (Schindler, 1974; Hughes and Sibeck, 1987; Slavin et al., 2003; Eastwood et al., 2005). A passage of the flux rope through the Earth’s magnetotail disturbs the ambient magnetic field causing a compression of both north and south lobes. The regions of the enhanced magnetic field known as TCRs (Travelling Compression Regions) can be identified by a bipolar variation of the BZ magnetic field component, by an enhancement of the magnetic field magnitude and by an absence of the plasma fast flow (Slavin et al., 1984, 1993). Kiehas et al. (2009, 2012) have shown the direct relation between TCRs and a presence of flux ropes in the middle tail. According to several statistical studies, the distant neutral line (DNL) of the Earth’s magnetotail is located at XGSM ∼ −100 RE (e.g., Zwickl et al., 1984; Slavin et al., 1985; Richardson et al., 1987a; Moldwin and Hughes, 1992). Plasmoids are considered as a dominant substorm signature beyond this distance (Nagai et al., 1994). Their typical tailward velocities are several hundreds of km/s (Richardson et al., 1987b; Ieda et al., 1998). The relationship between a substorm expansion and magnetopause motion in the far tail was suggested by Sauvaud et al. (1996). The authors have presented a pass of the ISEE-3 spacecraft from the distant magnetosheath into magnetosphere at XGSM  205 RE due to the plasmoid ejection from the near-Earth plasmasheet. The opposite motion of the magnetopause was detected by Sauvaud et al. (2011) and they explained shrinking of the far tail at 255 RE shortly (14 minutes) after a very weak substorm activity by a large-scale wave propagating inside the lobe. In this study, we analyze the IP shock propagation through the solar wind to the Earth’s magnetosphere and to the tail. Based on observations of the spacecraft located in well separated places of the geospace, we present an example of the magnetotail deformation as a response to the IP shock motion along the tail.

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2. Observations

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On December 7, 2003, a fast forward IP shock was registered by many spacecraft in different locations around the Earth. We trace the IP shock propagation from the L1 point through the solar wind (SOHO, ACE) and the foreshock (Geotail), its modification in the magnetosheath (Cluster), its impact on the Earth (geomagnetic indices) and we investigate a complicated response of the magnetotail on its arrival to the Wind position (−230 RE ). 3

Spacecraft location on December 7, 2003 -100 SOHO

-50

YGSE [Re]

GEOTAIL

WIND

4 S/C

0 ACE CLUSTER

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100 200

100

XGSE0 [Re]

-100

-200

Mar 23 2013 kostia C:\Users\kostia\Desktop\Advanced\for paper\orbit.ps

Figure 1: Spacecraft locations at the time of the IP shock arrival on December 7, 2003. The black line shows a global shock orientation calculated using the 4 s/c method. The local shock orientation at WIND is marked by the blue line.

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2.1. Overview of the event Fig. 1 presents locations of the spacecraft in mentioned regions. We used Vinas and Scudder (1986) and Szabo (1994) techniques to find shock parameters (i.e., the local shock orientation and shock velocity) from a full set of the RankineHugoniot (R-H) conditions and spacecraft data. The results of shock parameter calculations are shown in Table 1 (together with the spacecraft coordinates). The local estimations are complemented with calculations of the shock plane using a four-spacecraft (4 s/c) technique. For this method, we use times of the shock arrival to SOHO, ACE, Geotail, and Cluster-4 because they were located upstream of the Earth in the solar wind or dusk flank magnetosheath. The deceleration of the shock in the magnetosheath (Koval et al., 2005) was omitted. As it can be seen from the table, all methods provide similar results. However, the 4 s/c method reflects a large scale shock geometry and thus we use these shock parameters for a prediction of the IP shock propagation down the tail. The average shock parameters are: the shock speed in the spacecraft frame, vSH ≈ 450 km/s, shock normal, n = (−0.63; −0.61; 0.48), and the Alfv´enic Mach number, MA = 5.5. An overview of IP shock observations at two locations in the solar wind 4

