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ISSN 0016 7932, Geomagnetism and Aeronomy, 2013, Vol. 53, No. 2, pp. 147­156. © Pleiades Publishing, Ltd., 2013. Original Russian Text © V.N. Obridko, Kh.D. Kanonidi, T.A. Mitrofanova, B.D. Shelting, 2013, published in Geomagnetizm i Aeronomiya, 2013, Vol. 53, No. 2, pp. 157­166.

Solar Activity and Geomagnetic Disturbances
V. N. Obridko, Kh. D. Kanonidi, T. A. Mitrofanova, and B. D. Shelting
Pushkov Institute of Terrestrial Magnetism, Ionosphere, and Radiowave Propagation, Russian Academy of Sciences (IZMIRAN), Troitsk, Moscow oblast, 142190 Russia e mails: obridko@izmiran.ru; kanonidi@izmiran.ru; tamara_m@izmiran.ru; shelting@izmiran.ru
Received August 23, 2011; in final form, December 14, 2011

Abstract--An analysis of IZNIRAN magnetic observatory data indicated that geomagnetic storms with sud den and gradual commencements form two independent populations with respect to the disturbance occur rence time and character because the solar sources of these disturbances are different. Storms with sudden and gradual commencements are caused by coronal mass ejections and high speed solar wind streams from coronal holes, respectively. DOI: 10.1134/S0016793213010143

1. INTRODUCTION The aim of this work is to study the distribution of magnetic storms with gradual and sudden commence ments (1950­2010) during five solar activity cycles. In addition, it is to a certain degree interesting to con sider the annual variations (distribution over months) in magnetic storms of different origins. The task of this work is to specify the features of dif ferent magnetic disturbances in order to determine the interrelationship between solar activity cycles. We also had to determine the seasonal variations in magnetic storms and their singularities. It is known that two geomagnetic sources exist (Legrand and Simon, 1981, 1989; Venkatesan et al., 1982, 1991; Feynman, 1982; Simon and Legrand, 1989; Gonzalez et al., 1990; Echer et al., 2004). One of the sources (coronal mass ejections or CMEs) more or less agrees with solar cycle variations since it is related to strong closed local fields characterized by the sunspot number, which is the most well known index. High speed solar wind streams are the second source. They are related to coronal holes, which largely depend on large scale and global magnetic fields. Large scale fields also follow the 11 year cycle; however, their maximum is displaced relative to the sunspot cycle maximum (Obridko and Shelting, 1992, 1999; Tlatov and Makarov, 2005; Nagovitsyn, 2006; Obridko et al., 2009, 2011). In many cycles, geomag netic disturbances reach the maximal occurrence slightly later than the sunspot number maximum is formed (Svalgaard, 1977; Legrand and Simon, 1981, 1985; Sargent, 1985; Simon and Legrand, 1989; Tsu rutani et al., 1995, 2006; Wang et al., 2000; Richardson et al., 2002; Schwenn, 2006). Geomagnetic disturbances are also divided into two types: storms with gradual and sudden com mencements. In the present work, we will indicate that

the difference between these disturbances consists in that they are related to different solar agents and there fore form uncorrelated populations independent of each other. 2. DATA Our statistical analysis was based on the data of the geomagnetic observatory for 1950­2010. All data on the geomagnetic field were obtained at IZMIRAN. Data on solar activity were obtained from special literature and the Internet. Data on coronal mass ejections were taken from the catalog presented in (Gopalswamy et al., 2010). Information on coronal holes was obtained from the Jaan Alvestad site Coro nal Hole History (since Late October 2002) and directly from EUV observations on the SOHO space station for an earlier period (Obridko et al., 2011). 3. COMPARISON OF THE MAGNETIC STORM OCCURRENCE WITH THE SUNSPOT NUMBER All initial data are presented in Tables 1 and 2. During the considered period occurred 2607 geo magnetic disturbances of different intensities and character. It turned out that 614 storms (i.e., 24%) had sudden commencements. The remaining 1993 storms (76%) had gradual commencements. Thus, the num ber of storms with sudden commencement was three times as small as that of storms with gradual com mencement. But the number of large and very large storms in the first and second groups considered by us is almost identical. An analysis indicated that the number of magnetic storms with sudden commencement increases in pro portion with an intensification of solar activity and

