Geoeffectiveness of magnetic clouds occurred during solar cycle 23

ARTICLE IN PRESS Planetary and Space Science 58 (2010) 741–748

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Geoeffectiveness of magnetic clouds occurred during solar cycle 23 Santosh Kumar a, Amita Raizada b,n a b

Department of P.G. Studies and Research in Physics and Electronics, R.D. University, Jabalpur, Madhya Pradesh-482001, India Department of Physics, DAV (PG) College, Dehradun, Uttarakhand-248001, India

a r t i c l e in fo

abstract

Article history: Received 31 July 2009 Received in revised form 21 November 2009 Accepted 27 November 2009 Available online 24 December 2009

Highly variable conditions prevail in the geospace environment due to the variations in Solar activity. The characteristics of the magnetic clouds (MCs) and their effects on geosphere, which have occurred during the period January 1996 to December 2006; have been investigated. No systematic trend has been observed between MCs and Solar activity cycle which is analyzed on the basis of maximum Sunspot number in that particular year. 85% MCs are found to be geoeffective. MCs are divided into two major classes: unipolar and bipolar. Unipolar MCs are of south (S) or north (N) type while bipolar MCs are of south–north (SN) or north–south (NS) type. During Solar cycle 23, SN-type MCs dominated over NS-type MCs. Highly intense geomagnetic storms (GMSs) of Dst o 300 nT follow from SN or S-type MCs. No preference is observed for right handed (RH) or left handed (LH) clouds for being geoeffective. MCs of very high speed lead to intense GMSs. The correlation coefficient (r) of southward component of magnetic field (Bz), total magnetic field (B) and their products with plasma flow speed (VB and VBz) with Dst are observed to be r= 0.78, 0.81, 0.79 and 0.82, respectively, which suggests that these parameters are reliable indicators of the strength of GMS. SN clouds do not always lead to more fall in Dst value (or lead to high strength of GMS) than NS clouds for similar value of Bz minimum associated with both type of MCs. & 2009 Elsevier Ltd. All rights reserved.

Keywords: Geomagnetic storm Magnetic cloud Solar wind Interplanetary magnetic field

1. Introduction The term magnetic cloud (MC) is originally used by Parker (1957) in a much broader sense in his theoretical study of the dynamics of hydromagnetic gas clouds ejected from the Sun into the interplanetary (IP) space. Later on, it is used by Burlaga et al. (1981) who defined MCs as a region of magnetic field strength higher than the average, low proton temperature and smoothly changing (rotating) magnetic field through a large angle. MCs are the IP manifestations of coronal mass ejections (CMEs) (Gopalswamy, 2006; Huttunen et al. 2005; Lepping et al. 2006; Singh and Badruddin, 2007). Interplanetary CMEs (ICMEs) are identified using magnetic compositional and energetic plasma particle signatures, which include bidirectional streaming of superthermal electrons and ions, unusual abundances and charge states, low electron and proton temperatures, strong-magnetic fields with flux rope structures and leading to cause Forbush decreases. All the signatures are not present in all the events (Gopalswamy, 2006 and references there in). The percentage of MCs contained within ICMEs apparently depends on specific conditions as shown by Richardson and Cane (2006).

n

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0032-0633/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.pss.2009.11.009

MCs are relatively common phenomena in the Solar wind (SW). They are found to have low proton density and temperature. Because the total pressure inside the cloud is higher than outside, the cloud expands as it moves outward away from the Sun through the Solar system (Wilson, 1987). An MC expands until a near equilibrium state with the surrounding pressure is established (Lepping et al., 2006). MCs are often but not always associated with IP shocks. MCs that strike the Earth may cause intense geomagnetic storms (GMSs) (Bothmer and Schwenn, 1998). The orientation of MC is an important factor in GMSs (Burlaga, 1988; Wang et al., 2005). The time of onset of geomagnetic activity coincides with the arrival of MC when the magnetic field is oriented southward at cloud onset and it occurs later during the cloud when magnetic field is oriented northward at cloud onset (Zhang and Burlaga, 1988). The duration of MC lies between 7 and 48 h at 1 AU (Lepping et al., 2006). The arrival times of MCs lagged the shocks by 5–25 h with an average delay of about 11 h (Bothmer and Schwenn, 1998). Geomagnetic response of MCs has been studied on the basis of disturbance storm time (Dst) index. Dst index represents the deviation in the horizontal component of the Earth’s magnetic field at the equator relative to its value in a quiet day (Cid et al. 2004). In the present investigation, the GMSs have been classified as: weak ( 30 nT4DstZ 50 nT), moderate ( 50 nT 4DstZ 100 nT) and intense with Dst r 100 nT as suggested by Gonzalez et al. (1994).