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(ACE and Geotail) is shown in Fig. 2 that covers the time interval from 1320 to 1520 UT. The arrival of the IP shock is marked by the red vertical line for ACE and by the blue line for Geotail. Abrupt changes of both interplanetary magnetic field (IMF) and plasma parameters after the shock arrival can be clearly seen. The IMF BZ was directed mostly southward during a hour prior to the shock arrival with the mean value of −3 nT. Following the shock, the IMF BZ component becomes more negative (-5 to -11 nT); the IMF BX component exhibits the same tendency, and the BY component slightly fluctuates. As a summary, the magnetic field magnitude as well as all components are enhanced in the downstream region but we would like to stress out the behavior of the plasma velocity. The VZ component is about 20 km/s prior to the shock but it reaches a value of 100 km/s within several minutes after the shock arrival and exceeds 50 km/s for a following 40 minutes. Similar (but negative) values are reached by the VY component. We can conclude that the solar wind flow downstream the shock is highly deflected; the angle between pre- and post-shock velocity is as high as 15o . Last but not least, we would like to note that, despite the fact that Geotail was located near the bow shock, the observations of Geotail and ACE are very similar. In the last two panels of Fig. 2, the substorm activity that accompanied the December 7, 2003 event is presented. A positive SI up to 20 nT (it is marked by the green dashed line) is a clear signature of the fast forward IP shock arrival registered in the SYM-H index (available at http://wdc.kugi.kyoto-u.ac.jp/). An enhancement of the AE index from -100 up to 600 nT (which indicates a moderate storm) was detected between 1420 and 1430 UT. Although Geotail was located at +4 RE , the enhanced geomagnetic activity precedes the shock arrival into its location. This effect would be connected with a slight inclination of the shock normal from the Sun-Earth’s line (see Table 1 and Fig. 1). To check this suggestion, we present Cluster observations at the dusk flank magnetosheath. The plasma and magnetic field parameters measured by Cluster-4 as a representative of the Cluster fleet are shown in Fig. 3. The IP shock registration at 1416 UT, i.e., prior to the geomagnetic activity enhancement, is distinguished with the black vertical line. One can note two steps in the density and magnetic field profiles resulting from the IP ˇ ankov´ shock interaction with the bow shock (Samsonov et al., 2006; Safr´ a et al., 2007) but the most distinct features in the Cluster-4 data are two jumps of all parameters at 1421:40 and at 1424:20 UT marked by the two dashed lines. They are related to outward/inward bow shock crossings that often follow IP shock ˇ ankov´ observations in the magnetosheath (Safr´ a et al., 2007). 2.2. Wind observations in the magnetotail Wind was located near the L2 point ((−232.9, −23.4, −13.3) RE in GSM coordinates) at the dawn side southward of the ecliptic plane during the studied time interval. According to the ”contour map of the probability of the observing magnetotail” from Maezawa and Hori (1998), and the Wind location, the spacecraft has more than 50 % probability of encountering the geomagnetic tail. Fig. 4 shows Wind plasma and magnetic field measurements in the time 5

TP (eV)

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AE (nT)

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UT

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Figure 2: Observations of the December 7, 2003 IP shock. Top four panels show the density, temperature, and VX component of the velocity and magnetic field magnitude as measured by ACE (red) and Geotail (blue). The shock arrival to a particular location is denoted by the vertical line of a corresponding color. The next two panels present VY and VZ velocity and all magnetic field components (ACE). The AE and SYM-H indices can be found in the last two panels. The substorm onset is marked by the green dashed line.

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CLUSTER-4 14:00-14:40 7/12/2003

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UT Figure 3: Cluster-4 observations in the magnetosheath (MSH) and solar wind (SW). The arrival of the IP shock is marked by the solid line and two bow shock crossings are depicted by two dashed black lines.

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Table 1: Observational times of the IP shock in the solar wind, magnetosheath, on the Earth’s surface and in the magnetotail (in the GSE coordinate system). Parameters of the shock are calculated locally from R-H conditions for each spacecraft and using the 4 s/c technique.