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Table 1. Magnetic storms of different types (1950­2010) Number of storms Year Total 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 32 59 51 29 23 27 50 53 47 43 58 47 41 21 27 20 30 38 40 31 28 28 33 44 37 34 30 31 45 35 35 Storms with Storms with SC GC 19 52 46 29 22 23 32 29 31 25 42 36 36 18 23 13 19 25 30 17 11 17 23 42 32 31 28 26 27 12 20 13 7 5 0 1 4 18 24 16 18 16 11 5 3 4 7 11 13 10 14 17 11 10 2 5 3 2 5 18 23 15 Number of sunspots Year Total 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 Total 46 73 64 56 37 35 35 46 67 41 53 43 47 54 53 26 30 43 59 72 58 63 109 55 63 40 33 27 7 25 2607 Number of storms Number Storms with of sunspots Storms with SC GC 26 59 50 51 33 31 32 38 37 25 38 33 39 50 49 26 21 38 43 47 37 45 93 42 53 37 30 25 7 22 1993 20 14 14 5 4 4 3 8 30 16 15 10 8 4 4 0 9 5 16 25 21 18 16 13 10 3 3 2 0 3 614
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83.9 69.4 31.5 13.9 4.4 38.0 141.7 190.2 184.8 159.0 112.3 53.9 37.6 27.9 10.2 15.1 47.0 93.7 105.9 105.5 104.5 66.6 68.9 38.0 34.5 15.5 12.6 27.5 92.5 155.4 154.6

140.5 115.9 66.6 45.9 17.9 13.4 29.2 100.2 157.6 142.6 145.7 94.3 54.6 29.9 17.5 8.6 21.5 64.3 93.3 119.6 111.0 104.0 63.7 40.4 29.8 15.2 8.0 3.0 3.1 16.5

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SOLAR ACTIVITY AND GEOMAGNETIC DISTURBANCES Table 2. Distribution of the number of magnetic storms over months (1950 2010) Month I II III IV V VI VII VIII IX X XI XII Total Average monthly value Rms deviation from the average Storms with SC 180 163 179 164 160 148 150 145 184 198 171 151 1993 217.25 19.22 Storms with GC 34 54 59 62 56 53 54 60 52 56 43 31 614 51.17 9.96 All storms 214 217 238 226 216 201 204 205 236 254 214 182 2607 166.08 16.60

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Table 3. Characteristics of magnetic storms Storm characteristic Small Moderate Large Very large D 100­139 140­200 201­290 291 H 80­125 126­200 201­270 271 Z 40­90 91­140 141­250 251

Note: D, H, and Z are the magnetic declination and the horizontal and vertical geomagnetic field components, respectively.

their maximums completely coincide. This is demon strated by Fig. 1a. Solar activity cycles for 1950­2010 and a plot of geomagnetic storms with sudden commencement during the same period are shown here. The behavior of storms with gradual commence ment is different. Their maximum is formed two to three years after the formation of a solar activity max imum (Fig. 1b). We can calculate the correlation between the appearance of these two storm types and the sunspot number. The correlation coefficient between storms with sudden commencement and the sunspot number is rather large (0.872 ± 0.06). The correlation coeffi cient between storms with gradual commencement and the sunspot number is on the contrary very small and is virtually zero (0.014 ± 0.13). However, the cor relation coefficient is increasing if we construct a cor relation function by shifting the sunspot number plot forward (see Fig. 2). If the shift is three years, the correlation coefficient reaches its maximum and is 0.41 ± 0.12.
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Figure 3 shows regression plots for storms with sud den commencement (open circles) without a shift and for storms with gradual commencement (filled circles) at a shift of three years. The numbers of sunspots are shown along the abscissa. We should pay attention to the fact that the angular coupling coefficients almost coincide on the diagrams. They are 0.115 and 0.119 for storms with gradual and sudden commencements, respectively. This difference is statistically insignificant and indicates that the mechanisms by which oncoming solar wind streams interact are identical in both cases. Differences between these two storm types (namely: occurrence frequency, time shift, and disturbance profile) depend on the properties of solar agents responsible for their occurrence. 4. TWO STORM TYPES: DIFFERENT POPULATIONS In this section, we will compare the occurrence fre quency of different power storms with the sunspot number. The magnetic storm value was determined in
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150 120 100 Number of storms 80 60 40 20

