Earthq Sci (2011)24: 365–372 365 doi:10.1007/s11589-011-0799-x Statistical study on great geomagnetic storms during solar cycle 23∗ Qi Li1, Yufen Gao1 Peiyu Zhu2 Huaran Chen1 Xiuling Zhang3 1 Institute of Geophysics, China Earthquake Administration, Beijing 100081, China 2 Chongming Observatory, Earthquake Administration of Shanghai Municipality, Shanghai 202164, China 3 Beijing Observatory, Institute of Geophysics, China Earthquake Administration, Beijing 100095, China Abstract Characteristics of great geomagnetic storms during solar cycle 23 were statistically investigated. Firstly, we focused on the uniqueness of solar cycle 23 by analyzing both the great storm number and sunspot number from 1957 to 2008. It was found that the relationship between the sunspot number and great storm number weakened as the activity of the storms strengthened. There was no obvious relationship between the annual sunspot number and great storm number with Dst≤−300 nT. Secondly, we studied the relationship between the peak Dst and peak Bz in detail. It was found that the condition Bz≤−10 nT is not necessary for storms with Dst≤−100 nT, but seems necessary for storms with Dst≤−150 nT. The duration for Bz≤−10 nT has no direct relationship with the giant storm. The correlation coefficient between the Dst peak and Bz peak for the 89 storms studied is 0.81. After removing the effect of solar wind dynamic pressure on the Dst peak, we obtained a better correlation coefficient of 0.86. We also found the difference between the Dst peak and the corrected Dst peak was proportional to the Dst peak. Key words: geomagnetic storms; solar cycle 23; sunspot number; 4.5 solar cycles; peak Dst and peak Bz CLC number: P318.2+2 Document code:A 1 Introduction four low-latitude stations. Gonzalez et al. (1994) defined the storms with −30 nT≤Dst≤−50 nT as weak storms, those with −50 nT≤Dst≤−100 nT as moderate storms It is well known that magnetic activity observed and those with Dst≥−100 nT as great or intensive mag- on the ground depends on solar activity. Chapman and netic storms, and the classification has been extensively Bartels (1940) found that geomagnetic activity is cor- adopted. related with the sunspot number, so the occurrence of The input energy of geomagnetic storms comes magnetic storms has the same period of the solar cycle. from the Sun, and the energy is transferred into the A magnetic storm is a global geomagnetic field dis- Earth’s magnetosphere by streams of solar wind. The turbance characterized by a distinct decrease in the hor- energy from solar wind is injected into the magneto- izontal component of the magnetic field at mid- and sphere only in the case when the interplanetary mag- low-latitudes. It is generally accepted that the source netic field (IMF) has a significant component parallel of the magnetic disturbance is a westward ring current to the terrestrial magnetic dipole; i.e., an approximately circling the Earth in the equatorial plane. The strength negative (southward) Bz component of the IMF (Rus- of a magnetic storm is usually estimated using the Dst sell et al., 1974; Perreault and Akasofu, 1978; Tsurutani index, which is derived from hourly values of the hori- and Gonzalez, 1997; Gonzalez et al., 2007; Echer et al., zontal component of the geomagnetic field recorded at 2008a, b). The mechanism for energy transfer between the solar wind and magnetosphere is the magnetic re- ∗ Received 12 May 2011; accepted in revised form 28 July 2011; published 10 August 2011. connection between the IMF and the Earth’s magnetic Corresponding author. e-mail: [email protected] field (Dungey, 1961), and the energy transfer efficien- The Seismological Society of China and Springer-Verlag Berlin Heidelberg 2011 cy is of the order of 10% during intensive magnetic 366 Earthq Sci (2011)24: 365–372 storms (Gonzalez et al., 1989). Gonzalez et al. (2011a) cycle 23. In section 3, we investigated the relationship reviewed the interplanetary causes of intensive geomag- between the peak Dst and peak Bz in detail. We be- netic storms, and pointed out that the most common lieve that these studies are important for space weather interplanetary structures leading to the development of forecasting. intensive storms were magnetic clouds, sheath fields, sheath fields followed by a magnetic cloud and coro- 2 The uniqueness of solar cycle 23 tating interaction regions at the leading fronts of high speed streams. Gonzalez and Tsurutani (1987) found 2.1 Data preparation that intensive