ARTICLE IN PRESS S. Kumar, A. Raizada / Planetary and Space Science 58 (2010) 741–748

2. Data and its analysis MCs that are listed on the Wind-magnetic field investigation (MFI) Website http://lepmfi.gsfc.nasa.gov/mfi/mag_cloud_pub1. html from January 1996 to December 2006 that satisfies the classic definition of an MC given by Burlaga et al. (1981) have been used. Ninety five MCs are observed and analyzed during the said period. During the duration of MCs at 1 AU, the nature of magnetic field is observed such that if the magnetic field is mostly (90% or more) northward or southward, MCs are called unipolar, otherwise bipolar. The values of Dst indices are taken from World Data Center, Japan (http://swdcwww.kugi.kyoto-u.ac.jp). The list of shock waves associated with MCs is taken from http:// www-ssg.sr.unh.edu/mag/ace/ACElists/obs_list.html. The electronic supplement available in Gopalswamy et al. (2008) is also considered. Interplanetary magnetic field (IMF) values are obtained from Wind data set. The minimum value of southward component of magnetic field (Bz) in the GSE coordinates, the maximum value of the total magnetic field (B) and the minimum value of Dst during the arrival and departure of MCs at the Earth distance is chosen in the present investigation.

3. Results and discussion 3.1. Solar activity and occurrence of MCs The Solar activity is analyzed on the basis of Sunspot numbers (SSNs), which are present over the Solar disk during Solar cycle 23. The largest value of the Sunspot acquired in a year has been plotted in Fig. 1. The entire period under consideration, is divided into three parts: ascending phase from 1996 to 1998, maximum phase from 1999 to 2002 and descending phase from 2003 to 2006, which is almost similar to Gopalswamy et al. (2008). MCs also show three part structure: 96–99, 99–03 and 03–06 in which their number increases and decreases thrice, which is clear from Fig. 1. During the year 1999, only four MCs are observed which is something unexpected since the year 1999 is close to the maximum phase; whereas, in 1997, which is in the ascending phase and close to minimum phase, largest number of MCs of 23rd Solar cycle is observed, which is almost similar to the observations of Wu et al. (2006) and Huttunen et al. (2005) but contrary to the findings of Lynch et al. (2003) who reported largest number of MCs in 2000. There is ambiguity in different results due to differences in method of identification of MCs (Huttunen et al. 2005). Thus, the MCs do not reveal any systematic relationship with Solar activity. However, similar to the three part structure of MCs, Gopalswamy et al. (2007) has observed three part structure for front side Halo CMEs occurring during Solar cycle 23. Thus, MCs may not have one to one relationship with

RDVV, Jabalpur

20

300

16 200 12 8 100

Maximum SSN

During the passage of MCs, both northward and southward magnetic fields are observed. However, southward field is crucial for the reconnection process. This has been established by now that GMSs occur when the southward component of interplanetary magnetic field (IMF), Bz impinges upon the Earth’s magnetosphere and reconnect (Gonzalez et al., 1989). Large numbers of theoretical and experimental studies have been done in Solar-terrestrial physics; still the identification of Solar and IP factors and their order of magnitude dependence that causes magnetospheric disturbances are not satisfactorily known. The purpose of this paper is to investigate the different characteristics of MCs, which have occurred during Solar cycle 23 and to study their effect on geomagnetosphere so as to have better understanding of the association of GMSs with MCs.

Number of Events/MCs

742

4 0

0 1996

1999

2002 Year

2005

Fig. 1. Yearly occurrence of MCs (&) and yearly variation of Solar activity observed in terms of maximum SSN (~) during the period 1996–2006.

