The Relationship between the Magnetic Cloud Boundary Layer … Relationshi… · In addition to geomagnetic storms, magnetospheric substorms are another most inten-sive geomagnetic

Solar Phys (2007) 242: 167–185DOI 10.1007/s11207-007-0407-3

The Relationship between the Magnetic Cloud BoundaryLayer and the Substorm Expansion Phase

P.B. Zuo · F.S. Wei · X.S. Feng · F. Yang

Received: 18 January 2007 / Accepted: 27 May 2007 / Published online: 6 July 2007© Springer 2007

Abstract The magnetic cloud boundary layer (BL) is a disturbance structure that is locatedbetween the magnetic cloud and the ambient solar wind. In this study, we statistically an-alyze the characteristics of the magnetic field Bz component (in GSM coordinates) insidethe magnetic cloud boundary layers as well as the relationship between the magnetic cloudboundary layers and the magnetospheric substorms based on 35 typical BLs observed byWind from 1995 to 2006. It is found that the magnetic field Bz components are more turbu-lent inside the BLs than those inside the adjacent sheath regions and the magnetic clouds.The substorm onsets are identified by the auroral breakups that are the most reliable sub-storm indicators by using the Polar UVI image data. The UVI data are available only for17 BLs. The statistical analysis indicated that 9 of the 17 events triggered the substormswhen BLs crossed the magnetosphere and that the southward field in the adjacent sheathregion is a necessary condition for these triggering events. In addition, the SF-type BLs,which are named by their features of the Bz components inside the BLs and adjacent sheathregions, can easily trigger the substorms during their passage of the magnetosphere. SF-typeBLs are characterized by sustained strong southward magnetic fields persisting for at least30 minutes in the adjacent sheath regions and at least one change in the polarity of the Bz

component inside the BL. In this study, 7 out of 8 such SF-type BL events triggered the sub-storm expansion phase, suggesting that the SF-type BLs are another important interplanetarydisturbance source of substorms.

P.B. Zuo (�) · F.S. Wei · X.S. Feng · F. YangSIGMA Weather Group, State Key Laboratory for Space Weather, Center for Space Science andApplied Research, Chinese Academy of Sciences, Beijing 100080, Chinae-mail: [email protected]

P.B. Zuo · F. YangCollege of Earth Sciences, Graduate University of the Chinese Academy of Sciences, Beijing 100049,China

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1. Introduction

The magnetic cloud (MC), a term introduced by Burlaga et al. (1981) as an impor-tant solar wind transient, has been intensively investigated by many people in the pastdecade with a focus on the magnetic and plasma structures, its solar origins, and ge-omagnetic effects (e.g., Bothmer and Schwenn, 1994; Osherovich and Burlaga, 1997;Tsurutani and Gonzalez, 1997; Farrugia, Burlaga, and Lepping, 1997; Wu and Lepping,2002; Lynch, Zurbuchen, and Fisk, 2003; Crooker et al., 2004; Huttunen et al., 2005;Lepping et al., 2006). The outstanding characteristics of a magnetic cloud are enhancedmagnetic field strength, low proton temperature, low plasma β value, and a smooth ro-tation in the direction of the magnetic field (Burlaga, 1995). Some fast magnetic cloudscould drive an interplanetary shock with a speed faster than that of the MC. In addition,the magnetic clouds that do not drive shocks are usually associated with a pressure-pulsediscontinuity, i.e., a sharp rise in density or velocity. The region between the shock orpressure-pulse discontinuity and the magnetic cloud is often defined as the sheath regionwhere the plasma is compressed and heated, so is usually hot and dense and the mag-netic field is extremely turbulent (Tsurutani et al., 1988; Tsurutani and Gonzalez, 1997;Wu and Lepping, 2002). The magnetic cloud and/or the driven shock and sheath region arethe major triggers of intense geomagnetic storms (Dst < −100 nT) as the consequence ofthe long-duration and strong southward magnetic fields.