OBRIDKO et al. (a) 200 180 160 140 120 100 80 60 40 20 0 200 180 160 140 120 100 80 60 40 20 0 200 180 160 140 120 100 80 60 40 20 0

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0

120 100 Number of storms 80 60 40 20

(c)

2005

2000

1965

1995

1960

1990

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Fig. 1. Distribution of magnetic storms over years (1950­2010): (a) storms with SC, (b) storms with GC, and (c) all storms.

nanoteslas (10­5 oersted), according to the regulations of the International Committee, for the parameters shown in Table 3. All (strong, moderate, and weak) storms with sud den (Fig. 4, solid curves) and gradual (Fig. 5, solid curves) commencements are compared with the sun spot number (dashed line) in Figs. 4 and 5. To simplify the consideration and increase the statistics, we com bined strong and very strong storms in one class. A comparison of Figs. 4 and 5 indicates that storms with all intensities follow the rule formulated by us above:

storms with sudden commencement are in good agreement with the sunspot number curve, and storms with gradual commencement are shifted by one to three years. In this case, the number of strong storms of both types is approximately identical; however, the number of moderate and weak storms with gradual commencement is much larger than a number of storms with sudden commencement of the same power. It is interesting that the plots show a smooth increase in geomagnetic activity with gradual com
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SOLAR ACTIVITY AND GEOMAGNETIC DISTURBANCES Number of magnetic storms 0.5 Correlation coefficient 0.4 0.3 0.2 0.1 0 0 1 2 3 4 Shift in years 5 6 60

151

40

20

0 0 50 100 150 Number of sunspots 200

Fig. 2. Cross correlation function for average annual val ues of sunspots and numbers of storms with GC. The shift in years is shown on the abscissa.

Fig. 3. Regressions of storms with SC (open circles) with out a shift and storms with GC (filled circles) at a shift of three years. The numbers of sunspots are shown on the abscissa.