CMEs; their number may have good correlation with the occurrence frequency of these CMEs. 3.2. Classification of MCs In the present study, MCs are classified into two major categories, as unipolar and bipolar depending upon the change in magnetic field direction between arrival and departure of MC as observed at 1 AU. Unipolar MCs have either southward (S) or northward (N) magnetic field for most of the time of MC’s duration at 1 AU which is almost similar to Gopalswamy et al. (2008). Bipolar MCs have magnetic field either changing from southward to northward (SN) or vice versa (NS). During Solar cycle 23, 40% MCs are observed to be unipolar while 60% are bipolar, as are shown in Fig. 2(a) which depicts the yearly occurrence of unipolar and bipolar MCs. Mulligan et al. (1998) found that unipolar MCs are most frequently observed in the declining phase of Solar activity; whereas, bipolar MCs; during the minimum and rising phases. Contrary to this, not much difference in unipolar and bipolar MCs are observed during rising and declining phases of present study; however, in the maximum phase, the number of bipolar MCs are almost double the number of unipolar MCs. No definite trend is seen from Fig. 2(a). They seem to appear throughout the Solar cycle 23. S- or N-type unipolar MCs are found to be equal in number during the Solar cycle 23, as shown in Fig. 2(b); whereas, SN-type of bipolar MCs are more dominating than NS type with their pretty high abundance of 61%, which is similar to the observations of Zhang and Burlaga (1988), who stated that SN rotation of magnetic field vector is more frequent. As depicted in Fig. 2(c), S- and N-type MCs are almost equally distributed during rising phase. However, N-type MCs dominate in maximum and S-type in declining phase. SN-type MCs are more in rising and maximum phases, which is clear from Fig. 2(d); whereas, NS are more in declining phase, which confirms the behavior of odd no. Solar cycle (Gopalswamy et al. 2008), where magnetic field in MCs rotates from S to N in rising phase (Huttunen et al. 2005). 3.3. Cloud handedness Bothmer and Schwenn (1998) have considered four different magnetic configurations which result from two possible directions of the field axis and two possible values of magnetic helicity. In the present investigation, eight different magnetic configurations are considered resulting from four types of MCs as discussed

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in Section 3.2 and two possible values of magnetic helicity. The clockwise rotation of magnetic field has been considered as left handed (LH) rotation; whereas, counter clockwise is taken as right handed (RH) rotation. For unipolar MCs, the rotation is from East to West through North or South or vice versa. For bipolar MCs, the rotation is from North to South through East or West or vice versa.

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According to Lepping et al. (2006), RH cases are expected to arise in the southern hemisphere of the Sun and LH cases from the northern hemisphere. In the present investigation, 43% RH and 57% LH MCs are observed during Solar cycle 23. More LH cases are observed during maximum phase; however, there is no major difference in the frequency of occurrence of LH and RH MCs in the rising and declining phases, as it is observable from Fig. 3.

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40 Unipolar Bipolar

Number of MCs

Number of Events

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2

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0 1996

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NS S Types of MCs

SN

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Year

N

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SN type MCs

S type MCs

NS type MCs

No. of SN / NS MCs

No. of S / N MCs

N type MCs

4

2

0

6

4

2

0 1996

1999

2002

1996

2005

1999

2002

Year

2005

Year

Fig. 2. (a) Yearly occurrence of unipolar and bipolar MCs, (b) total number of MCs of SN, NS, S and N type that occurred during Solar cycle 23, (c) yearly occurrence of S- or N-type unipolar MCs and (d) SN- or NS-type bipolar MCs.

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SN type MCs

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NS type MCs

LH MCs

8

No. of Events

Cloud handedness

RH MCs

S type MCs

8

N type MCs

4

4 0 (a)

0 1996

1999

2002 Year

Fig. 3. Yearly occurrence of RH- and LH-type MCs.

2005

(b)

(c)

(d)

(e)

(f)

(g)

Dst Range Fig. 4. Different types of MCs leading to fall in Dst value in various ranges, e.g. (a) 30 to 50 nT, (b) 51 to 100 nT, (c) 101 to 150 nT, (d) 151 to 200 nT, (e) 201 to 300 nT, (f) 301 to 400 nT and (g) 401 to 500 nT.

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Weak Moderate Intense

No. of GMSs

6

4

2

0 1996

1999

2002

2005

Year Fig. 5. Yearly occurrence of different types of GMSs.

3.4. Geomagnetosphere’s response to MCs Out of the total MCs, 85% MCs are observed to be geoeffective and 79% of them are associated with shock. In the present investigation, the analysis related to sheath is not taken into account. 15% MCs, which do not lead to GMS are observed to have low velocity (o350 km/s) or high magnitude of Bz component (4 7 nT) or are of less duration. Majority of these non-geoeffective MCs are observed during rising phase. 71% of these non-geoeffective MCs, in spite of having southward Bz component, do not lead to initiate GMS. Wilson (1987) also noted that not all MCs caused a GMS, even though all clouds may have a large southward component of the magnetic field at some point during their existence at Geosphere. This suggests that some clouds may either lack sufficient energy to initiate a storm or that other influence like corotating interaction regions (CIRs) may have a disturbing effect on a cloud’s ability to generate a storm (Wilson, 1987). Moreover, no preference is observed for cloud handedness for being geoeffective in the present study. Different types of MCs leading to fall in Dst value in various ranges have been plotted in Fig. 4. It is observable from Fig. 4 that all types of MCs lead to GMSs in the lower (up to

11%

18%

41%

37% 26%

18%

26%

23%

SN

NS

11%

21%

5%

32%

58%

26%

26% 21% S Intense GMSs

Moderate GMSs

N Weak GMSs

No GMSs

Fig. 6. Different types of GMSs caused due to (a) SN, (b) NS, (c) S and (d) N types of MCs.

