Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1372–1379

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Journal of Atmospheric and Solar-Terrestrial Physics

1364-68

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jastp

On the relationship between magnetic clouds and the great geomagneticstorms associated with the period 1995–2006

M.A. Hidalgo a,n, J.J. Blanco a, F.J. Alvarez b, T. Nieves-Chinchilla c

a Space Research Group. Departamento de Fısica. Universidad de Alcala, Spainb Departamento de Fısica. Universidad de Alcala, Spainc NASA Goddard Space Flight Center, USA

a r t i c l e i n f o

Article history:

Received 6 April 2010

Received in revised form

2 February 2011

Accepted 17 February 2011Available online 2 March 2011

Keywords:

Geomagnetic storms

Magnetic clouds

Corotating interaction regions

26/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jastp.2011.02.017

esponding author. Tel.: þ34 91 8854958; fax

ail address: miguel.hidalgo@uah.es (M.A. Hid

a b s t r a c t

The fact that magnetic clouds are one of the main sources causing geomagnetic storms is a well-

established fact. One of the issues is to establish those features of magnetic clouds determinant in the

intensity of the Dst corresponding to geomagnetic storms. We examine measurements of geoeffective

magnetic clouds during the period 1995–2006 providing geomagnetic storms with Dst indexes lower

than �100 nT. These involve 46 geomagnetic storm events. After establishing the different character-

istics of the magnetic clouds (plasma velocity, maximum magnetic intensity, etc.) we show some

results about the correlations found among them and the storms intensity, finding that maximum

magnetic field magnitude is a determinant factor to establish the importance of magnetic clouds in

generating geomagnetic storms, having a correlation as good as the electric convective field.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

A basic problem in magnetospheric physics is the cause of thegeomagnetic storm (GS) phenomenon. Characterized by a depres-sion of the magnetic field at the Earth’s surface for hours or days,the GSs begin with a sudden impulse that indicates the arrival ofan interplanetary structure, like a magnetic cloud (MC), a cor-otating interaction region (CIR), an interplanetary shock or a morecomplex structure resulting in a combination of some of them.This usually coincides with the onset of a period of increased rampressure, the initial phase (absent in some storms), just followedby southward interplanetary fields (corresponding to the mainand recovery phases; Gonzalez et al., 1994; references therein). Infact, this southward field in the interplanetary sheath causes themain phase of the storm through magnetic reconnection.

There are several origins of such southward interplanetarymagnetic field in the high-speed stream region (Tsurutani andGonzalez, 1997; Webb et al., 2000; Richardson et al., 2002). Relation-ships have been found between the geomagnetic perturbations andthe interplanetary shocks (Tsurutani et al., 1992; Gonzalez et al.,1994; Jurac et al., 2002), the CMEs speed (Gonzalez et al., 2004), theMCs (Webb et al., 2000; Wu and Lepping, 2002; Leamon et al., 2002),their polarities (Vieira et al., 2001), and also CIRs (Cid et al., 2004).

ll rights reserved.

: þ34 91 8854942.

algo).

One of the main indexes to quantify the intensity GSs is the Dst

index, obtained from the perturbation in the horizontal componentof the geomagnetic field at equatorial latitudes as a consequence ofmagnetic disturbances recorded in four magnetic stations. A well-established classification considered as a great storm one with Dst

index values lower than �100 nT; a moderate one when Dst hasvalues between �100 and �50 nT; and a weak storm if Dst is inbetween �50 and �30 nT (Gonzalez et al., 1994).

Alexeev and Feldstein (2001) developed a model for the behaviorof the geomagnetic field during GSs where the magnetopause wasrepresented by a paraboloid of revolution, concluding that Dst

indexes depend not only on the ring current but also on othercontributions like the field of the currents on the magnetopause andthe field of the magnetospheric tail current. Moreover, Hakkinenet al. (2002) estimated the effects of currents induced in Earth on theDst index, showing that during the storm main phase the internalcontribution is about 30%, and about 20% during the storm recoveryphase. This difference is a consequence of the inductive effectsarising from rapid variations in the magnetic field, strongest duringthe main phase (Ganushkina et al., 2002).