One of the unresolved questions is how to identify the boundary between the mag-netic cloud and the ambient solar wind. Many signatures have been used to define theboundary, such as a temperature decrease, a density decrease, a directional discontinu-ity, magnetic hole, a bidirectional streaming of suprathermal electrons, a deviation fromthe Maxwell distribution of the electrons, and an abrupt decrease in the intensity oflow-energy protons and plasma β (e.g., Burlaga et al., 1980; Marsden et al., 1987;Gosling et al., 1987; Osherovich et al., 1993; Farrugia et al., 1994; Tsurutani et al., 1988;Tsurutani and Gonzalez, 1997; Burlaga, 1995; Lepping et al., 1997). However, as Burlaga(1995) and Zwickl et al. (1983) indicated, there was no consistency among those various ap-proaches. The boundary of the MC is a problem related to the interaction of the MC with thesolar wind, which has been highlighted in recent years. Based on a statistical analysis of theboundary physical states of 80 magnetic clouds detected in 1969 – 2001, Wei et al. (2003a)pointed out that there exist front and tail boundary layers between ambient solar wind and themagnetic clouds and called these structures magnetic cloud boundary layers (BLs). TheseBLs could provide some clues in solving the puzzle of why the three-part structure of theCME (the bright outer loop, the dark cavity, and the filament) has not been identified byspacecraft near 1 AU. Whereas MCs are usually regarded as an interplanetary manifestationof the dark cavity material by some people (e.g., Tsurutani et al., 1988), the front boundarylayers that are located between the magnetic cloud and the sheath region could be associatedwith the outer loop of CMEs (Wei et al., 2003a). The BL could be characterized by the mag-netic signatures (the intensity drop and the abrupt directional change in azimuthal angle of�φ ∼ 180◦ as well as the latitudinal angle change of �θ ∼ 90◦ in the magnetic field) (Wei etal., 2003b) and the corresponding plasma change (relatively high proton temperature, highproton density, and high plasma β). The BL is often a non-pressure-balanced structure witha magnetic pressure decrease that is affected by certain dynamical interactions between theMC and the solar wind and is not a simple “transition layer” (Wei et al., 2006). In addition,Wei et al. (2003a) suggested that the BL might play an important role in the initiation ofcloud – magnetosphere coupling. The size of BLs is approximately 400 Earth radii on aver-age at 1 AU in terms of its typical time scale of 1.7 – 3.1 hours. When one BL crosses the

Magnetic Cloud Boundary Layers and the Substorms 169

magnetosphere, the Earth can be immersed in the layer for a few hours. Our concerns arewhat kinds of geomagnetic activity takes place and how the magnetosphere will be affectedduring the passage of BLs.

In addition to geomagnetic storms, magnetospheric substorms are another most inten-sive geomagnetic disturbance with many observational phenomena in the nightside mag-netosphere. These substorms are characterized by brilliant auroral displays, polar mag-netic bays, the disruption of the magnetotail current and the formation of the substormcurrent wedge, near-magnetotail magnetic reconnection, and magnetic field dipolariza-tion and energetic particle injections at geosynchronous orbit (e.g., Meng and Liou, 2004;Akasofu, 2004; and references therein). Whether the occurrence of substorms is sponta-neous or is triggered by abrupt changes of the external solar wind conditions is still acontroversial issue. Although there is evidence showing that substorms can occur duringstable solar wind and IMF conditions, there is more evidence indicating that substorm oc-currences are related to IMF and plasma variations (Meng and Liou, 2004). Lyons (1995)proposed that the substorm expansion phase resulted from a reduction in the large-scaleelectric field imparted to the magnetosphere from the solar wind, after a growth phase whenthe electric field enhanced for at least 30 minutes. This theory can explain the facts thatsubstorms are triggered frequently by northward-turning IMF or variations of IMF By com-ponents after a southward magnetic field persisting for at least 30 minutes (Lyons, 1996;Lyons et al., 1997). In addition, interplanetary shocks tend to cause substorms when theIMF is oriented southward prior to the arrival of the shock (Akasofu and Chao, 1980;Liou et al., 2003). As we know, the physical mechanisms of substorm triggering are notvery clear yet, and it is a common understanding that the energy of substorms results fromthe solar wind. Thus we believe that it is significant from the point of view of space weatherprediction to investigate the relationship between special structures in the solar wind – suchas corotating streams, interplanetary shocks, magnetic clouds, and the corresponding sheathregions as well as BLs – and magnetospheric substorms. In this study, 35 typical magneticcloud boundary layers detected by Wind in 1995 – 2006 are investigated. We try to find thecommon characteristics of the magnetic cloud boundary layer as well as the potential con-nections between the BLs and the substorm expansion phase.

