www.ann-geophys.net/24/3383/2006/ © European Geosciences Union 2006

We investigated geomagnetic activity which was induced by interplanetary magnetic clouds during the past four solar cycles, 1965-1998. We have found that the in- tensity of such geomagnetic storms is more severe in solar maximum than in solar minimum. In addition, we affirm that the average solar wind speed of magnetic clouds is faster in solar maximum than in solar minimum. In this study, we find that solar activity level plays a major role on the intensity of geomagnetic storms. In particular, some new statistical re- sults are found and listed as follows. (1) The intensity of a geomagnetic storm in a solar active period is stronger than in a solar quiet period. (2) The magnitude of negative Bzmin is larger in a solar active period than in a quiet period. (3) Solar wind speed in an active period is faster than in a quiet period. (4) VBsmax in an active period is much larger than in a quiet period. (5) Solar wind parameters, Bzmin, Vmax and VBsmax are correlated well with geomagnetic storm in- tensity, Dstmin during a solar active period. (6) Solar wind parameters, Bzmin, and VBsmax are not correlated well (very poorly for Vmax) with geomagnetic storm intensity during a solar quiet period. (7) The speed of the solar wind plays a key role in the correlation of solar wind parameters vs. the intensity of a geomagnetic storm. (8) More severe storms with Dstmin 100 nT caused by MCs occurred in the solar active period than in the solar quiet period.


Introduction
Many scientists have tried to understand the fundamental mechanisms that explain the geomagnetic storm since storm Correspondence to: C.-C. Wu (wuc@cspar.uah.edu)patterns were explored by Chapman (1919).The primary cause of a magnetic storm is long duration, intense southward interplanetary magnetic fields which interconnect with the Earth's magnetic field and allow solar wind energy transport into the Earth's magnetosphere (e.g.Tsurutani et al., 1988;Gonzalez et al., 1994).Magnetic clouds (Mcs) are major sources of long-lasting strong southward interplanetary magnetic field (IMF) B z and, therefore, usually cause magnetic storms (Burlaga et al., 1981;Wilson, 1990).However, the largest (in duration) Mcs do not necessarily have the most intense field strengths (Farrugia et al., 1997).Nor are the most intense (strong |B|) clouds the most geoeffective, especially if the MC of interest is highly inclined with respect to the ecliptic plane and possesses a positive axial field.Then there will be little or no negative B z (Lepping et al., 2003).A geomagnetic storm might be driven by a MC itself or by the sheath fields upstream of a MC if an upstream exists (e.g., Lepping and Berdichevsky, 2001;Wu and Lepping, 2002a).
Various changes in the IMF are well known to play a key role in regulating geomagnetic activity (e.g., Fairfield and Cahill, 1966).In particular, the variation of the north-south component of the IMF (B z ), when rendered in the geocentric solar-magnetospheric (GSM) coordinate system, plays a crucial role in determining the amount of solar wind energy that is transferred to the magnetosphere (e.g.Arnoldy, 1971).Using Explorer 12 measurements of the magnetic field outside the magnetosphere and comparing with ground magnetograms from arctic observatories, Fairfield and Cahill (1966) showed that an IMF with a southward component tends to be associated with a ground disturbance, whereas a northward IMF is associated with a quiet condition.The early studies of "solar wind-magnetosphere coupling" have concentrated on a single coupling function (e.g.Perreault and Akasofu, 1978;Kan and Lee, 1979;Doyle and Burke, 1983;Gonzalez et al., 1989) which contains either a single or several solar wind parameters (e.g.Gonzalez et al., 1994).