Spacecrafts ACE SOHO Ground CLUSTER GEOTAIL WIND 4 s/c (ASGC)

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UT 13:41:27 14:01:15 14:13:19 14:15:56 14:22:40 14:50:47 -

X(RE ) 242.1 198.5 0.8 3.6 -232.9 -

Y(RE ) 4.4 -79.6 18.8 -24.1 -20.4 -

Z(RE ) 3.5 8.9 -1.1 5.1 -17.5 -

Normal (−0.56, (−0.74, (−0.76, (−0.63,

−0.48, 0.68)

−0.37, 0.56) −0.47, −0.46) −0.61, 0.48)

interval of 1340–1530 UT. We can see from top to bottom: the plasma density, the proton temperature, the strength and components of the magnetic field, and components of the plasma velocity. The last but one panel shows thermal, magnetic, and total pressures and the last panel presents the plasma beta, β (a ratio between the thermal and magnetic pressures) calculated using protons, electrons, and alpha-particles (Mullan and Smith, 2006) from MFI and 3DP instruments onboard Wind (Lepping et al., 1995; Lin et al., 1995). All regions that Wind visited in this time period are marked in the magnetic field magnitude panel. Note that the region marked as SW can be either a pure solar wind or magnetosheath (or boundary layer) because it is very difficult or even impossible to distinguish these regions in such distances. At 1348:50 UT, Wind entered the south lobe and scanned the whole magnetotail until 1406:11 UT. The tail lobes are characterized by the high and steady magnetic field strength, and the BX component has a large, low-variance negative value in the south lobe and positive one in the north lobe. To distinguish the lobe intervals, the typical lobe BX value of 8.3 nT is shown by the horizontal dashed line in the fifth panel of Fig. 4. The periods when the spacecraft was located in tail lobes are also confirmed by the plasma data (lower proton densities and temperatures than observed in the magnetosheath/solar wind or in the plasmasheet, and plasma β < 1). The total strength of the lobe magnetic field (8.9 nT) is in an agreement with previous statistical studies (Slavin et al., 1985; Fairfield and Jones, 1996) at such far distances. Between 1354:13 and 1400:21 UT, a tailward fast plasma flow with a mean velocity of 770 km/s is observed in the central plasmasheet (at the same time, β rises up to 100). This flow velocity is high but a statistical study of Slavin et al. (1985) has shown that the velocities in the range of 700–1000 km/s can be observed for 10 − 15% of time at these distances. From 1400:21 UT to 1406 UT, Wind crosses a north lobe despite that it is located southward of the ecliptic plane. This magnetotail crossing occurred during the quiet time (there was no notable geomagnetic activity within three hours prior to the IP shock signature on the Earth’s surface at 1418 UT which is marked by the green dashed line). Note that all regions the spacecraft has passed are marked in the fourth panel of Fig. 4. 8

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WIND 13:40-15:30, 7/12/2003

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Figure 4: WIND magnetic field and plasma observations in the magnetotail from 1340 to 1530 UT. Abbreviation: SL and NL – south and north lobes, respectively; PS – central plasmasheet; BL – boundary layer; EM – crossing of the magnetotail; SW - solar wind; MSH – magnetosheath. The green dashed line presents the substorm onset on the Earth’s surface, the red dashed line marks the IP shock arrival, and black lines separate particular regions.

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B

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VYACE Z (km/s)

VY Z (km/s)

VX(km/s)

BY Z (nT)

BX (nT)

BTOTAL (nT)

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1500

UT Figure 5: WIND magnetic field and plasma observations from 1443 to 1505 UT. Abbreviation: SL and NL – south and north lobes, respectively; PS – central plasmasheet; BL – boundary layer; SW – solar wind; MSH – magnetosheath; PL – plasmoid; FPF – the fast tailward plasma flow. The red dashed line marks the IP shock arrival and two blue dashed lines indicate magnetopause crossings.