2. Observations

2.1. A Typical Magnetic Cloud and the Corresponding Magnetic Cloud Boundary Layers

One typical magnetic cloud was detected by Wind on 23 – 25 December 1996. Figure 1shows the magnetic field and plasma characteristics and the Dst index during the passageof this magnetic cloud. The magnetic field data are obtained from Magnetic Field Inves-tigation (MFI) and the plasma data are obtained from the Solar Wind Experiment (SWE).These instruments onboard Wind are described by Lepping et al. (1995) and Ogilvie et al.(1995), respectively. Four remarkably different regions can be found in sequence: the sheathregion, the front BL, the MC body, and the tail BL. The sheath region starts with a pressure-pulse structure, which is denoted by “S” in the figure where the number density N and thedynamic pressure Pdy increase sharply and the field strength Bt decreases sharply at thesame time. Obviously this magnetic cloud belongs to “N-S” type MC characterized by Bz

northward first and then smoothly turning southward. Its trailing part triggered a weak storm(Dstmin = −33 nT) owing to the moderate southward magnetic fields (Bzmin = −7 nT) thatstarted at 05:24 UT on 25 December and persisted for nearly 6 hours. Between the MCbody and the ambient solar wind are front and tail boundary layers labeled by “Mf – Gf” and

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Figure 1 A typical magnetic cloud and the front and tail boundary layers observed by Wind on 23 – 25December 1996. The panels from top to bottom show total strength (Bt), latitudinal angle (θ ), and magneticazimuthal angle (φ) of magnetic field, proton velocity (Vp), proton temperature (Tp), number density (N ),plasma dynamic pressure (Pdy), plasma parameter β , and the Dst index. The front and tail BL are labeled byMf – Gf and Mt – Gt, respectively.

“Mt – Gt,” respectively. Inside the BLs, the total magnetic field strength decreased sharply,reaching a minimum near the center just as the magnetic field behaved in the magnetic holesand the magnetic field direction rotated about 145◦ and 176◦ through the front and tail BLs,respectively. But BLs are obviously not magnetic holes since they are not pressure-balancedstructures. These magnetic characteristics are typical of BLs that possibly formed by recon-nection, as pointed out by Wei et al. (2003a). It also can be seen that the proton temperature,number density, plasma β , and proton velocity increased considerably compared with theplasma in the adjacent sheath region and the MC body.

2.2. Analysis Methods and the Magnetic Cloud Boundary Layer Event List

Substorm onsets can be determined by many substorm-related signatures, such as auroralbreakups, the sharp decrease in polar negative magnetic bays, transient Pi2 magnetic pul-sation, dispersionless plasma injections near the inner edge of the plasma sheet, and the


Figure 2 The magnetic field and plasma data and the Sym-H index detected simultaneously when the BLwas observed by Wind on 30 September 2002. The BL is the region labeled by Mf – Gf. The arrow shows thetime of SI of the geomagnetic field caused by the compression of the front boundary of the BL.