Published by Copernicus GmbH on behalf of the European Geosciences Union.Akasofu (1981) , proton thermal speed (V th ), plasma bulk speed (V ), proton density (N P ).Burton et al. (1975) presented a formula for predicting the D st index from knowledge of the velocity and density of the solar wind and the north-south component of the interplanetary magnetic field (in GSM coordinates).The intensity of a magnetic storm which is associated with a geomagnetic cloud is predictable (Wu and Lepping, 2005).Wu and Lepping (2002a) used the first four years of WIND magnetic field (Lepping et al., 1995) and plasma data (Ogilvie et al., 1995) to investigate the relationships between solar wind parameters and geomagnetic storms.They found that the intensity of geomagnetic storms (D st min ) is strongly related to the magnitude of solar wind parameters (B z , V B S or in Akasofu, 1981) within magnetic cloud complexes (including upstream sheath regions), and the occurrence tim-ing of a geomagnetic storm is associated with the occurrence timing of Bz min , (V Bs) max or max .D st min is minimum D st , Bz min is minimum B z , (V Bs) max is maximum V Bs, and max is the maximum (Akasofu, 1981) value observed during or in front of a MC event (i.e., in the sheath).(The word "event" here usually means the entire "sheath"/cloud complex.)B s is the southward component of the IMF (B s =|B z | for B z <0 and B s =0 for B z ≥0).Using the solar wind parameters of the apparent 135 magnetic clouds observed between 1965 and 1998 from the OMNI data set, Wu and Lepping (2002b) found that storm intensity (D st min ) is strongly related to the IMF in the z-direction (B z ) and to V Bs, but not well correlated with solar wind velocity alone.However, the relationship between storm intensity and Bz (or V Bs) shows better correlation as the solar wind velocity gets faster.Eighty-two interplanetary magnetic clouds were identified by the MFI team by using WIND solar wind plasma and magnetic field data from solar minimum through the maximum of solar cycle 23 (1995-2003) (Lepping et al., 2006).The occurrence frequency of MCs appears to be related neither to the occurrence of coronal mass ejections as observed by SOHO LASCO nor to the sunspot number (e.g.Wu et al., 2003Wu et al., , 2006)).In contrast, the intensity of geomagnetic storms related to magnetic clouds is affected by both solar activity and the occurrence frequency of CMEs, and more than ∼87% of MCs induced geomagnetic storms (Wu et al., 2003(Wu et al., , 2006)).Wu et al. (2006) also found that the occurrence of magnetic cloud-like structures (MCLs) or the occurrence rate of the joint set (MCs+MCLs) are correlated with both sunspot number and the occurrence rate of CMEs.This motivated us to study several relationships for solar activity vs. various solar wind parameters in order to understand the solar cycle effect on geomagnetic storms which is caused by MCs.
In Sect. 2 we present associated data analysis, storm event selection and the derived storm intensities, D st min , and prediction schemes, which include both velocity dependence and independence.The results will be discussed in Sect.3, and some conclusions are stated in Sect. 4.