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After 1406 UT, Wind passes through the mantle/boundary layer (BL) and remains in the solar wind till the IP shock arrival to its location at ≈ 1451 UT (red dashed line). The shock signatures are: the increases of the total magnetic field and proton density. The total plasma flow speed increases from 377 to 420 km/s, i.e., the velocity jump exceeds 15 %. Analyzing Wind measurements, we can conclude that the IP shock was observed in the magnetosheath because the velocity values are lower than those recorded by ACE (Fig. 2) but the relative jump is about the same. The shock parameters from Table 1 lead to a predicted time of the IP shock arrival to Wind around 137 s after its real observation at ≈ 1451 UT. Taking into account that the last reference point (Geotail) is separated by more than 230 RE , we think that this agreement is excellent. Several minutes after, Wind observes large fluctuations of all parameters that last for ≈ 5 minutes and it enters the solar wind with the same parameters as those measured by ACE (Geotail) downstream of the IP shock after 1510 UT. The solar wind velocity and magnetic field components keep the same signs in both upstream and downstream regions. Wind remained in the solar wind till 1900 UT when it enters the south lobe (not shown). The series of fluctuations prior to 1500 UT is marked as a crossing of the whole magnetotail (marked as EM in Fig. 4). In order to show that this interpretation is supported with observations, Fig. 5 presents a blow-up that covers the 1443–1505 UT time period. The order of all panels is the same as that in Fig. 4 except the second one from the bottom where the VY and VZ velocity components from ACE shifted to the Wind IP shock time are shown for reference. The visited regions are again indicated in the magnetic field panel. Wind started in the solar wind (magnetosheath) and observed the IP shock at 1451 UT. One can note a similar behavior of the VY and VZ components measured by Wind and those propagated from ACE. It is true for the VY component with an exception of large fluctuations of all Wind parameters between ≈1457 and 1459 UT. On the other hand, the VZ component stays near zero until 1457 UT but it reaches the ACE value after 1459 UT. We will return to this point later. 2.3. The tail activity from detailed Wind measurements Two minutes between ≈1457 and 1459 UT are denoted in Fig. 5 as plasmoid (PL) and fast plasma flow (FPF) observations. To illustrate this interpretation, we present a detail of Wind measurements between 1454 and 1502 UT in Fig. 6a and a corresponding sketch of a probable scenario in Fig. 6b. At 1454:04 UT, Wind enters the north lobe (the interval between 1-2 points) as the BX magnetic field component indicates. Between 1456:55-1457:33 UT (marked as the interval 2-3), Wind passes the plasmoid/flux rope. The signatures that we used to identification of the plasmoid/flux rope are similar to Borg et al. (2012): BX stays positive (the crossing occurs in the northern hemisphere) and changes from lower values on the edges of the rope to the peak in its center. The bipolar BZ signature varies from positive to negative values with an amplitude of 8.6 nT. The maximum of the BY component occurs at 11

WIND 14:54:00-15:02:00, 7/12/2003

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1

BTOTAL (nT)

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2

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10 5

0 15

BX (nT)

10 5 0 -5

-10 -15 10

BY Z (nT)

BZ

5 0 -5 B Y

-10 0

VX (km/s)

-200 -400 -600 -800

VY Z (km/s)

-1000 200 100 VZ

0

-100 V Y -200

1454

1456

1458

1500 (a)

(b) Figure 6: A possible interpretation of the magnetotail motion during the Wind passage around a plasmoid. (a) Wind magnetic field and velocity measurements focused on the time interval of 1452 and 1502 UT. Vertical black lines separate particular regions under discussion; (b)  a sketch of the magnetotail cross-section in the XZ plane. VSW and VSW – the solar wind velocity vectors in the XZ plane prior to and after the tail crossing, respectively. Black lines represent magnetic field lines and the numbered red line shows the spacecraft trajectory. The numbers in both parts of Fig. 6 correspond to each other.

