FarMagnetotail Observations of an Interplanetary Shock · (nT) (km/s) -10-5 0 5 10 B ACE (nT)-100-50 0 50 100 V ACE (km/s)-200 0 200 400 600 AE (nT) 1330 1400 1430 1500 UT 5 10 15

Far Magnetotail Observations of an Interplanetary

Shock

K. Grygorov, L. Prech, J. Safrankova, and Z. Nemecek

Charles University in Prague, Faculty of Mathematics and Physics,Prague, Czech Republic.

Abstract. We present a study of the impact of the interplanetary (IP) shockon December 7, 2003 in the distant tail of the Earth’s magnetosphere. Using thedata from the several spacecraft located in the solar wind in front of the Earth, wemonitor the propagation of the IP shock from the L1 point to the magnetosphereand to magnetotail. Taking into account its velocity and beginning of the substormonset from the ground, we considered the plasma parameters on the Wind spacecraftlocated downstream at XGSM ≈ −230 RE. Shortly after the shock arrival, Windcrossed consequentially southern and northern lobes in between which observed aflux rope and the tailward fast plasma flow (≈ 780 km/s) in the plasmasheet. Wefound that a orientation of the nominal tail axis is affected by VY components ofthe solar wind. Also, according to Wind data, we present a sketch how the distanttail moves with respect to the IP shock passage.

Introduction

Interplanetary shocks are an important phenomenon in the solar wind, which can interactwith the Earth’s magnetosphere causing its compression/expansion and modifying its currentsystem. They may originate by several sources, e.g., by coronal mass ejections/solar flares andother solar transients in the solar corona and by the corotating interaction regions. Due tothe large scale of these events, interplanetary shocks are, in general, considered as a planarstructure [Russell et al., 2000]. They can be classified as fast/slow shocks or forward/reverseshocks [Burlaga, 1971] depending on changes of plasma parameters and interplanetary magneticfield strength.

One of the most typical signatures of the substorm expansion on the Earth surface is theobservation of two-dimensional magnetic field line loops referred as plasmoids [Schindler, 1974;Hones, 1977; Moldwin and Hughes, 1994; Nagai et al., 1994]. They are formed, probably, inassociation with magnetic reconnection between opposite sides of the current sheet and movein both tailward and earthward directions. A plasmoid formation and propagation scenarioin the Earth magnetotail were proposed by Baker et al. [1987, 1996] and Hones [1976, 1979].Plasmoid events are supported by high-speed plasma flows in the central plasmasheet [Moldwinand Hughes, 1992; Mukai, 1996]. Also, the helical magnetic flux ropes can be produced bymultiple X-line reconnection [Schindler, 1974; Slavin et al., 2003; Eastwood et al., 2005].

The main goal of this study is to analyze the IP shock propagation through the solar windto the Earth’s magnetosphere and to the far magnetotail. We present an example of an IPshock influence on the far magnetotail dynamics.

Observations

The near-Earth environment

On December 7, 2003, a fast forward IP shock [Berdichevsky et al., 2000] was registered bymany spacecraft located in different near-Earth locations. We trace the IP shock propagationfrom the L1 point through the solar wind (SOHO, ACE), a passage through the foreshock region(Geotail), its modification in the magnetosheath (Cluster), its impact to the Earth (geomagneticindices) and investigate following combined response of the magnetotail (Wind) on its arrival.

Fig. 1 presents locations of the spacecraft in all mentioned regions. We used Vinas and

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WDS'13 Proceedings of Contributed Papers, Part II, 20–29, 2013. ISBN 978-80-7378-251-1 © MATFYZPRESS

GRYGOROV ET AL.: THE IP SHOCKS OBSERVATION IN THE FAR MAGNETOTAIL

WINDGEOTAIL

CLUSTER

ACE

SOHO

4 S/C

Spacecraft location on December 7, 2003

200 100 0 -100 -200XGSE [Re]

100

50

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YG

SE [

Re]

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

Figure 1. Locations of the spacecraft in the solar wind and magnetosheath at time of the IPshock arrival on December 7, 2003. The black line shows a global shock orientation calculatedusing the four-spacecraft technique. The local shock orientation at WIND is marked by theblue line.