However, it is generally accepted that changes in the ring currentdue to the passage of an interplanetary structure through it is themain cause for the global decrease in the Earth’s equator magneticfield. The constituents of the ring current are energetic magneto-spheric particles, localized near-Earth plasma sheet and out-flowingionospheric plasma (Burch et al., 2001), which flow around the Earthfrom east to west with an approximate toroidal geometry, extendingfrom geocentric distances of about 4–7 RE. The storm ring current

M.A. Hidalgo et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1372–1379 1373

condition is thought to be a consequence of two accumulated effects:the injection of energetic magnetospheric particles by frequentoccurrence of magnetospheric substorms, and the enhanced magne-tospheric convection. Both ring current strengthenings occur duringthe storm main phase. Afterwards, in the recovery phase, there wasno increase in the current population without the appearance of oneof these two phenomena (or both) (Lui et al., 2001).

In the present work we are interested mainly in the relationshipbetween the MCs and the GSs with Dsto�100 nT during the timeperiod 1995–2006; although some comparisons with the relationshipwith CIRs will also be made. Because MCs are sources of intensesouthward magnetic field many authors have emphasized thisrelationship (Burlaga et al., 1987; Tsurutani et al., 1988; Farrugiaet al., 1997). More recently, large statistical analysis has beenpublished; Wu and Lepping (2002) suggest that a geomagnetic stormcan be induced by (1) a sheath, (2) the front part region of a cloud,(3) the trailing part of a cloud and (4) both sheath and cloud regions,with an occurrence percentages of storms of 17.6%, 44.1%, 5.9% and20.6%, respectively. Leamon et al. (2002) study 46 solar coronaleruptions associated with sigmoids, relating the properties of thesesigmoids to in situ measurements at 1 AU and geomagnetic storms,and concluding that erupting sigmoids tend to produce geoeffectiveMCs: 85% of the erupting sigmoidal structures studied spawned atleast a ‘‘moderate’’ (Dsto50 nT) GS. Srivastava and Venkatakrishnan(2004) examine 64 coronal mass ejections (CMEs), which producedmajor GSs (Dsto�100 nT), showing that fast full-halo CMEs asso-ciated with strong flares and originating from a favorable location, i.e.,close to the central meridian and low and middle latitudes, are themost potential candidates for producing strong ram pressure at theEarth’s magnetosphere; even more, that the intensity of GSs dependsmost strongly on the southward component of the interplanetarymagnetic field, followed by the initial speed of the CME and the rampressure.

Zhang et al. (2007) identified the source of 88 major storms(Dsto�100 nT) during the period 1996–2005, classifying thosesources in the following: (1) S-type, in which the storm isassociated with a single ICME and a single CME at the Sun;(2) M-type, in which the storm is associated with a complex solarwind flow produced by multiple interacting ICMEs arising frommultiple halo CMEs and (3) C-type, in which the storm isassociated with a CIR formed at the leading edge of a high-speedstream originating from a coronal hole. For the 88 major storms,the S-type, M-type and C-type events number 53 (60%), 24 (27%)and 11 (13%), respectively.

Echer et al. (2008a, 2008b) look for the interplanetary condi-tions leading to superintense geomagnetic (Dsto�250 nT), alsoduring solar cycle 23, concluding that about 1/3 of the super-storms were caused by MCs, 1/3 by a combination of sheath andMC and the last 1/3 by sheath fields alone. They established thatthe interplanetary parameter best correlated with peak Dst, i.e.,the total energy transferred from the solar wind to the magneto-sphere, is the time integral of the convective electric field,Ey¼�vSW�Bs, (where Bs is the southward magnetic field andvSW the maximum solar wind velocity), during the storm mainphase (this is defined from the time when Dst starts to decrease tothe peak negative Dst).

