Correlation between continuous lobe reconnection in the mid magnetotail and substorm expansion onset

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www.scichina.com www.springerlink.com 2795

Chinese Science Bulletin 2006 Vol. 51 No. 22 2795—2804 DOI: 10.1007/s11434-006-2198-6

Correlation between continuous lobe reconnection in the mid magnetotail and substorm expansion onset ZHANG Hui1, PU Zuyin1,2 , CAO Xin1, FU Suiyan1, XIAO Chijie3, LIU Zhenxing2, A. Korth4, M. Frazen4, ZONG Qiugong5, H. Reme6, K. H. Glassmeier7, R. Friedel8, G. D. Reeves8 & M. W. Dunlop9

1. School of Earth and Space Sciences, Peking University, Beijing 100871, China;

2. Center for Space Science and Applied Research, Chinese Academy of Sciences, Beijing 100080, China;

3. National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100081, China;

4. Max-Planck-Institute for Solar System Research, Katlenbury-Lindau, 37191, Germany;

5. Center for Atmospheric Research, UML, Lowell MA01854, USA; 6. CESR/CNRS, Toulouse F-31028, France; 7. IGM, Braunschweig D-38106, Germany; 8. Los Alamos National Laboratory, Los Alamos NM 87545, USA; 9. Rutherford Appleton Laboratory, Chilton OX110 OX, UK Correspondence should be addressed to Pu Zuyin (email: zypu@ pku.edu.cn) Received June 28, 2005; accepted August 31, 2005

Abstract Data on plasma sheet crossing meas-ured by Cluster/HIA and Cluster/FGM during the pe-riod from July to October in 2001―2003 are analyzed. Based on previous work on the characteristic fea-tures of continuous lobe reconnection (CLR) de-scribed in reference, two case studies and a statisti-cal analysis were carried out on correlation between CLR in the mid magnetotail and substorm expansion onset for the events occurring during this period. It is found that almost all CLR events are in close con-nection with substorms. The beginning of CLR is al-most always a few minutes ahead of substorm activi-ties seen in the near Earth magnetotail and on the ground-based stations. This provides a clear indica-tion that CLR is the virtual cause of substorm expan-sion onset during the period of continuous southward interplanetary magnetic field. Keywords: lobe reconnection, magnetospheric substorm, high-speed flow, substorm onset.

The process responsible for substorm expansion on-set is one of the hottest topics intensively studied in the

solar-terrestrial community. There are numbers of mod-els concerning the physical mechanism of expansion onset which has not been clearly understood yet. The most well-known and competitive models are the near Earth neutral line (NENL) model[1,2] and near Earth current disruption (NECD) model[3]. In the NENL model magnetic reconnection (MR) is thought to be the virtual cause for substorm expansion onset. The high speed flow ejected by MR carries energy from the MR region to the substorm trigger region and finally trig-gers substorm onset in the inner magnetotail. On the other hand, in the view of NECD advocators, the cross-tail current instability causes the current disrup-tion triggering the expansion phase; meanwhile a rare-factive wave excites in the trigger region, henceforth propagates tailward and induces MR in more distant tail. To resolve this controversy and get insight of the underlying physics, the precise timing of MR in the mid magnetotail and substorm onset in the near-Earth tail becomes the key issue, since the timing provides an indication of their causality if they are related.

Burst Bulk Flow (BBF) is a signature of MR in the magnetotail[4,5]. During the period from 2001 to 2003, Cluster observed more than 300 BBF events when the constellation was traversing through the plasma sheet in the mid tail. Substorms did not always occur during BBF events, and vice versa; BBFs could be observed before or after substorm onsets. It seems that the sub-storm processes are so complicated that one should not try to explain the expansion onset using a single model. It is likely that there are different substorms with dif-ferent underlying mechanisms. Alternatively, that en-ergy stored in the near-Earth magnetotail can explo-sively release due to some instabilities, leading to cur-rent disruption and dipolarization at substorm onset[3], is the possible mechanism. In addition, the northward turning of the interplanetary magnetic field (IMF) also seems a cause of some substorm expansion. Neverthe-less, the opinion is prevailing, that MR is the primary mechanism to release energy in the tail and that the BBF deriving from MR carries a great amount of the energy into the inner magnetotail, which are of crucial importance for the ultimate onset of substorm expan-sion phase.