intensification of the auroral kilometric radiation (Liou et al., 1999). In previous studies, thepolar negative bay, which represents an auroral westward electrojet AL index, is typicallyregarded as the main proxy for a substorm. But in recent years, it has been found that sub-storms are not a sole cause of the bay signature since the enhancement of convection andthe compression of the magnetosphere can also drive the magnetic bay (Meng and Liou,2004). Like polar negative bays, none of the other onset signatures have a one-to-one cor-respondence with an auroral breakup, especially during geomagnetic active periods (Mengand Liou, 2004). For these reasons, Meng and Liou (2004) pointed out that other commonlyused onset proxies including the negative bay may not always be associated with substorms,and the auroral breakups were the primary and most reliable onset indicators. The Ultravi-olet Imager (UVI) onboard the Polar satellite, which was launched on 24 February 1996, iscapable of taking a 2D image every ∼37 s with typical 30 – 40-km spatial resolution whenPolar was positioned at its apogee of ∼9 RE (Torr et al., 1995). The large number of re-turned auroral images are suitable and have been widely used to study aurora and substormphenomena. In the present study, auroral breakups are visually identified using the UVIimages on the basis of the classical auroral substorm scheme (Akasofu, 1964): a suddenbrightening of auroras in the nightside sector of the auroral oval followed by a subsequentrapid poleward and azimuthal expansion.

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Figure 3 The representation of each type of front BL: (A) 3 April 1995 event for N type; (B) 18 September1997 event for S type; (C) 24 December 1996 event for F type. The BLs are labeled by Mf – Gf.

Based on the MC events identified by Lepping et al. by using about 12 years’ of Winddata (1995 – 2006) (http://lepmfi.gsfc.nasa.gov/mfi/mag_cloud_pub1.html), 35 typical frontBLs and 10 tail BLs were identified with the BL concept and identification criteria (Wei etal., 2003a). Then we checked the Polar UVI images obtained during the intervals when BLscrossed the magnetosphere to judge whether or not a substorm was triggered and determineits onset time. It is unfortunate that the UVI data are available only for 18 events: 17 frontBLs and 1 tail BLs. As the tail BL events are rare and the UVI data are unavailable for majortail BLs, we include only the 35 front BLs in this study.

To discuss the relationship between the change of solar wind plasma and IMF and sub-storms triggering, the transit time for solar wind propagating from solar wind monitors to themagnetopause must be estimated. As applied by many people, it can be simply calculated byusing the measured solar wind velocity and the distance in the direction of the GSM X axisbetween the spacecraft and the nose of magnetopause (X = 10 RE, Y = 0). However, thismethod will yield a timing error of at least as much as 10 – 20 minutes (Ridley, 2000) sincethe subpolar position is fixed and the deceleration of the solar wind as it crosses the bowshock is neglected. In this study, we take the Wind spacecraft as the monitor of solar wind.To our knowledge, the front boundary of major BLs coincides with a pressure pulse. Whenthe magnetosphere lacks geomagnetic storm activity (Dst > −30 nT), this pressure pulsecan usually cause a sudden impulse (SI) in the geomagnetic field when the front boundaryof BL passes the magnetosphere. Figure 2 shows an example of such an event, where the


Table 1 The BL list with the observations by Wind and Polar UVI.

No. BL (UT) MC Type Transit time UVI

(minutes)