Data analysis
Four data sets are used in this study.The first data set, OMNI solar wind plasma and magnetic field data, which were obtained from NSSDC/NASA-GSFC, were used for most earlier events.The second data set, WIND solar wind plasma and magnetic field data, were obtained from the WIND SWE and MFI groups for the events after 1995.Both the third data set (the geomagnetic activity index, D st ) and the forth data set (sunspot number) were obtained from National Geophysical Data Center, Boulder, Colorado, USA.

Magnetic cloud event selection
The MC events used in the present study have been extracted from three previously published reports (Klein and Burlaga, 1982;Bothmer and Rust, 1997;Lepping and Berdichevsky, 2000).Hourly averages of solar plasma and magnetic field data from OMNI and D st during these cloud events were compiled into a data base.Thus, a total of 135 events which cover the period from 1965 to 1998 were obtained for this analysis.(It should be understood that this set of 135 events must be considered a subset of the full set of MCs occurring over the period 1965-1998, since there were many data gaps due to spacecraft tracking limitations and from times when the spacecraft were behind the bowl shock.)This data set has been used to study the effect of solar wind speed on magnetic cloud-associated magnetic storm intensity (Wu and Lepping, 2002b).
Figure 1 shows two examples of MCs which occurred during 22-24 November and 9-11 February 1997.Note that each MC produced a two-step (two main phases) geomagnetic storm.In Fig. 1, we define the following: max is the maximum value of the Akasofu [1981] observed during a cloud event.Note that D st min is the minimum value of D st observed during a cloud "event" (where "event" here and below usually means the entire "sheath"/cloud complex.),V Bs max is the maximum value of V Bs observed during a cloud event, where Bs is the southward component of the IMF (Bs=|B z | for B z <0 and Bs=0 for B z ≥0); Bz min is minimum B z value observed during or in front of a MC event (i.e., in the sheath region, if there is an upstream shock).
The yearly averages of sunspot number and geomagnetic activity index (D st ) for the years 1965-2002 are shown in Fig. 2. The solid line represents yearly averaged D st and the triangles represent yearly averaged sunspot number.The correlation coefficient (c.c.) between yearly sunspot number and D st is -0.59. Figure 2 also shows that solar minima occurred in 1964, 1976, 1986 and 1996 and solar maxima occurred in 1969, 1980, 1991 and 2000.Figure 3 shows plots of D st min vs. |Bz min |, V max and V Bs max with respect to D st and resulting correlation coefficients for the solar active periods (years of 1968, 1969, 1979, 1980, 1981, 1990, 1991, 1992) and quiet periods (years of 1965, 1966, 1975, 1976, 1985, 1986, 1996, 1997).There were 32 MCs observed during the active period and 33 MCs observed during the quiet period.Table 1 summarizes the correlation coefficients of D st min vs. three different solar wind parameters.It shows clearly that all three solar wind parameters are well correlated with D st min during solar maximum but have a poor correlation in solar minimum.For example, the c.c. is quite high (-0.93) between |Bz min | and D st min for the active period.
Figures 4 and 5 show the histograms of V max , D st min , Bz min and V Bs max for both solar quiet and active periods, respectively.Table 2 summarizes the results related to Figs. 4  and 5.Both figures show that: (1) the solar wind speed is faster in solar maximum than in solar minimum; (2) |Bz| at Bz min is greater in solar maximum than in solar minimum; (3) the averaged storm intensity, <D st min > is more intense in solar maximum than the averaged <D st min > in solar minimum; and (4) averaged V Bs max in solar maximum is two times larger than what it is in solar minimum.