An extensive study about the relationship between CIRs andassociated Dsto�100 nT GSs in the period 1996–2004 was carriedout by Richardson et al. (2006), who find that among 79 GSs withDsto�100 nT, only in nine cases (around 11%) the storm driverappears to have been purely a CIR, without any contribution fromcoronal mass ejection-related material (interplanetary coronal massejections (ICMEs)). More recently Choi et al. (2009) have accom-plished a study of the relationships among coronal holes, CIRs and GS(with Dst index below �50 nT) in the period 1996–2003 from astatistical point of view. They identify 123 CIRs, also finding that best

correlation of Dst index is obtained with the maximum value of theconvective electric field Ey in the solar wind, more than other solar orinterplanetary parameter. Even more, Barkhatov. et al. (2009) statethat the clouds located close to the Sun–Earth line and having smallinclination angles to the ecliptic plane are more geoeffective, that is,the probability of occurrence of a geomagnetic storm for such cloudsis higher than for other events.

In the present study we show how the values of maximummagnetic field magnitude correlate with the geomagnetic fieldmagnitude, Dst, being as good as the electric convective field.

2. Data

All data used in the present work for plasma and magneticfield components have been obtained from WIND spacecraftdatabase and the Dst data form World Data Center for Geomag-netism in Kyoto (Japan).

To identify the MCs in the data we have used the well-established criteria given by Burlaga et al. (1981): the appearanceof an intense magnetic field module, the rotation of one of thecomponents of the magnetic field (usually the z-GSE component)and a depression in the temperature of protons at the interval ofthe cloud.

As we mention above, it is considered that any storm is ingeneral caused by a southward-directed interplanetary magneticfield related to MCs, CIRs or a combination of sheath and MC(Vieira et al. 2004; Echer et al., 2008a, 2008b; Choi et al., 2009).However, in all the present text we globally classified the origin ofany GS in CIRs or MCs. Of course not always is possible to havesuch a clear distinction of the interplanetary structure causing aGS; more complex structures should be a combination of both,requiring a deeper study, out of the scope of the present work,where we mainly are interested in the MC–GS relationship.

We analyze in detail GSs with Dsto�100 nT all of themassociated with MCs. In Table 1 relevant characteristic parametersof the MCs are detailed, all of them related to GSs: maximummagnetic field magnitude at the GS time interval, the wholevariation of the magnetic field at the same interval, the rotatingcomponent of the magnetic field, the storm intensity, the max-imum proton solar wind velocity, the whole variation of theproton velocity at the GSs interval and, finally, the duration ofevery MC. There are several MCs where it is difficult to decide therotated component of magnetic field; in those cases we havelabeled the corresponding component with a question mark. Incases where some characteristics of MCs in the interplanetarydata are not well-defined, as established Burlaga et al. (1981)criteria, we label them with an asterisk.

In every case the maximum value of the magnetic fieldmagnitude that appears at the corresponding forward-shock ofthe cloud is analyzed, or, if the MC has no sign of it at themagnetic field data, the maximum is located at the cloud interval(see below).

From the 89 storms found at the time interval studied(1995–2006) with intensity smaller than �100 nT, 28 are unambigu-ously associated with CIRs (a clear increment of the proton solar windvelocity at the interval of the event appears, with absence ofdepression in the behavior of the plasma pressure or temperature,and without a defined rotation in any component of the magneticfield, in fact a lack of structure in the components of the magneticfield), and 35 with MCs (although 46 could be related to them). Fromthe rest, there are 18 storms, most probable associated with CIRs dueto the lack of any apparent structure in the magnetic field compo-nents (of course, although with certain ambiguity), and 8 with MCs.

MCs without sign of a forward shock in their magnetic field datapresent a Dst index characterized by a wide and smooth main phase,

Table 1Geomagnetic storms associated with MCs during the period 1995–2006. From left to right it is shown the origin (MCn in case it is not well-defined (see text)); the

maximum magnetic field at the interval of the GS; the whole variation of the magnetic field magnitude at the time interval of the MC; the maximum proton solar wind

velocity, the whole variation of the proton velocity and, finally, the duration of every MC. There are several MCs in which to know the rotated component of magnetic field

is difficult; in those cases we have labeled the corresponding component with question mark.