Scudder [1986] and Szabo [1994] techniques which allow us to find shock parameters (i.e., thelocal shock orientation and shock velocity) from a full set of the Rankine–Hugoniot (R–H)conditions and particular spacecraft data. The results of shock parameter calculations areshown in Fig. 1 and in Table 1 (where the coordinates of all spacecraft are also displayed).These estimations are added with calculations of the shock plane using a four-spacecraft (4 s/c)technique. For this 4 s/c method, we used data from SOHO, ACE, Geotail and Cluster-4spacecraft because they were located upstream of the Earth in the solar wind or dusk flankof the magnetosheath. The average shock parameters are: the shock speed, vSH = 430 km/s,shock normal, ~n = (−0.730;−0.450;−0.450), and the Alfvenic Mach number, MA = 5.5.

An overview of IP shock observations by ACE and Geotail in two locations in the solarwind is shown in Fig. 2 and it covers the time interval from 1320 to 1525 UT. The arrivalof the IP shock is marked by the solid line for ACE and by the red dashed line for Geotail.Abrupt changes in both interplanetary magnetic field (IMF) and plasma parameters after theshock arrival can be clearly seen in first six panels (ACE by red, Geotail by blue). Followingthe shock, the IMF BZ component becomes more negative increasing from −5 to −11 nT; theIMF BX component exhibits the same tendency, and the BY component slightly fluctuates. InFig. 2, one can see that both ACE and Geotail (despite the fact that it is located near the bowshock) observations are very similar.

In the last panels of Fig. 2, the substorm activity that accompanied the December 7, 2003event is present. A positive Sudden Impulse [Siscoe et al., 1968; Smith et al., 1986] up to 30 nT(it is marked by the green dashed line in the seventh panel) is a clear signature of the fastforward IP shock arrival registered in the SYM-H index (available at http://wdc.kugi.kyoto-u.ac.jp/). An enhancement of the Kyoto AE index from −100 up to 600 nT (which indicates amoderate storm) was detected around 1418 UT (the last panel of Fig. 2).

Four Cluster spacecraft were located near the dusk flank in the Earth’s magnetosheathprior to the shock arrival. The plasma and magnetic field measurements of Cluster-4 as arepresentative of magnetosheath observations are shown in Fig. 3. The most distinct featuresin the Cluster-4 data are two jumps of all parameters at 1421:40 UT and at 1424:20 UT (aremarked by the two dashed lines), they can be related to bow shock crossings [Safrankova et al.,2007].

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VZ

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Figure 2. Observations of the IP shock in plasme and magnetic field data on December 7,2003 by ACE (solid line) near the L1 point (the ACE data are marked by red lines in the first 4panels) and Geotail (red dashed line) near the bow shock (blue lines in the same panels). Thesubstorm onset is marked by the green dashed line.

Wind observations in the magnetotail

During the studied time interval, Wind was located near the L2 point (i.e., ≈ (−232.9,−23.4,− 13.3) RE in GSM coordinates) in the far magnetotail, on the dawn side, southward of theecliptic plane. According to “contour map of the probability of the observing magnetotail” fromMaezawa and Hori [1998], and the Wind location, the spacecraft has more than 50 % proba-bility of encountering to the geomagnetic tail in the ecliptic plane. Fig. 4 shows plasma andmagnetic field measurements in the time interval of 1340–1530 UT (from top to bottom: theplasma density (both in logarithmical and linear scales), the proton temperature, the strength

Table 1. Observational times of the IP shock in the solar wind, magnetosheath, on the Earth’ssurface and in the magnetotail (in the GSE coordinate system). Parameters of the shock arecalculated locally from R–H conditions and by using four-spacecraft technique in the solar wind.