1 8 February 1995 02:52 – 03:23 8 – 9 February 1995 FF 27 Unavailable

2 4 March 1995 10:54 – 11:42 4 – 5 March 1995 NF 70 Unavailable

3 3 April 1995 06:29 – 07:43 3 – 4 April 1995 NN 62 Unavailable

4 18 October 1995 18:20 – 19:01 18 – 20 October 1995 FF 40 Unavailable

5 16 December 1995 03:01 – 04:49 16 December 1995 SF 8 Unavailable

6 27 May 1996 12:10 – 14:50 27 – 29 May 1996 SF 30 Unavailable

7 24 December 1996 01:01 – 03:02 24 – 25 December 1996 NF 25 No substorm

8 21 April 1997 10:16 – 12:05 21 – 23 April 1997 SF 54 Substorm

9 15 May 1997 07:32 – 09:51 15 – 16 May 1997 SF 37 Substorm

10 15 July 1997 04:45 – 06:12 15 July 1997 FF 13 No substorm

11 3 August 1997 10:05 – 13:51 3 – 4 August 1997 SF 35 Substorm

12 18 September 1997 02:55 – 03:52 18 – 20 September 1997 SS 21 No substorm

13 21 September 1997 21:40 – 22:14 21 – 22 September 1997 SF 17 Unavailable

14 7 November 1997 04:36 – 05:28 7 – 8 November 1997 FF 28 Substorm

15 22 November 1997 11:20 – 13:50 22 – 23 November 1997 SF 36 Substorm

16 6 January 1998 22:06 – 24:00 7 – 8 January 1998 SF 47 Substorm

17 4 February 1998 02:20 – 05:00 4 – 5 February 1998 FF 50 Unavailable

18 2 June 1998 08:18 – 10:29 2 June 1998 FF 40 No substorm

19 20 August 1998 04:51 – 09:12 20 – 21 August 1998 NF 6 No substorm

20 19 October 1998 03:58 – 04:21 19 October 1998 SF 16 Unavailable

21 18 February 1999 11:11 – 12:18 18 – 19 February 1999 NF −2 No substorm

22 9 August 1999 04:55 – 10:20 9 – 10 August 1999 SS 21 Substorm

23 21 February 2000 01:55 – 05:25 21 – 22 February 2000 FF 35 No substorm

24 3 October 2000 16:34 – 17:18 3 – 4 October 2000 NF −5 Unavailable

25∗ 6 November 2000 17:40 – 22:25 6 – 7 November 2000 SF Substorm

26 31 October 2001 17:03 – 21:21 31 October 2001 – 2November 2001

SF 1 Substorm

27 19 May 2002 02:46 – 03:20 19 May 2002 FF 22 Unavailable

28 1 August 2002 11:19 – 11:45 1 August 2002 SF −9 Unavailable

29 30 September 2002 19:06 – 21:25 30 September 2002 – 1October 2002

NF 24 Unavailable

30 20 March 2003 12:00 – 12:48 20 March 2003 FF 16 Unavailable

31 22 July 2004 12:58 – 13:54 22 July 2004 NF 51 Unavailable

32 9 November 2004 18:25 – 20:30 9 – 10 November 2004 SF 23 Substorm

33 20 May 2005 06:04 – 06:46 20 – 21 May 2005 SF 25 Unavailable

34 12 June 2005 14:41 – 15:02 12 – 13 June 2005 NF 56 Unavailable

35 31 December 2005 12:33 – 13:48 31 December 2005 – 1January 2006

NF 54 Unavailable

* For this event, the Sym-H data are unavailable, so the transit time cannot be calculated using our method.

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Figure 4 (A) Panels from top to bottom are the time-shifted total magnetic field strength (Bt), Bz compo-nent, proton temperature (Tp), number density (N ), and auroral westward electrojet indices (AU, AL, andAE). The BL is labeled by Mf – Gf. The arrow marks the onset time of the polar negative bay. (B) A sequenceof auroral images in the LBHL band from the Polar UVI taken when the BL crossed the magnetosphere.Auroral breakup occurred at ∼05:04 UT.

panels from top to bottom show the intensity and two directional angles of magnetic field,the solar wind speed, proton temperature, the number density, the dynamic pressure, andthe Sym-H index. The Sym-H index measures the mean longitudinally symmetric compo-nent of the magnetic disturbances and can be used as a high-resolution Dst index (James


Figure 4 (Continued).