mencement in the course of time, previously indicated in (Kishcha et al., 1999). In this case, this increase completely depends on an increase in the number of weak storms with gradual commencement and is com pletely absent in storms with sudden commencement. To study the relationship between the occurrence of different storm types with different magnitudes, we calculated their cross correlation. It turned out that the correlation within either type of storms is very high. For storms with gradual commencement, the correlation between the total list of all storms and storms with different magnitudes is 0.84, 0.78, and 0.62 for weak, moderate, and strong storms, respec tively. For storms with sudden commencement, the correlation between the total list of all storms and storms with different magnitudes is 0.78, 0.88, and 0.82 for weak, moderate, and strong storms, respec tively. But the internal correlation between the occur rence frequencies of the weakest and strongest storms decreases slightly, it is 0.24 and 0.37 for storms with gradual and sudden commencements, respectively. A complete absence of correlation between occurence of storms with gradual and sudden com mencements is of prime importance for understanding the nature of these storms. The correlation coefficient is no more than 0.10 with an error of 0.15. Thus, storms of these types form two independent popula tions, which is undoubtedly is determined by a differ ent nature of solar agents exciting storms. 5. SEASONAL VARIATIONS At the next stage of analysis, we distributed the num ber of the studied storms over months. We constructed the corresponding plots (see Table 3; Figs. 6a­6c). It is known that the relative position of the Earth's magnetic dipole axis and the average IMF direction change sys tematically during a year, as a result of which the geoef
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fectiveness of solar wind disturbances changes. There fore, a semiannual wave, with maximums near the spring and autumnal equinoxes and minimums in June and December near the summer and winter solstices, is observed in geomagnetic activity. Figure 6 shows the distribution of magnetic storms occurrens over months (1950­2010) divided by monthly mean for the entire period. The region where the observed occurrence frequency values differ from the mean uniform value by less than one rms deviation () is shaded. It is evident that the effect of an increase in the number of magnetic storms during the autumnal equinox months is actually observed but is rather weak (close to being unreliable). Only storms with gradual commencement (and, as a consequence, all storms) in October have a maximum (~2). For storms with sud den commencement, the maximum in April is very feeble and the minimum in December­January is clearly defined. Legrand and Simon (1985) confirmed the effect based on an analysis of especially strong storms in 1868­1980. However, in their work, a posi tive deviation (~2) from the average is only reached in March, and the negative deviation (1.5) is sub stantially smaller in June and December. It is unclear why the seasonal effect is so insignifi cant. Possibly, its value is in general overstated in liter ature. However, this effect possibly depends on the polarity of the general solar magnetic field (Obridko et al., 2002, 2004). In such a case, we cannot at all consider the seasonal effect by averaging data over sev eral cycles at once. It is necessary to take into account polarity reversals of the general magnetic field and to combine the years when the general solar magnetic field had the same sign. On the other hand, we should note that the occur rence frequency seasonal curves are sharply different for storms with gradual and sudden commencements.
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152 30 Number of storms 25 20 15 10 5 0 1960 12 Number of storms 10 8 6 4 2 0 1960 1980 Years 2000 1980 Years Moderate storms 2000 All storms

OBRIDKO et al. 200 Number of sunspots 150 100 50 0 Number of storms 14 12 10 8 6 4 2 0 1960 12 200 Number of sunspots Number of storms 150 100 50 0 10 8 6 4 2 0 1960 1980 Years 2000 1980 Years Small storms 2000 200 150 100 50 0 Number of sunspots Number of sunspots Number of sunspots 0 100 50 150 Strong storms 200 Number of sunspots

Fig. 4. Annual number of all storms and strong, moderate, and weak storms with SC (solid curves) as compared to the number of sunspots (dashed line).

100 Number of storms 80 60 40 20

All storms

200 Number of sunspots 150 100 50 Number of storms

12 10 8 6 4 2

Strong storms

200 150 100 50

Number of sunspots

0 0 1950 1960 1970 1980 1990 2000 2010 Years 35 Moderate storms 200 30 Number of storms 25 20 15 10 5 0 0 1950 1960 1970 1980 1990 2000 2010 Years 100 50 150

0 0 1950 1960 1970 1980 1990 2000 2010 Years 60 Small storms 200 50 Number of storms 40 30 20 10 50 150 100

0 0 1950 1960 1970 1980 1990 2000 2010 Years

Fig. 5. Annual number of all storms and strong, moderate, and weak storms with GC (solid curves) as compared to the number of sunspots (dashed line). GEOMAGNETISM AND AERONOMY Vol. 53 No. 2 2013


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II

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VI VII VIII Months (b)

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1.0 0.8 0.6 I 1.2 II III IV

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VI VII VIII Months (c)

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1.09 1.04 0.99 0.94 0.93 0.94 0.99 0.84

1.0
0.99

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0.8 0.6 I II III IV V VI VII VIII Months IX X XI XII

Fig. 6. Distribution of magnetic storms over months (1950­2010) divided by monthly mean for entire period: (a) storms with SC, (b) storms with GC, and (c) all storms.