ARTICLE IN PRESS S. Kumar, A. Raizada / Planetary and Space Science 58 (2010) 741–748

150 nT) and intermediate ( 151 to 300 nT) range of Dst index; however, the number of GMSs has considerably reduced in intermediate range. Only SN- and S-type MCs lead to highly intense GMSs with Dst o 300 nT.(Fig. 4) The yearly occurrence of different types of GMSs caused due to MCs has been plotted histographically during the period 1996–2006 in Fig. 5. It is clear from Fig. 5 that 20%, 24% and 41% MCs lead to weak, moderate and intense GMSs, respectively. Majority of the intense (79%) GMSs have occurred during maximum and declining phases of Solar activity; whereas, majority of the weak (53%) GMSs are observed during rising phase of Solar activity. However, moderate GMSs are more or less equally distributed among all the three phases of Solar activity. Fig. 6(a–d) clearly shows that majority of the MCs lead to intense GMSs, which agree with the observations of Zhang and Burlaga (1988), Huttunen et al. (2005) and Gopalswamy (2006); followed by moderate and then weak GMSs. The % number of weak, moderate and intense GMSs caused by different types of MCs are shown in Fig. 7(a–c). Further, 79% N-type MCs lead to GMSs which could be due to the presence of southward component in the sheath region (Lepping et al., 2006) as they are not geoeffective themselves (Huttunen et al., 2005; Gopalswamy et al., 2008). 33% of the total such non-geoeffective N-type MCs lead to intense GMSs which is contradictory to Zhang et al. (2004) who concluded that N-type MCs are predominantly associated with moderate GMSs.

3.5. Speed distribution of MCs As shown in Fig. 8(a), the number of MCs having high speed is less. Maximum number of MCs has speed in the range

745

300–500 km/s which is also observed by Gopalswamy (2006). The average speed of MCs is observed to be 475.7 km/s; whereas, the average speed of MCs leading to GMSs is 491.5 km/s as depicted in Fig. 8(b), which is considerably higher than that of slow Solar wind of average speed 440 km/s (Gopalswamy et al. 2008). Only 73% of the slow speed (301–400 km/s) MCs and 89% with intermediate speed (401–600 km/s) cause GMSs; whereas, all the MCs with speed 4600 km/s lead to GMS; indicating that speed of MC is also one of the parameters responsible for occurrence of GMSs. MCs with speed 4800 km/s causes intense GMSs only, which implies that MCs of very high speed lead to high intensity GMSs which agrees with the observations of Zhang and Burlaga (1988). Fig. 8(c–e) shows the velocity distribution of MCs causing weak, moderate and intense GMSs, respectively. The average speed of MCs for weak GMSs is least; for moderate GMSs, it is intermediate and for intense GMSs, it is high which suggests that speed of MC has significant impact on the intensity of GMS; however the anti-correlation observed between Dst and V (r = 0.61) is not very significant. Moreover, Fig. 8(a–e) also shows standard error (s) observed in the present investigation along with average speed.

3.6. Effect of IMF on GMSs The major aim of Solar-terrestrial physics is to understand the mechanism of transference of Solar wind (SW) energy into the magnetosphere. Internal correlation between various parameters is one of the problems in the study of SW–magnetospheric interactions. In order to understand the response of the RDVV, Jabalpur

N 17%

N 26%

SN 39%

SN 48%

S 22%

S 5% NS 21%

NS 22%

N 15% SN 34% S 28% NS 23%

Fig. 7. Different types of MCs leading to (a) weak, (b) moderate and (c) intense GMSs.

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40

40 Avg. V = 475.7 Km/s σ = 126.65 Km/s

No. of GMSs

No. of MCs

30 20 10 0

30 20 10 0

400 500 600 700 800 900 1000 V (Km/s)

400 500 600 700 800 900 1000 V (Km/s)

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18 Avg. V = 419.9 Km/s σ = 178.25 Km/s

8

4

0

No. of Mod. GMSs

No. of Weak GMSs

Avg. V = 491.5 Km/s σ = 110.9 Km/s

Avg. V = 462.4 Km/s σ = 52.17 Km/s

12

6

0

400

500

700

800

400

500

V (Km/s)