Date Origin Btotalmax (nT) DBtotal (nT) Component of

B crossing

Storm Intensity vSW (km/s) DvSW (km/s) Dtcloud (h)

1995/10/18 MC 24 0 Bz �130 400 0 20

1997/04/21 MC 13 0 Bx? �110 420 100 18

1997/05/15 MCn 25 15 Bz �110 450 50 12

1997/10/10 MC 13 4 Bz �125 450 75 20

1997/11/07 MC 17 0 Bz �110 450 25 18

1998/02/17 MCn 21 0 Bz �100 400 0 12

1998/05/04 MCn 38 10 Bx? �200 800 150 12

1998/08/26 MC 15 10 Bx �150 830 250 18

1998/09/25 MC 25 2 Bz �200 820 140 9

1998/10/19 MC 25 12 Bz �110 420 70 24

1998/11/08 MCn 34 14 By? �140 630 130 12

1998/11/09 MCn 14 10 Bx? �130 450 70 24

1998/11/12 MCn 20 7 Bz �125 400 0 24

1999/02/18 MC 27 10 Bx �120 750 150 24

1999/09/22 MC 26 13 Bx? �175 600 100 19

2000/07/15 MC 49 45 Bz? �300 1000 400 18

2000/08/12 MC 33 10 Bz �230 600 50 12

2000/09/17 MC 36 30 Bz? �200 800 150 24

2000/10/28 MC 18 10 Bz �125 400 60 20

2000/11/06 MC 24 0 Bz �150 600 100 12

2001/03/20 MC 21 10 Bx? �140 450 150 28

2001/04/12 MC 35 20 By �275 700 100 18

2001/04/18 MCn 22 5 Bx? �110 500 100 35

2001/08/18 MC 30 17 Bx �100 550 100 18

2001/11/24 MC 49 15 Bx �225 1000 400 24

2002/04/18 MCn 27 2 Bz �125 500 50 18

2002/05/23 MC 30 0 Bz? �100 800 400 24

2002/08/02 MC 15 2 �100 500 50 18

2002/08/20 MC 10 0 By? �110 480 80 18

2002/09/07 MC 22 10 Bz �180 550 100 18

2002/10/04 MCn 11 5 Bz? �140 400 0 24

2003/05/30 MCn 28 0 Bz? �145 750 200 12

2003/08/18 MC 21 0 Bz �150 500 0 12

2003/10/31 MCn 35 18 By? �380 1200 200 12

2003/11/20 MC 55 42 Bz �400 700 200 10

2004/01/22 MC 24 5 By? �150 650 100 12

2004/04/04 MC 18 10 Bz �110 500 100 18

2004/07/23 MC 17 7 Bx �100 650 150 12

2004/11/07 MC 45 16 By? �350 700 100 18

2004/11/10 MC 39 10 By? �290 800 200 9

2005/05/08 MC 15 0 Bx, By? �120 800 200 18

2005/05/15 MC 53 30 By �270 950 200 15

2005/05/30 MCn 18 0 Bx? �135 450 0 18

2005/06/13 MC 17 10 Bz �110 450 0 15

2005/09/11 MC 18 10 By �140 1000 200 20

2006/12/15 MC 18 12 Bx, Bz? �140 890 290 18

M.A. Hidalgo et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1372–13791374

as it is shown in the cloud of October 13, 1997, Fig. 1, which presentsa smooth increase in the magnetic field intensity, reaching a max-imum value of 12 nT, and with Dst¼�135 nT. In Figs. 1–4, the Dst

index corresponding to the geomagnetic storm associated with eachcloud, the proton solar wind velocity, the magnetic field magnitudeand the z-magnetic field component are shown from up to down,.The same effect can be seen in the MC of November 7 (2000), with amaximum magnetic field magnitude of 25 nT and a Dst around�155 nT (Fig. 2).