Spacecrafts UT X(RE) Y(RE) Z(RE) Normal Vsh

ACE 13:41:27 242.1 4.4 3.5 (−0.555,−0.478, 0.679) 427SOHO 14:01:15 198.5 −79.6 8.9 — —Ground 14:13:19 — — — — —CLUSTER 14:15:56 0.8 18.8 −1.1 — —GEOTAIL 14:22:40 3.6 −24.1 5.1 (−0.737,−0.372, 0.564) 479WIND 14:50:47 −232.9 −20.4 −17.5 (−0.759,−0.465,−0.456) 4244 s/c (ASGC) — — — — (−0.631,−0.606, 0.484) 452

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Figure 3. Cluster-4 observations in the magnetosheath (MSH) and solar wind (SW). Thearrival of the IP shock is marked by the solid line and bow shock crossings are depicted by twodashed lines.

and components of the magnetic field, and components of the plasma flow velocity). The lastpanel presents the plasma beta, β (the ratio between thermal and magnetic pressures) calcu-lated using protons, electrons, and alpha-particles [Mullan and Smith, 2006] from MFI and 3DPinstruments onboard Wind. Note that all regions that Wind crossed in this time period aremarked in the figure caption.

At 1348:50 UT, Wind began to enter the south lobe and scanned the whole magnetotailuntil 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 positivein the north lobe, respectively. In fifth panel of Fig. 4, the horizontal dashed bars mark periodswhen the absolute value of BX (in GSM) is sustained weakly varying near the maximal level ofabout 8.3 nT. Periods when the spacecraft was located in the tail lobes are also confirmed by theplasma data (lower proton densities and temperatures than observed in the magnetosheath/solarwind or in the plasmasheet, and low plasma β (less than 1)). The total strength of the lobefield (8.9 nT) is in an agreement with previous statistical studies [Slavin et al., 1985; Fairfieldand Jones, 1996] at such far distances.

Between 1354:13 and 1400:21 UT, a tailward fast plasma flow with the mean velocity of770 km/s is observed in the central plasmasheet (at the same time, β rises up to 100). From1400:21 UT to 1406 UT, Wind crosses a north lobe (note that Wind is located southward ofthe ecliptic plane). In the forth panel, we list all regions the spacecraft has passed through near

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GRYGOROV ET AL.: THE IP SHOCKS OBSERVATION IN THE FAR MAGNETOTAIL

8.3 nT

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a par

amet

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Figure 4. WIND magnetic field and plasma observations in the distant magnetotail near theL2 point from 1340 to 1530 UT. Notation: SL and NL – south and north lobes, respectively; PS— central plasmasheet; BL — boundary layer; EM — Earth’s magnetotail; SW – solar wind,MSH — magnetosheath. The green dashed line depicts the substorm onset on the Earth’ssurface. The red dashed line marks the IP shock arrival.

the L2 point. We also note that Wind made the first passing through the magnetotail duringthe quite time (there is no significant geomagnetic activity in three hours prior to the IP shocksignature on the Earth’s surface at 1418 UT, which is marked by the green dashed line).

After 1406 UT, Wind passes through the mantle/boundary layer (BL) and remains in thesolar wind till the IP shock arrival to the L2 point at 1451 UT (red dashed line). At thistime, the increases of the total magnetic field and proton density were observed. The totalplasma flow velocity increases from 377 km/s to 420 km/s, i.e., the whole velocity jump equals10 %. Analysing Wind measurements, we can conclude that the IP shock was observed in themagnetosheath/boundary layer. The use of shock parameters from Table 1 leads to a predictedtime of the IP shock arrival to the Wind location around 137 s after its real observation at≈ 1451 UT.

After several minutes, Wind crosses the magnetotail totally and enters the solar wind withthe same parameters as in the downstream region as described for ACE and Geotail. The solarwind velocity and magnetic field components keep the same signs in upstream and downstreamregions. In the solar wind region, Wind stayed till 1900 UT crossing the south lobe afterwards(not shown). A zoom of the 1443–1505 UT time period in Fig. 4 is shown in Fig. 5. The orderof all panels is the same except the last but one that shows the calculated magnetic (red curve),and thermal (green curve) pressures and their sum (blue curve).