As mentioned by Cao et al.[6], during the continuous southward IMF, a great amount of energy is stored in the magnetotail due to the significantly enhanced con-vection in the tail, and MR comes as the times required to release surplus energy and magnetic flux into the

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inner tail. The magnetic field in the lobe region, as well as that in the plasma sheet, will be involved in MR process during continuous southward IMF, and so plasma sheet MR evolutes into CLR. CLR shows the following characteristic features in observations: 1) typical signatures of MR near the X-line, such as si-multaneous reversals of Bz and Vx; 2) plasma bubbles with low density and temperature, manifesting that the plasma sheet is significantly thin and that magnetic field and plasma of lobe region have be involved; 3) earthward/tailward convective high-speed flows ex-ceeding 400 km/s, which carry a great amount of mag-netic flux out of the MR site; 4) enhanced temperature (even higher than that before CLR) and lower density in the end of CLR event. More energy is supposed to be ejected into inner region since CLRs typically occur during continuous southward IMF, and hence it tends to cause more intensive substorms; that is, if MR proceeds just inside the plasma sheet, it can cause only small substorms or pseudo-breakups, or even nothing, be-cause of comparably less energy released than CLR does.

magnetopause. Since the velocity of solar wind ob-served by Geotail was −360 km/s, it will take about 1.03 min from

xv ≈x = 13.5 RE to x = 10 RE, i.e., to

the average location of the magnetopause. Thus all data should be shifted forward about 1.03 min if using the Geotail data as the monitor of the interplanetary condi-tion just outside the magnetopause. Fig. 1 shows IMF kept southward for several hours up to 1420UT with its minimum of −15 nT. This set the required condition for CLR[6].

Fig. 2 presents the observatory structure of CLR on October 14, 2002. Before 1405UT, Cluster/SC1 was located in the plasma sheet. At 1405UT, a tailward convective high-speed flow was observed, implying that a MR was going on in the central plasma sheet and neutral line lay inside SC1. The decreasing density and temperature indicates the plasma might flow in from the outer plasma sheet. The lobe characteristics were detected between 1424UT and 1440UT with the very low density and temperature, we refer to these observa-tions as the CLR bubble region[6], and so MR devel-oped into CLR. 25 min later the temperature and den-sity recovered; the temperature was higher than that before CLR, while the density came to be still lower than that before CLR. An earthward high-speed flow was observed when SC1 returned back into the plasma sheet earthward the neutral line. The similar results from SC3 and SC4 confirm these analyses. Note that SC3 was closest to the central region of the plasma sheet, so the duration of CLR bubble observed by SC3 was shortest, and what is more, at 1425UT SC3 ob-served a convective earthward high-speed flow while the others did not.

From 2001 to 2003, Cluster crossed the plasma sheet more than 100 times, with its apogee being 19.6 RE in the mid tail. 37 CLR events are selected and studied[6]. The main point of the present paper is to investigate the correlation between the 37 CLRs and the corresponding substorm onsets.

1 Case study of the timing of CLR and substorm expansion onset

1.1 The event of October 14, 2002

From 1100UT to 1500UT on October 14, 2002, Geo-tail was located at (13.5, 14.1, −5.2) RE, just outside

Fig. 1. The Bz component of the IMF outside the magnetopause during the event of October 14, 2002.

2796 Chinese Science Bulletin Vol. 51 No. 22 November 2006

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Fig. 2. Features of CLR observed by Cluster/SC1. From top to bottom are: the magnetic field, the perpendicular velocity and Vx, the number density, the temperature. Before 1405UT, Cluster/SC1 was located in the plasma sheet. CLR started at 1405UT (the first vertical line). Until 1452UT, SC1 came back into plasma sheet (the second vertical line).

We used data from the space and ground-based sta-

tions to identify the initial time of substorm expansion, i.e., the expansion onset.

(1) The aurora observed by IMAGE is shown in Fig. 3 in the Geomagnetic Coordinate System. Gray scale denotes the intensity of aurora and the deep ones mark the brightening region. At 1407UT aurora suddenly brighten at the night side; henceforth the brightening region expanded polarward, eastward and westward; and in 25 min, the brightening region moved westward from 2100MLT to almost 1800MLT. We refer to 1407UT as the time of substorm expansion onset[7], which is later than the beginning of CLR (at 1405UT).

(2) AL and AU index observed from ground-based stations are given in Fig. 4. A sudden decrease of AL started on a disturbed background at 1420UT, indicat-ing the initiation of geomagnetic substorm during which the westward electrojet significantly enhanced.

(3) Fig. 5 shows the low energy electrons flux meas-ured by geostationary orbit spacecraft LANL 1991-080 and LANL 1994-084, which were located at 3.4MLT and right 24MLT, respectively. Both of these two satel-lites observed a dispersionless electron injection at ~1409UT and 1410 UT. About 10 min later, a disper-

sive electron injection event was also observed by LANL-01A located at 15MLT (not shown). We infer that the original region heating electrons was most likely to be near the midnight and the heated electrons drifted eastward and gradually dispersed. The initial time of the electron injection event, 1409UT, is very close to the auroral substorm onset.