The correlation coefficient between these curves is 0.016 ± 0.29, i.e., being strictly zero. This emphasizes once more that these two storm types are different and directly indicates that the solar agents responsible for their appearance are different. 6. SOLAR AGENTS: CORONAL HOLES AND CORONAL MASS EJECTIONS Geoeffective solar agents are various but can be conditionally divided into two groups. Coronal mass ejections are often related to flares and represent a magnetic cloud that approaches the Earth at a rela tively high velocity (600­1000) km/s. These ejections usually have a sharp leading front and result in the origination of magnetic storms with sudden com mencement (SC). Since these agents are more fre quently related to active regions on the Sun (the so
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called local magnetic fields), we can anticipate (and this is confirmed by statistics) that the correlation between storms with SC and the sunspot number is high. High speed solar wind streams propagate at a velocity of 450­700 km/s and result in the origination of storms with gradual commencement. They are related to solar coronal holes, which reach their max imal development 5­6 years after the sunspot number maximum. Therefore, we can anticipate (and this is confirmed by statistics) that the number of storms with gradual commencement (GC) is shifted relative to the sunspot numbers by a half cycle. Figure 7 shows the distribution of coronal mass ejections, sunspot number, and storms with SC over years in cycle 23. Figure 7 is based on data from (Gopalswamy et al., 2010), but the data of only two catalogs, which are considered to be the most reliable ones, were used: the catalog prepared by the Coordi
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250 Number of coronal mass ejections W > 120° 200 CDAW CACTUS

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100

50

0 1996 1998 2000 2002 2004 2006 2008 Years 25 200 180 160 20 140 120 15 100 80 10 60 40 5 20 0 0 1996 1998 2000 2002 2004 2006 2008 2010 Years Number of storms with SC Number of sunspots
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Fig. 7. Distribution of coronal mass ejections, sunspot number, and storms with SC over years in cycle 23.

nated Data Analysis Workshop (CDAW) and the Computer Aided CME Tracking (CACTus) catalog (Robbrecht and Berghmans, 2004). A solid line on the lower panel in Fig. 7 shows the distribution of the occurrence of storms with SC over years and the sun spot number distribution as a histogram. The coinci dence of all three curves is evident. Figure 8 shows a similar comparison of the distri butions of coronal holes, sunspot numbers, and storms with GC. The similarity of these curves is doubtless; the correlation coefficient is 0.6 ± 0.17, although the number of magnetic storms decreases faster than that of coronal holes as a minimum is approached. This is possibly a specific feature of cycle 23, when the num ber of equatorial coronal holes was unusually large during the decline phase (Obridko and Shelting, 2009). The secondary maximum in the occurrence fre quency of strong storms three years after the sunspot number maximum was previously registered by Leg

rand and Simon (1985); however, these researchers did not relate this maximum directly to coronal holes, which were hardly known at that time. Tsurutani et al. (1995) assumed that this shift is related to cyclic varia tions in open solar magnetic fields and subsequently (Tsurutani et al., 2006) directly indicated that corotat ing flows are related to coronal holes. 7. CONCLUSIONS An analysis, performed based on long series of uni form data, indicated that the manifestations of geo magnetic disturbances with sudden and gradual com mencements are much different. Both disturbance types form two independent populations with respect to the disturbance occurrence time and character. The occurrence of these disturbance types differently depends on the season of the year because their solar sources are different, are differently localized on the solar disk, and show different cyclic variations. Storms with SC and GC are caused by coronal mass ejections
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SOLAR ACTIVITY AND GEOMAGNETIC DISTURBANCES 50
46

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45 Number of solar coronal holes 40 35
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28 28 27 25 23 18 22

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80 60 40 20

10 5 0 1996 1998 2000 2002 2004 2006 2008 2010 1997 1999 2001 2003 2005 2007 2009 0 140
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26 21 38 37 47 43 45

120 100
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80 60
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0 1996 1998 2000 2002 2004 2006 2008 2010 1997 1999 2001 2003 2005 2007 2009 Years

0

Fig. 8. Distribution of coronal holes, sunspot number, and storms with GC over years in cycle 23.

and high speed solar wind streams from coronal holes, respectively. At the same time, the physical mechanisms by which solar agents interact with the Earth's mag netosphere are apparently identical. REFERENCES
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