Three minutes after the IP shock passage through the Wind location, it passed throughthe inner regions of the magnetotail. A possible interpretation of this crossing in the XZ planeis shown in the sketch (Fig. 6a). The solar wind vectors in the XZ plane prior to and after the

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GRYGOROV ET AL.: THE IP SHOCKS OBSERVATION IN THE FAR MAGNETOTAIL

MSH/SW PLNL FPF SL SW

UT

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1445 1450 1455 1500

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a p

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eter

Figure 5. WIND magnetic field and plasma observations in the magnetotail near the L2 pointfrom 1443 to 1505 UT. Notation: SL and NL — south and north lobes, respectively; PS —central plasmasheet; BL — boundary layer; SW — solar wind, MSH — magnetosheath; PL —plasmoid event; FPF — the fast tailward plasma flow. The red dashed line marks the IP shockarrival. The blue dashed lines (right) indicate magnetopause crossings.

tail crossing are shown by VSW and V ′

SW, respectively. The detail corresponding plasma and

magnetic field data are shown in Fig. 6b.At 1454:04 UT, Wind begins to enter the north lobe (the interval between 1–2 points)

as the decelerated VX plasma flow component and the BX magnetic field component indicate.Between 1456:55–1457:33 UT (marked as the interval 2–3), Wind passes the plasmoid/flux rope.The signatures that we used to identify plasmoid/flux rope are similar to Borg et al. [2012]: BX

stays positive (the crossing occurs in the northern hemisphere) and changes from lower valueson the edges of the rope to the peak in its centre. The bipolar BZ signature varies from positiveto negative values with an amplitude of 8.6 nT. The maximum in the BY (GSM) componentoccurs at the center of the bipolar BZ signature and the maximum of the total magnetic fieldis 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 ≈ 2.1 RE. A time delaybetween the substorm onset at 1418 UT and a response of the tail at the Wind location was43 min. It is in a good agreement with other studies of the distant magnetotail [Slavin et al.,1984; Moldwin and Hughes, 1993]. As example, Hones et al. [1984] found a time response of 30min 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. At this plasmoid/flux rope event, a

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GRYGOROV ET AL.: THE IP SHOCKS OBSERVATION IN THE FAR MAGNETOTAIL

(a)

1 2 3 4 4’ 5 6

BZ

BY

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

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(b)

Figure 6. A possible interpretation of the distant magnetotail dynamics due to the IP shock-substorm relation between 1454 and 1502 UT: (a) detail sketch in the XZ plane, Black linesrepresent magnetic topology. (b) corresponding Wind magnetic field and plasma flow compo-nents (according to the numbers). VSW and V ′

SW— the solar wind vectors in the XZ plane

prior to and after the tail crossing, respectively

total pressure enhancement (TPE) is increased due to the enhancement of the magnetic pressure(red curve in the panel 9 in Fig. 5). The flux rope/plasmoid event also satisfies the requiredcondition of more than a 10 % enhancement of the baseline of the magnetic pressure [Ieda etal., 1998]. Besides it, the plasma beta reaches a local minimum value during this event (thelast panel in Fig. 5). The strongest magnitude of the lobe magnetic field in comparison to thequiet time crossing was detected between 1348:50–1406:11 UT. In both north and south lobes,the magnitude increased by a factor 1.2 and 1.7, respectively. It is consistent with the fact thatduring substorms a large magnetic field magnitude in the lobe was observed in association withenhancements in the solar wind pressure [Tsurutani, 1995; Kokubun et al., 1996].

After the plasmoid/flux rope event, between 1457:33–1458:41 UT (interval 3–4’), the tail-ward fast plasma flow event (FPF) occurs in the plasmasheet boundary layer (according tothe low plasma beta value). The mean plasma velocity of the FPF event is 781 km/s. Suchvelocities are typical for post-plasmoids regions. For example, Richardson et al. [1987] reportedthat the post-plasmoid plasmasheet has typical velocity of 840 km/s at a distance of ≈ 200 RE

in the distant tail. Unfortunately, there is a data gap in the magnetic field measurements inthe interval 4–4’. During this data gap, the BZ and BX components change their sign to theopposite one, i.e., Wind crosses the south lobe (between 1458:44–1459:28 UT, interval 4–5). Weattribute all these crossings to change of the plasma flow direction, because the VY componentsharply increases up to 150 km/s. After all crossings of the magnetotail, Wind entered to the

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GRYGOROV ET AL.: THE IP SHOCKS OBSERVATION IN THE FAR MAGNETOTAIL

solar wind (interval 5–6) with the parameters similar to ACE and Geotail in the downstreamshock region.