(4) Fig. 6 plots the magnetic field variations ob-served by geostationary spacecraft GOES 10. At 1420UT the simultaneous decrease in the northward component and increase in the radial component indi-cate the magnetic field dipolarization, which is one of the major manifestations for substorm expansion onset.

The relevant substorm phenomena described above illustrate that substorm expansion onset started about 2 to 5 min after the beginning of CLR. This was an in-tense substorm with luminous aurora brightening, in-tensive variations of AE index in excess of 1000nT, and electron flux enhancements over 2―3 orders.

1.2 The event of August 27, 2001

Another typical CLR event was observed by Cluster from 0401UT to 0430UT on August 27, 2001. Before and during the event, the IMF outside the magneto-


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Fig. 3. The aurora sequence during the CLR of October 14, 2002. At 1407UT aurora suddenly brightened on the night side, indicating the aurora sub-storm expansion onset.

Fig. 4. The AL and AU index during CLR of October 14, 2002. AL suddenly decreased at 1420UT and reached the maximum of 600nT soon, which indicates the development of an intense geomagnetic substorm. This plot is provided by the WDC-C2 KYOTO AE index service.


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Fig. 5. Electron flux at geostationary orbit observed by LANL 1991-080 and LANL 1994-084. LANL 1991-080 and LANL 1994-084 were located at 3.4MLT and right 24MLT, respectively. Both of these two satellites observed a dispersionless electron injection, respectively at ~1409UT and 1410UT.

Fig. 6. Magnetic field variations from GOES10 observations. From top to bottom are: B_total, the magnetic field magnitude; B_Earthward and B_North, the radial and northward component of magnetic field; B_ East, the azimuthal component of magnetic field. The vertical line mark the dipo-larization at 14:20UT.


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pause was continuously southward lasting for almost 2 hours as inferred from the data of ACE and Geotail. At 0408UT, 7 minutes after the beginning of CLR, an aurora substorm onset burst. Simultaneously GOES10 observed a dipolarization at the geostationary altitude. At 0412UT the AE index suddenly enhanced and rap-idly reached the maximum value of 600nT. In addition, the LANL satellites observed dispersive electron injec-tions because the locations of these satellites are all far from the midnight.

The analysis of above two CLR events shows that during the continuous southward IMF, CLR develops and has a close association with substorm expansion onset; CLR is most likely to be the virtual cause of substorm onset in the events studied.

1.3 Periodical substorms and CLRs

On October 1, 2001, the IMF kept southward con-tinuously from 0500UT to 1700UT with the minimum

of −15 nT. During this interval the repeated CLR events happened with the period of 2―3 h (Fig. 7). The sub-storm phenomena associated with these CLRs are: the multiple AE index enhancements; periodic electron injections (Fig. 8) and dipolarizations detected by geo-stationary spacecraft, LANL and GOES. Relevant in-formation is listed in detail in Table 1. Note that in CLR event of 1213UT, CLR was observed later than that of electron injection. It is most likely that Cluster missed the initial observation of CLR due to its inappropriate position.

The quasi-periodical CLRs are in one-to-one corre-spondence with the quasi-periodical substorms, and the beginnings of CLRs tend to be earlier than the substorm onset. Numbers of quasi-periodical CLR events can be seen in Cluster data during 2001―2003. They are all corresponding to periodical substorms.

Fig. 7. A periodical CLR event on October 1, 2001. Three CLRs are shown to begin, respectively, at 0926UT, 1213UT and 1549UT, with the quasi-period of 2―3 h. All the formats are the same as Fig. 2 and the vertical lines indicate the beginnings of CLRs.

Table 1 Timing analysis of relevant substorm phenomena during periodical CLRs CLR (T0) AE (T1) T1―T0 (min) Dipolarization (T2) T2―T0 (min) Injection (T3) T3―T0 (min) Expansion onset

0926UT 0936UT 10 0931UT 5 0927UT 1 0927UT

1213UT 1218UT 5 1216UT 3 1202UT −11

1549UT 1621UT 30 1550UT 1 1550UT


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Fig. 8. Periodical electron injection events on October 1, 2001. Dispersionless electron injection events began at 0927UT, 1202UT and 1550UT, re-spectively, corresponding to the CLRs in Fig. 7.