It is also possible to find MC without forward shock appearing inthe magnetic field data but with an abrupt increase in its intensity,like the one of October 19 of 1998, with a maximum magnetic field of25 nT and a Dst of �110 nT, and where the main phase time durationis hardly 1 h, as seen in Fig. 3. Similar characteristics are present inthe MC of May 16 of 2005 as seen in Fig. 4, although the magneticfield strength is 50 nT and the Dst around �275 nT.

Now we describe some selected intense storms, not all of themwith a clear origin, i.e., associated with MCs or CIRs, because their

structures (plasma and magnetic field behaviors) do not establishtheir character. This description shows the difficulties in finding theorigin of many storms.

1997: The May 15 of 1997 corresponds to a �110 nT Dst indexstorm. The profile of the field, rotating the z-component, and thetemperature behavior would indicate a storm associated with aMC. However, the behavior of solar wind velocity at the stormtime interval corresponds with a CIR. The events of November 22and 23 are complex phenomena (three successive storms appear-ing, one of them with Dsto�100 nT), although apparentlyassociated with CIR.

1998: The storm with Dsto�100 nT of February 17 of 1998 isapparently associated with a MC (rotation in the Bz component of themagnetic field), although without forward-shock, whose consequenceis a long storm main phase. The event of May 5 provides a �200 nTstorm. It has an intense forward-shock clearly coincident with themain phase. Although a decrease in the solar wind velocity appears inthe recovery phase, the behavior of the magnetic field components

Fig. 1. October 13, 1997, magnetic cloud. From up to down, the Dst index for the corresponding geomagnetic storm, the proton solar wind velocity, the magnetic field

magnitude and the z-magnetic field component are shown.

Fig. 2. Same as in Fig. 1 for the magnetic cloud of November 7 of 2000.

M.A. Hidalgo et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1372–1379 1375

Fig. 3. Same as in Fig. 1 for the October 19 of 1998 magnetic cloud.

Fig. 4. May 16 of 2005 magnetic cloud. The figures are the same as in Fig. 1.

M.A. Hidalgo et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1372–13791376

and the plasma pressure inside time interval seem to be related to aCIR. In the interval between November 6 and 15 three intense storms,of the order of �150 nT, are generated. The structure of data is

complex and is really difficult to establish the origin of each one,although the two first could be associated with two MCs and the lastone with a CIR.

M.A. Hidalgo et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1372–1379 1377

2000: During October 4–5, three successive events appear,similar to the events observed during November 22, 1997; one ofthe three storms is related to CIRs and the other two could berelated to MCs, although the analysis of these GSs are difficultbecause the storms are consecutive. In October 13, the corre-sponding storm has a Dst of �100 nT, after a previous one of�60 nT with a complex structure. The behavior of the compo-nents of the magnetic field has a complex structure; although

Fig. 5. (a) Dst index for GSs related to CIRs as a function of the maximum magnetic field

at a magnetic field of 30 nT and horizontally at the Dst of �200 nT. (b) Same as (a)

1995–2006. (d) Dst as a function of the maximum solar wind velocity for GSs associat

a low temperature and decrease in the velocity seems to mean tobe originated in a MC. The event of November 29 is associatedwith a MC without forward-shock appearing in the magnetic fielddata, which provides an extent main phase.

2001: The event of April 18 has a magnetic field and plasmabehaviors completely similar to the one observed in May 4, 1998,which was generated by a CIR. Although the shape of the Dst iscomplex, the corresponding storm of April 23 has its origin in a MC.

magnitude for storms with a Dsto�100 nT. Reference lines are traced, vertically

but for the Dst related to MCs. (c) All GSs with Dsto�100 nT during the period

ed with MCs. (e) Dst vs the convective electric field given by vSW�Btotalmax .