Discussion

In this study, we have shown an example of the magnetotail dynamics as a response to theIP shock arrival on December 7, 2003 between 1330 and 1500 UT.

The global IP shock parameters calculated from mutual timing of four spacecraft (the lastrow in the Table 1) assume to be the best approximation of the observed shock. Using theseparameters in the solar wind, we predicted of the IP shock arrival time to the Wind locationin the magnetotail. The prediction and Wind observations differ; Wind registers the IP shockarrival approximately 2 min 17 s after its real observation. This difference can be attributedto the slight shock deviation from the planarity at such large distance between the L1 andL2 points. To check this idea, we have calculated shock parameters from Wind (see Table 1),however, changes of Wind parameters are not so strictly accentuated as in the solar wind. Weattribute it to the Wind location in the magnetosheath boundary layer. Moreover, the Windshock normal shows a higher inclination toward duskside in the XY plane comparing to the 4s/c shock normal. The shock propagation velocity along the Wind normal is 424 km/s, thusthe shock seems to be slightly decelerated by factor 0.94 comparing to the 4 s/c method. Suchdecelerations have been already published by Koval et al. [2005, 2006] and they range from 0.82to 0.97 of a shock propagation velocity in the near-Earth magnetosheath.

In the downstream shock region, the VY component of the plasma flow grows up to−80 km/s, which indicates a dawnward expansion of the Earth’s magnetotail. Ho and Tsu-rutani [1997] have found that more than 70 % of tail crossings can be attributed to the changesof the tail size caused by the pressure balance and solar wind directional changes. Owen etal. [1995] has reported that strong and variable VY and VZ components cause the whole tailtilting. This fact has been considered as one of the main causes of moving of the tail regionswith respect to a spacecraft located inside the tail.

Summary

The unusual location of the spacecraft between the L1 and L2 points gives us the opportu-nity to obtain a very large-scale picture of the Earth’s magnetotail dynamics resulting from solarwind variations. To our knowledge, this study is the first to describe the connection betweenthe IP shock passing and the whole far tail crossings. Due to the fortunate IP shock orientationin the interplanetary medium and a good Wind spacecraft location, we were able to observethe far magnetotail under both substorm and IP shock conditions. Unfortunately, there is onlyone spacecraft in the Earth’s magnetotail. Another significant point to remember is that thespacecraft may not have observed all events we have described from start to end due to therapid tail motion back and forth. We can summarize the main results of our study as it follows:

• The fast forward IP shock on December 7, 2003 has southward IMF BZ , which causes notonly compression of the whole magnetosphere, but also triggered the moderate substormactivity on the Earth’s surface. The observed IP shock can be considered as the nearlyplanar structure at distances between L1 and L2 points. The small deceleration of theshock velocity by factor 0.94 in the far magnetosheath/boundary layer is observed.

• The orientation of the tail axis in this event was strongly affected by the downstream VY

component of the solar wind velocity. This led to a dawnward expansion of the wholemagnetotail. The VZ component in this region varied around zero in contrast to thestrongly northward increase near the L1 point. The values became equal in the solar windafter exiting far magnetotail for the second time.

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GRYGOROV ET AL.: THE IP SHOCKS OBSERVATION IN THE FAR MAGNETOTAIL

• The substorm activity leds to multiple X-line reconnection and ejection at least one plas-moid/flux rope event, surrounded by tailward streaming plasma. For the second magne-totail crossing, we made a sketch to interpret data from Wind. The time delay (≈ 48 min)between the substorm onset and spacecraft observation of the plasmoid/flux rope as wellas its parameters in the magnetotail are in a good agreement with previous studies.

• The ratio between the time of flight of the first magnetotail crossing to the second one is3.2. Thus, the IP shock can lead to constriction of the far magnetotail by an increase ofthe outside solar wind pressure. The magnetic field magnitudes in both north and southlobes increase by factor 1.2 and 1.7, respectively.

Acknowledgments. The authors thank all spacecraft teams for the magnetic field and plasmadata. The data were obtained through the CDAWeb service. The present work was supported bythe Czech Grant Agency under contract 205/09/0170. We also thank to the Grant Agency of CharlesUniversity for the support (GAUK 1096213).

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