2 Statistical analysis of correlation between CLR and substorm onset

During 2001―2003, the Cluster constellation trav-ersed through the plasma sheet about 150 times; the crossing points were located in the range of −20RE <X<−10RE and −10RE<Y<10RE (GSM). 37 CLR events are observed[6]. We have made a statistical analysis of relationship between these CLRs and substorms as listed in Table 2. The following interesting facts are clear:

(1) Almost all CLRs are in one-to-one correspon-dence with substorms.

(2) Among all the 37 events, 34 events began aver-agely 8.7 min ahead of the substorm onsets. While in the other 3 events, the timing is difficult to determine since Cluster did not observe the exact beginning of CLR due to its inappropriate location.

(3) In the 34 events, dipolarization occurred after CLR began averagely with time delay of 9.5 min; the electron injection did with delay of 9.3 min; and AE enhancement came in 12.3 min, Fig. 9 illustrates such a statistic result of AE index for 13 events in 2001. The second vertical line indicates a sudden increase of AE index, generally the CLR (the first line) began 8 min ahead of sudden increase in AE index.

Fig. 9. The statistical unitary AE index during the superposed period 30 min around CLRs for 13 events in 2001. The first vertical line marks the beginning of CLR. The second one marks the sharp enhancement of AE index. In average the interval between CLR and substorm onset (sharp enhancement of the AE index) is almost 8 min.

With continuous support of energy, the plasma sheet

MR will develop into CLR during which more energy and magnetic flux are engaged, hence with larger spa-tial scale and longer duration. This makes difference between CLR and plasma sheet MR[6]. Usually at the initial phase of CLR, MR is going on inside the plasma


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Table 2 Timing analysis of relevant substorm phenomena and CLRs from 2001 to 2003 Event number Beginning of CLR Substorm onset First phenomenon Delay time

1 2001-07-29 0843UT 0855UT dipolarization 12 2 2001-07-29 1222UT 1224UT AE INDEX 2 3 2001-07-31 2320UT 2342UT AE INDEX 22 4 2001-08-03 1328UT 1358UT dipolarization 30 5 2001-08-13 0220UT 0250UT dipolarization 30 6 2001-08-17 1623UT 1626UT electron injection 3 7 2001-08-22 0920UT 0945UT dipolarization 25 8 2001-08-27 0401UT 0408UT dipolarization 7 9 2001-08-29 1032UT 1039UT AE INDEX 7

10 2001-09-14 2355UT 0000UTa) AE INDEX 5 11 2001-09-17 0715UT 0720UT dipolarization 5 12 2001-10-01 0926UT 0928UT electron injection 2 13 2001-10-01 1213UT 1202UT electron injection −11 14 2001-10-01 1549UT 1550UT electron injection 1 15 2002-08-02 0010UT 0027UT AE INDEX 17 16 2002-08-02 0215UT 0220UT AE INDEX 5 17 2002-08-04 0228UT 0230UT AE INDEX 2 18 2002-08-13 2229UT 2231UT electron injection 2 19 2002-08-18 1838UT 1844UT dipolarization 6 20 2002-08-18 2231UT 2233UT AE INDEX 2 21 2002-08-20 2242UT 2240UT AE INDEX −2 22 2002-08-21.0754UT 0742UT AE INDEX −12 23 2002-09-01 2252UT 2258UT dipolarization 6 24 2002-09-11 0838UT 0845UT dipolarization 7 25 2002-09-13 1802UT 1810UT electron injection 8 26 2002-10-03 0047UT 0048UT AE INDEX 1 27 2002-10-14 1403UT 1410UT electron injection 7 28 2002-10-14 1700UT 1721UT AE INDEX 21 29 2003-07-22 0815UT 0840UT AE INDEX 25 30 2003-07-22 1235UT 1300UT AE INDEX 25 31 200307-24 2240UT 2242UT AE INDEX 2 32 2003-07-29 1810UT 1812UT electron injection 2 33 2003-08-01 0548UT 0550UT electron injection 2 34 2003-08-24 1834UT 1837UT electron injection 3 35 2003-08-29 1930UT 1933UT AE INDEX 3 36 2003-09-01 0450UT 0451UT dipolarization 1 37 2003-09-24 1420UT 1421UT electron injection 1

a) 0000UT on 2001-09-15.

sheet. If Cluster spacecraft are in the outer region of plasma sheet, they will not be able to observe the right beginning of CLR (or MR). We think it is why the 3 events listed in Table 2 have the opposite timing.