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800

No. of Int. GMSs

18 Avg. V = 539 Km/s σ = 160.32 Km/s

12

6

0 400 500 600 700 800 900 1000 V (Km/s) Fig. 8. (a) Total number of MCs occurred during Solar cycle 23. Number of MCs leading to (b) all type of GMSs, (c) weak GMSs, (d) moderate GMSs and (e) intense GMSs, in different speed ranges.

magnetosphere to IP conditions, IMF strength (B) and its north– south component (Bz) with Dst are plotted in Fig. 9(a, b). As shown in Fig. 9(a), the anti-correlation coefficient between B and Dst for those MCs which lead to GMSs is r = 0.81. The correlation coefficient between Bz and Dst for those MCs which lead to GMSs as well as the events which have Bz o0 is observed to be r =0.78 which is shown in Fig. 9(b). The significantly high value suggests that when an MC has southward component of Bz, coupling between the magnetosphere and the SW occurs and energy enters into the magnetosphere, resulting in increased geomagnetic activity. Thus, the magnitude of Dst index, which is a measure of the ring current, increases with increase in the magnitude of IMF component, Bz. Hence, B and Bz may be treated as the reliable predictors of GMS’s strength. As suggested by Zhang and Burlaga (1988), for almost same value of Bz minimum, the fall in Dst may be different for different type of MCs, i.e. the cloud having southward component of magnetic field on front of the MC leads to more fall in Dst value than those having northward component on the front. However, the present study suggests that this feature is not always applicable. Seven such events are observed in the present investigation where Bz minimum value is found to be same; whereas, they are associated with different types of MCs. Out of the seven events, only three such events satisfy

the criteria given by Zhang and Burlaga (1988). However, the speed of the MC seems to be more responsible for taking Dst to its minimum value, which is suggested by Zhang and Burlaga (1988), as well. As stated earlier, V, B and Bz show significant correlation with Dst index. Since the GMS is the response of the magnetosphere to IP phenomena arising as a consequence of a Solar event, the coupling between the Sun–Earth parameters seem essential so as to forecast the magnitude of an impending GMS. The correlative study between Dst and the product of V with IMF parameters are helpful in understanding the energy transform from SW to magnetosphere. The strength of the southward IMF or more accurately the dawn–dusk component of the electric field, E= VB describes this process efficiently (Zhang et al., 2007). VB and VBz also show a strong correlation with Dst index as shown in Fig. 9(c, d). These high values are in agreement with the findings of Wu and Lepping (2005), Gonzalez and Echer (2005) and Gopalswamy et al. (2008). Thus, the geomagnetic activity responds to magnetospheric changes brought about by SW and IMF fluctuations. Correlative studies have clearly demonstrated the usefulness of geomagnetic indices as a tool for understanding the coupling between magnetosphere and IP medium. Hence, Plasma flow speed, magnetic field and its southward component play major role in deciding the strength of GMS.

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-100 Dst (nT)

Dst (nT)

-100

-300

-300

Corr. Coeff. = - 0.81

Corr. Coeff. = 0.78

-500

-500 0

20

40

60

-50

-30

B (nT)

-10 Bz (nT)

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Dst (nT)

-100

-300

-300

Corr. Coeff. = - 0.79

Corr. Coeff. = 0.82

-500

-500 0

20

40

60

-35

-25

V.B (*1000)

-15

-5

V.Bz (*1000)

Fig. 9. The dependence of Dst index on (a) B, (b) Bz, (c) VB and (d) VBz.

4. Conclusions Based on the present analysis of the characteristics of the MCs observed from January 1996 to April 2006 and their geomagnetic consequences, the following conclusions have been derived: (1) No systematic trend is observed between SSNs and MCs. (2) SN type of MCs is dominating over NS type of MCs during Solar cycle 23. However, S- and N-type unipolar MCs are found to be almost equal in number. (3) 85% of the total MCs observed during Solar cycle 23 are found to be geoeffective. All the four types of MCs lead to GMSs; whereas, highly intense GMSs of Dst o 300 nT follow only from SN- or S-type MCs. (4) No preference is observed for RH or LH clouds for being geoeffective. (5) Geoeffective MCs generally have high speed. MCs of very high speed lead to intense GMSs. Speed of the MC appear to have significant impact on the strength of GMS. (6) B, Bz, VB and VBz show strong correlation/anti-correlation with Dst index. Thus, IMF (B), its southward component (Bz) and their products with Plasma flow speed (V) play a major role in deciding the strength of GMSs. (7) SN clouds do not always lead to more fall in Dst value (or lead to high strength of GMS) than NS clouds for similar value of Bz minimum associated with both type of MCs.

Acknowledgements The authors are highly indebted to various experimental groups for providing data on the Web. The authors are also

thankful to the anonymous referee for constructive comments and helpful suggestions.

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