and Kristin, 2006). Here we use the Sym-H index to search the SI caused by the dynamicpressure pulse. This front BL was observed by Wind from 19:06 UT to 21:25 UT on 30September 2002. As seen from Figure 2, the dynamic pressure jumped sharply from 5 to9.5 nPa across the front boundary, and the corresponding SI occurred at 19:30 UT owingto the compression of the magnetosphere. So if we do not consider the reaction time of themagnetosphere to the pressure pulse, which can be reasonably neglected, the time difference

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Figure 5 The same format as Figure 4 for the 21 April 1997 event. The auroral images were taken in theLBHS band. Auroral breakup occurred at ∼12:29 UT.

can be taken as the transit time of this BL from Wind to the magnetopause. This method isexpected to reduce the uncertainty considerably. As we know, a pressure-pulse discontinuityincluding the driven shocks is always in front of the sheath region. This pressure pulse canalways cause a SI. By using the property of the good one-to-one correspondence betweenthem, the propagation of the pressure pulse structure from Wind to the magnetopause canbe ascertained. The front boundary of some BLs do not cause a SI when the magnetosphere



exhibits geomagnetic storm activity. To such events, we take the transit time of the pressurepulse in front of the sheath region as that of the BLs.

Inside the BLs, the magnetic field Bz polarity has three states: entirely southward (Stype), entirely northward (N type), and bipolar (with the polarity changing at least once orBz fluctuating around the zero point; F type). For these 35 BLs, 2 events belong to the Ntype, 1 belongs to the S type, and 32 belong to F type. Figure 3 shows the representation of

178 P.B. Zuo et al.

each type where the magnetic-hole-like structures are given for the sign of BLs. Notice thatthe Bz component was more turbulent inside each type of BL than that inside the MC andthe adjacent sheath region. Also, we divide the state of the adjacent sheath region, whichcan be represented by the region 30 minutes in front of the front boundary of BLs, into threetypes, labeled N, S, and F, respectively, in terms of the same criteria. Finally, according tothe state of the adjacent sheath region and the BL itself, the BLs can be divided into ninetypes. For example, an SF-type BL means that the state of the adjacent sheath region is Sand the state of the BL is F.

Table 1 presents the corresponding information of all discussed BLs (35 events), includ-ing the time observed by Wind of the BLs and the corresponding magnetic clouds, the typeof BLs, the transit time for BLs from Wind to the magnetopause, and whether the substormtriggered. Among the 17 BLs for which the polar UVI data were available, 9 BLs triggeredsubstorms. For event No. 11, the substorm onset was triggered 7 minutes before the BLpassed the magnetosphere. In the following we will first discuss two triggering events indetail.

2.3. Two Potential Substorm-Triggering Events

One potentially triggering event was detected by Wind on 7 November 1997. Figure 4Ashows the magnetic field, plasma data, and the auroral westward electrojet AU, AL, andAE indices. The auroral westward electrojet indices are provided by the World Data Centerfor Geomagnetism at Kyoto University, Japan. The solar wind data have been shifted by 28minutes to account for the transit time for the BL from Wind to the magnetopause. Inside theBL, the magnetic field intensity decreased and rotated nearly 180◦, and the number densityand dynamic pressure notably increased. These are the fundamental characteristics of BLs.At 05:04 UT, when the front boundary of the BL crossed the magnetopause, the AL indexdecreased sharply from ∼−300 to ∼−1300 nT with the AE index increasing from ∼500to ∼1400 nT simultaneously. Then the indices kept a higher level for about 1 hour. Thisfeature of the magnetic bay is typical of the magnetic substorm expansion phase. The PolarUVI images, which are obtained using the Lyman-Birge-Hopfield long (LBHL) wavelengthfilter, are shown in Figure 4B. Each image is shown in the geomagnetic local time coordinatesystem with magnetic local noon on the top and dawn on the right. Although higher timeresolution data are available, we show images at nearly 3 minutes cadence to conserve space.A sudden auroral brightening occurred at ∼05:04 UT, at 60 – 62 MLAT, and near the localmidnight in the region of 2100 – 2400 MLT, which is the typical substorm expansion phaseonset location (Elphinstone et al., 1995). Then the aurora expanded poleward, westward,and eastward until ∼05:32 UT. Between 03:00 UT and 06:30 UT there is only one auroralsubstorm occurring, and the substorm expansion phase finished after the BL crossed themagnetosphere. So we inferred that the substorm was triggered by this BL.