Generally the convective high-speed flows derived from CLR with the velocity of exceeding 500 km/s, can reach the inner magnetotail (7―10 RE) in about 2 min. However, in our statistic study, the time interval be-tween CLR and substorm onset was typically 8―9 min or even longer. Why is the interval so long? We suggest that high speed flows do not trigger substorm onset by themselves and that the inner magnetotail needs time to adjust magnetic field configuration or plasma condition

when a great amount of energy is transported into the inner region. So the initial energy input may play a key role in triggering expansion onset. Therefore the inter-val between CLR and substorm onset must be longer than the traveling time of high speed flows.

3 Discussion and conclusions In sections 1 and 2 we present case studies and sta-

tistical analysis of the correlation between CLR and substorm expansion onset from 2001 to 2003. All the results indicate that CLRs during the period of con-tinuous IMF have one-to-one correspondence with sub-storms. CLR tends to occur several minutes before sub-

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storm onset, implying that the causality between CLR and substorm onset: the former is the cause and the lat-ter is the consequence. Under the continuous IMF con-dition, a great amount of energy is input from the solar wind into the tail lobe. CLR releases and transports the surplus free energy into the inner magnetotail. And so the close relation between CLR and substorm onset seems definitely reasonable.

Based on the observations in this paper and referen-ce[6], we can describe the global substorm process as follows: the MR happens in the dayside when the IMF turns southward, allowing the energy and magnetic flux entering the magnetosphere[1,2]. Most of the input en-ergy is stored in the tail lobe. The longer the IMF re-mains southward, the more energy will enter the mag-netotail. Since IMF remains southward typically for over 2 h in CLR events, a great amount of energy is deposited in the lobe region. Under this condition MR process in the tail generally remains longer time. It continuously releases the stored energy in the tail and finally develops into CLR. There are two situations in which MR is easily to occur. The first one is the case when the stored energy exceeds an energy threshold of the tail, MR emerges as times required to release the surplus free energy; this is the internal mechanism. The second case is variation of IMF[8], such as the north-

ward turning of IMF, this is the external cause. As to CLR, it would be the former. In the periodical CLR situation, the IMF continues to be southward for over 6 h in general. The existence of quasi-periodical CLR with period of about 2 h strongly suggests an ei-gen-behavior of the magnetotail. On the contrary, if IMF turns northward soon after the southward turning, relatively less energy is transported into the magnetotail. MR tends to release less energy than CLR does, it will lead to small substorm, or pseudo-breakup, or even nothing.

We presented a second event at 0215UT on August 27, 2001, during which less energy was stored in the magnetotail and released by MR, but no obvious sub-storm phenomena was observed. According to the IMF monitor ACE at L1 point, before 0215UT southward IMF lasted for about ~15 min. Probably the variation of IMF led to MR in the tail so that Cluster observed a convective high-speed flow in the central plasma sheet (Fig. 10). In this MR case substorm did not develop, AL index varied slightly; there was no electron injec-tion at all at the geostationary orbit; and there was no dipolarization detected, even though GOES-8 was lo-cated near midnight. It seems that energy stored in ~10 min was not enough to lead to even a seeming substorm in the inner magnetotail.

Fig. 10. High speed-flow observed by Cluster during 0218-0220 UT on August 27, 2001. The vertical line marks the beginning of the convective high-speed flow.


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In summary, we present several conclusions: (1) The causality between CLR and substorm onset

is clear for the events studied in this paper. CLRs are the virtual cause for the expansion onset of substorms which occur in continuous southward IMF condition. The high-speed flows derived from CLR transport en-ergy into the inner magnetotail and set up appropriate conditions for the onset of substorm expansion phase. We also noted the fact that the interval between CLR and substorm onset was 8 min in average, which is much longer than the traveling time of high-speed flows to propagate from the CLR region in the mid tail to the inner tail.

(2) MR in the magnetotail is not necessarily corre-sponding to substorms. The amount of energy released by MR is critical to substorm expansion onset. MR re-leasing less energy may not lead to substorms.

(3) There seems to be a possible energy threshold in magnetotail, when the stored energy exceeds the threshold MR will occur to release the excess energy. The periodical CLR events might be a case in point to reveal the existence of the threshold during continuous southward IMF.

Acknowledgements The authors are grateful to Japanese WDC-C2 KYOTO AE index service for presenting AU, AL and AE indices and to Goddard Space Flight Center, NASA for presenting data of the interpanaetary condition and aurora

data. Thanks are also given to Cluster and Double-Star Data Center for providing data used in the study. This work was supported by the National Natural Science Foundation of China (Grant No. 40390152), the State Key Basic Research Program (Grant No. G200000784), the XK100010404 of Beijing City, and the Space Weather Laboratory, Center for Space Science and Applied Research, CAS.

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Correlation between continuous lobe reconnection in the mid magnetotail and substorm expansion onset