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the center of the bipolar BZ signature and the maximum of the total magnetic field is also observed (marked by the black dashed line). According to the flow velocity (352 km/s) and the duration of its observations (38 s), the flux rope diameter was estimated as ≈ 2.1 RE . A time delay between the substorm onset at 1418 UT and a response of the tail at the Wind location was 43 minutes. It is consistent with other studies of the distant magnetotail (Slavin et al., 1984; Moldwin and Hughes, 1993). As an example, we can note that Hones et al. (1984) found a time response of 30 minutes to reach ISEE-3 at X ≈ −220 RE . Additionally, the basic quantities of plasmoids between −100 > XGSM ≥ −210 obtained by Ieda et al. (1998), such as the VX velocity (−590 ± 240 km/s), the magnetic field magnitude (5 ± 2 nT), the number density (0.11 ± 0.1 cm−3 ) and duration (1.8 ± 0.7 min) are also in a good agreement with our event. The flux rope/plasmoid event satisfies the required condition of more than 10 % enhancement of the magnetic pressure (Ieda et al., 1998). Beside it, the plasma β reaches a local minimum during this event (the last panel in Fig. 5). It is interesting to note that the observed plasmoid characteristics match well those reported in the Jovian magnetosphere (Vogt et al., 2014). The strongest magnitude of the lobe magnetic field in a comparison with the previous tail crossing was detected between 1348:50-1406:11 UT. In both north and south lobes, this magnitude is increased by a factor of 1.2 and 1.7, respectively. It is consistent with a fact that large magnetic field magnitudes in the lobe were observed in association with the enhancements in the solar wind pressure (Tsurutani, 1995; Kokubun et al., 1996) during substorms. After the plasmoid/flux rope, between 1457:33-1458:41 UT (interval 3–4’), the tailward fast plasma flow (FPF) occurs in the plasmasheet boundary layer (according to the low plasma β value). The average plasma velocity of FPF is 781 km/s and such velocity is typical for post-plasmoids regions. For example, Richardson et al. (1987b) reported that the post-plasmoid plasmasheet exhibits a typical velocity of 840 km/s at the distance of ≈ 200 RE in the tail. Unfortunately, there is a data gap in the magnetic field measurements in the interval 4–4’. During this data gap, the BZ and BX components change their sign to the opposite one, i.e., Wind entered the south lobe (between 1458:44 and 1459:28 UT, interval 4’–5). After the magnetotail crossing, Wind exited to the solar wind (interval 5–6) and registered the parameters similar to ACE and Geotail located upstream the Earth. One would expect a region occupied by bent detached magnetic field lines just behind the plasmoid at these distances. However, the crossing of this region (4–4’) corresponds to the gap in the magnetic field data, thus we do not speculate about the magnetic field orientation. Fig. 6b shows a sketch of a possible tail geometry with an embedded plasmoid that is consistent with discussed observations in Fig. 6a. The apparent Wind trajectory across this structure is shown by the red line. The arrows at upper and lower parts of the figure show the solar wind directions upstream (VSW ) and  downstream (VSW ) of the IP shock. Although it is expected that the magnetic field at the distant magnetotail would be aligned with the solar wind velocity, the sketch reveals a large angle (≈ 30o ) between the lobe magnetic field and solar wind velocity vectors. 13

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3. Discussion In this study, we have shown an example of the magnetotail motion (deformation) as a response to the fast forward IP shock arrival. We have chosen the IP shock observed on December 7, 2003 by many spacecraft located in different regions between L1 and L2 points. The global IP shock parameters calculated from mutual timing of four spacecraft (the last row in Table 1) are assumed to be the best approximation of the observed shock geometry. Using these parameters in the solar wind, we predicted of the IP shock arrival time to the Wind location in the magnetotail. The prediction and Wind observations differ approximately by 137 s. This difference can be probably attributed to a slight shock deviation from the planarity over such a large distance but it cannot influence our analysis. We have calculated shock parameters from Wind (see Table 1), however, changes of Wind parameters across the shock are not so distinct as those in the solar wind because Wind is located in the magnetosheath. Moreover, the Wind shock normal shows a higher inclination toward dusk in the XY plane comparing to the 4 s/c shock normal. The shock propagation velocity along the Wind normal is 424 km/s, thus the shock seems to be slightly decelerated by factor of 0.94 comparing with the 4 s/c method. Such shock deceleration in the magnetosheath has been already published by Koval et al. (2005, 2006) and it ranges from 0.82 to 0.97 of a shock propagation velocity in the near-Earth magnetosheath. The shock observation later than expected from a constant propagation speed in the tail is thus consistent with its deceleration in the magnetosheath. The IMF BZ component prior to the shock points southward and it becomes even larger downstream the shock. Such IMF orientation is favourable to initiate the substorms and indeed, a sudden impulse caused by the magnetospheric compression was followed by a relatively large (AE∼ 600 nT) substorm. This activity leads to multiple X-line reconnection and an ejection of at least one plasmoid/flux rope event surrounded by the tailward streaming plasma that was observed by Wind during its crossing of the tail from the solar wind, through the north lobe, plasmasheet, and south lobe to the solar wind southward of the tail. We present two Wind tail crossings in Fig. 4. First of them at ≈ 14 UT occurred prior to the IP shock arrival to the Earth and is probably caused by small fluctuations of the solar wind velocity direction (see Fig. 2). The total duration of this crossing was ≈30 minutes. By contrast, the second tail crossing that follows the IP shock arrival (after 1451 UT) lasted only ≈10 minutes, despite the fact that a plasmoid embedded into the plasmasheet would result in the tail thickening. It suggests much faster motion of the whole tail in the +Z direction. This motion is consistent with a sharp increase of the VZ component across the IP shock that was registered by ACE. However, the comparison of the velocity directions determined from ACE and Wind measurements (Fig. 5) reveals that whereas the Wind VY component even quantitatively follows that of ACE, the Wind VZ component stays near zero until the tail crossing. After it, both measurements of the perpendicular velocity components are nearly 14