Another BL event potentially triggering a substorm was detected by Wind on 21 April1997. Figure 5 shows the time-shifted solar wind data (the shifted time is 54 minutes),the auroral westward electrojet indices, and the Polar UVI images with the same formatas Figure 4. Before the BL crossed the magnetosphere, the AL index was very weak (largerthan −100 nT), which indicated that geomagnetic activity was low. When the front boundarylabeled Mf arrived at the magnetopause, the compression of the magnetosphere caused bythe sharply increased dynamic pressure triggered a negative bay. But at that time, auroralactivity was very low. Figure 5B shows that a sudden auroral brightening in the nightsidesector of the oval appeared at ∼11:55 UT. But this auroral breakup did not expand polarwardand azimuthally and there was no AL index disturbance associated with this auroral breakup.


Table 2 The substorm-triggering BL list.

BL (UT) Type Auroral breakup (UT) Bay onset (UT)

21 April 1997 10:16 – 12:05 SF 12:29 12:31

15 May 1997 07:32 – 09:51 SF 09:49

7 November 1997 04:36 – 05:28 FF 05:04 05:04

22 November 1997 11:20 – 13:50 SF 11:57, 14:08 11:57

6 January 1998 22:06 – 24:00 SF 23:43 23:48

9 August 1999 04:55 – 10:20 SS 10:16 10:17

6 November 2000 17:40 – 22:25 SF 21:32, 21:53, 22:20 21:40

31 October 2001 17:03 – 21:21 SF 20:12 20:18

9 November 2004 18:25 – 20:30 SF 19:45 19:47

So it should be classified as a pseudo-breakup. At ∼12:29, an auroral substorm was triggeredwith a clear auroral breakup at ∼63 MLAT and 01 – 02 MLT. Correspondingly, the AL indexdecreased sharply from ∼−100 to ∼−400 nT and the AE index sharply increased from∼150 to ∼450 nT at 12:31 UT. From the auroral substorm and the magnetic bay, we canconclude that the expansion phase and the recovery phase finished during the interval whenthe BL crossed the magnetopause. The direct IMF condition for this substorm-triggeringevent is the BL event.

2.4. One Quiescent Event: The BL Observed on 20 August 1998

On 20 August 1998, Wind detected a typical NF-type BL (see Figure 6). Inside the BL, thepolarity of the magnetic field Bz component changed many times. After 6 minutes, the BLpropagated from Wind to the magnetopause and interacted with the magnetosphere. It canbe seen from Figure 6 that when this BL crossed the magnetosphere, the AE index increasedslowly and kept a lower level. Furthermore, auroral activity was also low, especially in thenightside auroral oval, and there was no clear auroral breakup occurring, indicating that thisNF-type BL did not trigger a substorm.

2.5. Statistical Results and Discussion

Table 2 shows the nine triggering events. The four columns in sequence stand for the de-tected time and the type of BLs, the time of auroral breakup, and bay onset. Among thetriggering BLs, seven belong to the SF type, one belongs to the SS type, and one belongs tothe FF type. When the state of the adjacent sheath region is N, none of the BLs are associ-ated with substorms. This means that a southward field in the adjacent sheath is a necessarycondition for BLs triggering a substorm. In addition, there are 8 SF events in all for the17 BLs for which the polar UVI data are available, and 7 events trigger substorms. Theprobability for SF events triggering substorms is much higher. Then we examine remainingSF-type BL. This event was detected by Wind on 3 August 1997. The plasma and mag-netic field characteristics and the magnetospheric response are presented in Figure 7, wherethe solar wind data have been shifted by 35 minutes. It can be seen that the magnetic fieldBz component inside the BL fluctuated around zero and the magnetic field in the adjacentsheath was southward (about −4.5 nT on average) with the southward field lasting for ∼5