Discussion
Using 34 MCs that occurred during 1995-1998, Wu and Lepping (2002a) found that the correlation between storm intensity (D st min ) and the z-component of the interplanetary magnetic field, IMF (Bz), (or with V Bs) is very high.For solar cycle 23, previous studies have shown that the yearly average geomagnetic activity index (D st ) is not correlated well with solar activity (Wu et al., 2003), but the intensity of geomagnetic storms induced by MCs (observed during solar cycle 23) is related to solar activity (Wu et al., 2003(Wu et al., , 2006)).
(The correlation coefficient for the yearly averages of sunspot number vs. yearly averages of intensity of magnetic storms due to MCs is 0.83 (Wu et al., 2006).)Using data from 4 solar cycles, the results of this study show that the correlation coefficient is poor (c.c.= -0.59) between the yearly average geomagnetic activity index and sunspot number.This result reconfirms the reports of previous studies.Cane et al. (2000) studied the relationship between the intensity of geomagnetic storms (with D st min <−50 nT) and southward interplanetary magnetic field strength in ejecta or sheath regions for events associated with front-side halo CMEs during 1996-1999.They found that storm intensity versus the maximum southward magnetic field (Bs) in either the ejecta or the adjacent disturbed solar wind has a correlation coefficient of -0.74.Using 34 MCs observed between 1995 and 1998, Wu and Lepping (2002a) found that the D st min is strongly correlated with V Bs max and |Bz min | with c.c.= -0.79 and -0.77, respectively.Both of the earlier studies (Cane et al., 2000;Wu and Lepping, 2002a) show that the intensity of a storm is strongly correlated with the magnitude of B z .However, both studies cover less than 1/4 of solar activity cycle (where 22 years is a full solar cycle).The correlation coefficient of |Bz min | and D st min is similar for both of these earlier studies (i.e., -0.74 for .Using 135 MCs which occurred during 1965-1998 (33 years, one and a half full solar cycles), Wu and Lepping (2002b) found that the intensity of a geomagnetic storm is primarily related to the solar wind parameters B z and VBs.The results confirmed the two ear-  1965,1966,1975,1976,1985,1986,1996,1997.1968, 1969, 1979, 1980, 1981, 1990, 1991, 1992.lier studies (Cane et al., 2000;Wu and Lepping, 2002a).The c.c. of |Bz min | vs. D st min is -0.86 for that extended study which means that the long term study apparently gives a better correspondence than the short term studies for this correlation.In addition, Wu and Lepping (2002b) also found that the relationships between storm D st min and V max is poor (i.e., c.c. is -0.58), but the correlation for D st min vs.Bz min (or V Bs max ) increased dramatically when solar wind speed  1968, 1969, 1979, 1980, 1981,  1990, 1991 and 1992.b Events occurred during the years 1965, 1966, 1975, 1976, 1985, 1986, 1996, and 1997.increased.For example, the correlation of D st min with Bz min increases from c.c. = -0.86 for all events to c.c. = -0.93 for events with V max >600 km/s (Wu and Lepping, 2002b).That implies that solar wind speed also plays a role in the geomagnetic active but not in any simple way.
The result of this study (see Fig. 3) shows clearly that V max is well correlated with D st min in the solar active period (c.c.= -0.71).In contrast, during the solar quiet period, the correlation is very poor between V max and D st min (c.c.= -0.19).As expected, we notice that the solar wind speed within MCs on average is higher during the active period.The average of V max in the solar active period (522 km/s) is ∼10% higher than in the solar quiet period (468 km/s).
The correlation of D st min vs. different solar wind parameters (e.g., Bz min , V max or V Bs max ) became poor when the Sun became quiet (see Table 1).In addition, the intensity of a storm, in terms of average D st min during MC passage was more than ∼60% stronger in the active period (<D st min >= −100 nT) than in the quiet period (<D st min >= −61.9 nT).The average strength of Bz min in the active period (<Bz min >= -13 nT) is ∼60% greater than in the quiet period (<Bz min >= -7.9 nT).The average strength of V Bs max in the active period (<V Bs max >=6.6) is almost two times greater than that in the quiet period (<V Bs max >=3.3).The overall results show that southward IMF is important for causing a geomagnetic storm, as expected, but solar wind speed also plays a role.
Our earlier study showed that the most severe storms caused by MCs/MCLs (i.e., for D st min ≤−100 nT) during solar cycle 23 occurred in the solar active part of the cycle (Wu et al., 2006).This study shows longer term statistical results (33 years).More than twice as many MCs causing severe geomagnetic storms (D st min ≤-100 nT) occurred in the solar active period as in the quiet period.The result of this study is consistent with the earlier study by Wu et al. (2006).

Conclusions
Solar activity plays a major role in the average intensity of geomagnetic storms.Some new results were found as follows, where MC passage is understood in all statements.(1) The intensity of a geomagnetic storm in a solar active period is significantly stronger than in a solar quiet period.(2) The magnitude of negative Bz min is larger in a solar active period than in a quiet period.(3) Solar wind speed in an active period is faster than in a quiet period.(4) V Bs max in an active period is much larger than in a quiet period.(5) Solar wind parameters, Bz min , V max and V Bs max are well correlated with geomagnetic storm intensity, D st min , during a solar active period.(6) Solar wind parameters, Bz min , and V Bs max are not correlated well (very poorly for V max ) with geomagnetic storm intensity during a solar quiet period.( 7) The speed of the solar wind plays a role in the correlation for solar wind parameters vs. the intensity of a geomagnetic storm.(8) More severe storms with D st min ≤−100 nT caused by MCs occurred in the solar active period than in the solar quiet period.

Fig. 3 .
Fig.3.Relationships and correlation coefficients (c.c.) for storm intensity (D st ) vs. various solar wind parameters (see the text).The left column represents the results for the 32 events the during solar active period; the right column represents the results for the 33 events during solar quiet period.

Fig. 5 .
Fig. 5. Histograms of V max , D st min , Bz min and V Bs max for MCs during the active time of the solar cycle for the years of1968, 1969,  1979, 1980, 1981, 1990, 1991, 1992.

Table 1 .
Summarized correlation coefficient for D st min vs. |Bz min |, V max and V Bs max .
a Events occurred during the years

Table 2 .
Averages of V max , D st min , Bz min and V Bs max for solar quiet and active periods.

Table 3 .
The occurrence frequency of MCs which caused a geomagnetic storm during solar quiet and active periods.