storms with peak Dst≤−100 nT are pri- The one-hour Dst index obtained from the World marily caused by large fields with Bz≤−10 nT and a du- Data Center for Geomagnetism, Kyoto was used to ration longer than three hours. Kane and Echer (2007) characterize the intensity of a magnetic storm. The Dst examined the evolutions of severe geomagnetic storms index has been published since 1957, so we studied (Dst<200 nT) during solar cycle 23, and noticed that storms latter than that year. The Dst index is avail- the phase delay between the maximum of interplanetary able in its final form for the period from 1957 to 2003. negative Bz and the maximum of negative Dst varied in It should be noted that the Dst index after 2003 is pro- a very wide range from storm to storm. Zhang et al. visional, and that the Dst index after 2006 was given (2011) indicated that the product of solar wind density in real time when this work was undertaken, and hence and the south component of IMF determine the devel- the final index could be a little different. The interna- opment of the main phase of a geomagnetic storm. tional sunspot number was obtained from the National Solar cycle 23 started in April 1996 with a sunspot Geophysical Data Center of the National Oceanic and number of 8.0 and reached its peak in March 2000 with Atmospheric Administration. All magnetic storms with a maximum sunspot number of 120.8. Afterward, the Dst≤−100 nT from 1957 to 2008 were used. sunspot number for each year decreased toward a min- 2.2 Comparison between solar cycle 23 and imum. The maximum sunspot number of solar cycle 23 other cycles was lower than those of solar cycles 19, 21 and 22. How- There were 402 great storms with Dst≤−100 nT ever, cycle 23 was very active, and there were five giant from 1957 to 2008, with an average of 7.7 storms per magnetic storms with Dst≤−300 nT in the declining year; the maximum number of such storms was 20, both years 2001, 2003 and 2004. Furthermore, three of these in 1960 and 1989. There were 71 great storms with giant storms occurred within one month. In addition, Dst≤−200 nT during the same period, with an aver- there are more satellite data for the Sun and IMF in age of 1.4 storms per year; the maximum number of solar cycle 23 than that in previous solar cycles, which such storms was 7 in 1957. There were 22 giant storms is favorable for research work on space weather. Solar with Dst≤−300 nT, with an average of 0.4 storms per cycle 23 is very interesting and has been the subject year; the maximum number of such storms was three in of much research (Dal et al., 2004; Gopalswamy et al., 1957, 1958, 1960 and 2003. The years 1957, 1958 and 2005; Gonzalez and Echer, 2005; Gonzalez et al., 2007). 1960 were in solar cycle 19 and only the year 2003 was In this paper, we statistically investigated charac- in solar cycle 23. teristics of great geomagnetic storms during solar cycle Table 1 shows the annual occurrence rate of great 23. In section 2, we focused on the uniqueness of solar magnetic storms of different intensities. The table shows cycle 23 by analyzing both the great storm number and that the annual occurrence rate in cycle 23 was only low- sunspot number from 1957 to 2008, which covers 4.5 so- er than that in cycle 19 for storms with Dst≤−200 nT lar cycles from the latter part of solar cycle 19 to solar and Dst≤−300 nT. Table 1 Annual occurrence rate of great magnetic storms with different intensities Annual occurrence rate Intensity of storms All storms Cycle 19 Cycle 20 Cycle 21 Cycle 22 Cycle 23 Dst≤−100nT 7.7 10.9 4.4 7.9 9.5 7.3 Dst≤−200nT 1.4 2.5 0.75 1.3 1.1 1.5 Dst≤−300nT 0.4 1.25 0.08 0.3 0.2 0.5 Earthq Sci (2011)24: 365–372 367 Figure 1 shows the ratio of the storm number to from 1957 to 2008. Table 2 shows the occurrence date the maximum annual number of sunspots for each solar and times of these storms according to Dst. Among cycle. It can be seen that the ratio is the greatest in cy- them, 10 storms appeared in cycle 19 and six storms cle 23 for storms with Dst≤−100 nT or Dst≤−200 nT. appeared in cycle 23. Table 2 indicates that four of the There were 22 giant storms with Dst≤−300 nT 10 most intense giant storms appeared in cycle 23. (a) (b) ˉ ˉ Dst≤ˉ nT Dst≤ˉ nT Storm number/Sunspot maximum Storm number/Sunspot maximum Solar cycle Solar cycle Figure 1 Ratio of the storm number to the maximum annual number of sunspots in each solar cycle for magnetic storms with Dst≤−100 nT (a) and for magnetic storms with Dst≤−200 nT (b).