180 P.B. Zuo et al.

Figure 6 The same format as Figure 4 for the 20 August 1998 event. The auroral images were taken in theLBHL band.

hours. The AL index in front of the BL was very weak before the BL crossed the magne-tosphere. At 10:43 UT when the front boundary of the BL arrived at the magnetopause, theAL index decreased sharply from ∼−100 to ∼−500 nT with the AE index increasing from∼200 to ∼800 nT. As can be seen from Figure 7B, a sudden auroral substorm onset wastriggered at ∼10:33 UT, when the auroral breakup appeared at ∼65 MLAT, and near local



midnight in the region of 2100 – 2200 MLT. This substorm had been triggered before the BLarrived at the magnetopause, and the substorm was in the substorm expansion phase whenthe BL crossed the magnetosphere. The investigated SF-type BLs are all associated with thesubstorm expansion phase. From this discussion, we can conclude that the SF-type BLs areanother important interplanetary disturbance source for triggering substorms.

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Figure 7 The same format as Figure 4 for the 3 August 1997 event. The auroral images were taken in theLBHL band. Auroral breakup occurred at ∼10:33 UT.

3. Summary and Conclusions

It has been suggested that Bz is one of the most important solar wind parameters in con-trolling geomagnetic activity. In terms of the fact that many substorms have proven to betriggered by northward-turning IMF or southward-turning IMF after a sustained southwardmagnetic field in the solar wind, Lyons proposed that the expansion phase of a substorm



resulted from a reduction in the large-scale electric field imparted to the magnetospherefrom the solar wind (Lyons, 1995). This theory suggests that substorms can be triggered byIMF variations. However, as far as we know, not all IMF variations (including northward-and southward-turning Bz and a polarity change of the IMF By component) can trigger asubstorm. One open question involves the conditions under which the IMF variation can

184 P.B. Zuo et al.

trigger a substorm. This study concerned only the relationship between the BLs and the sub-storms. From our analysis, the Bz component of the magnetic field strongly fluctuates andthe polarity may change several times inside the BLs. Before a magnetic cloud crosses themagnetosphere, the adjacent sheath region will first interact with the magnetopause. When asouthward magnetic field is sustained in the adjacent sheath, the magnetic flux and the solarwind particles can easily enter the magnetosphere and will be stored in the tail lobes throughlarge-scale magnetospheric convection or magnetic reconnection. Because of the turbulenceand polarity changes of the Bz component inside BLs, the convection electric field willchange promptly when the BLs cross the magnetosphere. Thus in terms of Lyons’ substormtheory, this kind of magnetic structure probably can easily trigger a substorm. In the presentwork, the fact that all the SF-type BLs are associated with the substorm expansion phase canprove this idea.

This work reveals that BLs are important structures. The SF-type BLs are another impor-tant interplanetary disturbance source for triggering substroms. Magnetic cloud boundarylayers are complicated interaction regions between the magnetic cloud and ambient solarwind. When BLs interact with the magnetosphere, the state of the magnetosphere will bestrongly affected. Thus when we study the geomagnetic activity of a MC body, the contri-bution of BLs must be also considered. This will be our next consideration.

Acknowledgements We would like to thank NASA CDAWEB for providing the public Wind MFI andSWE data and Polar UVI data and the Kyoto University World Data Center C2 for providing the auro-ral electrojet indices, Dst indices, and Sym-H indices. We are also grateful to the referees for their helpfulcomments. This work is jointly supported by the National Natural Science Foundation of China (40621003,40336053, 40536029, and 40523006), 973 Program under Grant No. 2006CB806304, and the CAS Interna-tional Partnership Program for Creative Research Teams.

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