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identical. We assume that a reason is that the magnetospheric tail serves as an obstacle to the deflected solar wind flow and only slowly adapts to a new velocity direction. This suggestion is schematically shown in Fig. 7. The blue solid line shows a average magnetopause location prior to the IP shock arrival. The curved green line demonstrates a compression and global deflection of the magnetopause after the IP shock passage. The red arrow indicates an apparent Wind trajectory across the tail. Prior to the IP shock, the magnetospheric tail is aligned with the current solar wind velocity (blue arrow). Increased speed and density (Fig. 4) behind the IP shock compress the magnetosphere and the affected part of the tail shifts to a new position given by the solar wind direction downstream the shock. Since tail magnetic field lines should be continuous, the process leads to a huge deformation of the whole tail. At ≈1500 UT, Wind crosses the tail due to tailward propagation of this large kink. Note that the angle between the equilibrium tail orientation in the XZ plane prior to and after the IP shock arrival is larger than 30o . This interpretation can explain a large angle between the solar wind direction and lobe magnetic field lines shown in Fig. 6b as well as the slow increase of the Wind VZ component behind the shock. This scenario is consistent with previous studies because Ho and Tsurutani (1997) have found that more than 70 % of tail crossings can be attributed to the variations of the tail size caused by the pressure balance or to solar wind direction changes. Owen et al. (1995) have reported that strong and variable VY and VZ components cause the whole tail tilting. These effects have been considered as the main causes of a motion of the tail regions with respect to a spacecraft located inside. However, these studies compared the tail motion with smooth changes of the solar wind parameters and thus the speed of the tail motion needed to its adjustment to new conditions was low in a comparison with the Alfv´en speed. On the other hand, the abrupt turn of the solar wind direction across the IP shock leads to transient effects analyzed in the present study. The ratio of Wind times of flight across the magnetotail in the first and second cases shown in Fig. 4 is 3.2. The tail thickness would be depressed due to the pressure enhancement following the IP shock but, on the other hand, it would be enlarged by the plasmoid propagation. We think that the main reason of the short time of the second tail crossing is a kink propagating down the tail with the IP shock speed.

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4. Conclusion

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The fortunate location of many spacecraft between the L1 and L2 points on December 7, 2003 gives us the opportunity to study the effects of a strong fast forward IP shock on the Earth’s magnetotail. We have observed all expected features of the IP shock interaction with the magnetosphere. We can summarize that: (1) The interaction of the IP shock with the dayside bow shock leads to its deceleration and results into a pair of bow shock crossings observed by Cluster. (2) A similar (by a factor of 0.94) deceleration was observed even in the far magnetotail. (3) A compression of the magnetosphere results in the sudden 15

Figure 7: A sketch of a deformation of the far magnetotail caused by the IP shock. The blue solid and green dashed lines represent the magnetotail positions prior to and after the IP shock passage (dashed black line).

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impulse. (4) Probably due to prolonged southward IMF orientation prior to the shock arrival, the substorm is initiated. (5) The substorm activity leads to multiple X-line reconnection and ejection at least one plasmoid/flux rope event surrounded by tailward streaming plasma. All these features were observed by Wind at about 230 RE downstream the Earth. We have contrasted two Wind crossings of the whole vertical tail structure. Although the far tail is expected to be aligned with the solar wind flow, we have shown that a change of the solar wind direction across the IP shock results in a huge deformation of the whole tail that propagates downstream with the IP shock speed.

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Acknowledgements

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The authors thank all spacecraft teams for the magnetic field and plasma data. The data were obtained through the CDAWeb service. The present work was supported by the Czech Grant Agency under contract P209/13/22367J. O. Goncharov and K. Grygorov also thank to the Grant Agency of Charles University for the support (GAUK 1096213).

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