M.A. Hidalgo et al. / Journal of Atmospheric and Solar-Terrestrial Physics 73 (2011) 1372–13791378

The phenomena of October 22 and April are generated by CIRs, givingquite identical Dst. The event of October 28 has a complex magneticfield structure, leading to a complex Dst. We think it is associatedwith a CIR.

2005: The geomagnetic storm of May 30 has a long main phase,associated with the fact that the corresponding MC has not aforward-shock. In May 9, the �120 nT storm has a previous one of�90 nT. While the last one is coming from a CIR the more intense isthat of a MC.

3. Discussion

In Fig. 4 we show a typical case corresponding to the storm ofMay 16 of 2005. As it is seen and is usual in a GS related to a MC,the recovery phase matches with the own MC structure. Oneattempt to understand this relationship analytically was pub-lished by Hidalgo (2003a, 2003b), showing that the main con-tribution to the storm recovery phase corresponds to the effectsproduced in the ring current by the passage of the clouds;establishing a relationship between the MC structure and the GS.

There are several solar wind parameters determining any GSs.One of the most important is the southward magnetic field (Alveset al., 2006), although more recently it is accepted that theconvective electric field, Ey¼�vSW�Bs, has the best correlationwith Dst index (Echer et al., 2008a, 2008b).

In Fig. 5(a)–(e) we show the results obtained for the stormsconsidered (in the corresponding figures we have included eventhe non well-defined associated with MCs or CIRs). We representthe storm intensity as a function of different parameters asso-ciated with the phenomena studied in the present work, Table 1.In (a)–(b) the Dst indexes are represented for GSs related toCIRs and MCs, respectively, both as a function of the maximummagnetic field magnitude. Reference lines are traced, alsoin Fig. 5(c), vertically at a magnetic field of 30 nT and horizontallyat a Dst of �200 nT. (The cloud of November 8 of 1998, out of theboxes defined by the traced lines in Fig. 5(b), is most probably acomplex phenomenon where two MCs are superimposed.) Thecorrelation coefficient obtained for Fig. 5(a) is rE0.82, andfor Fig. 5(b) rE0.79.

Fig. 5(c) shows all storms (produced by CIRs and MCs) withDsto�100 nT. As it is seen no storms are produced due tointerplanetary structures with maximum magnetic field magni-tude below 10 nT.

Fig. 5(d) the Dst is represented as a function of the maximumsolar wind velocity, which shows a significant lack of correlation(it has a correlation coefficient rE0.58). Finally, Fig. 5(e) showsDst vs. the convective electric field, determined by the termvSW�Btotal

max , where vSW is the maximum value of the proton solarwind velocity at the cloud interval. For this figure a correlationcompletely similar as obtained for Fig. 5(b) is determined, i.e., ofthe order of rE0.79. Finally, we find that not only for MCs butalso for CIRs the correlations with the maximum magnetic fieldmagnitude are completely similar, if not better, than the con-vective electric field. This implies that this maximum magneticfield value is the determinant factor to establish the significanceof a MC or CIR in generating GSs with Dsto�100 nT.

As appears in Table 1, although the z-magnetic field compo-nent that rotates during the measurement along the spacecraftpath is present in most of the clouds, it is not always the case forthe most intense storms. Finally, although we do not show thecorresponding figures, we have not found any correlation at allbetween the Dst index and the others characteristics given in theremaining columns of Table 1.

Our next interest is to include in the study GSs withDst4�100 nT, and not only due to MCs but CIRs, determining in

case of the MC origins the orientations of the clouds, correlatingthem with the GSs intensity.

Acknowledgments

This work has been supported by the Comision Interminister-ial de Ciencia y Tecnologıa (CICYT) of Spain, grant ESP2006-08459.The authors thank to K. Ogilvie, R. Fitzenreiter and R. Lepping(Goddard Space Flight Center, Greenbelt, MD, USA) for thepermission to use the WIND data, and the World Data Centerfor Geomagnetism in Kyoto (Japan) for providing Dst data.

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