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How Different Are the Solar Wind-Interplanetary Conditions And Journal of Geophysical Research: Space Physics RESEARCH ARTICLE How Different Are the Solar Wind-Interplanetary Conditions 10.1029/2018JA025683 and the Consequent Geomagnetic Activity During Key Points: the Ascending and Early Descending Phases • Approximately 75% (34%) reduction in intense (moderate) storm of the Solar Cycles 23 and 24? occurrence rate during ascending and early descending phases of cycle 24 compared to cycle 23 is observed Rashmi Rawat1,2 , E. Echer1 , and W. D. Gonzalez1 • Cycle 24 showed no significant difference during equinoctial months 1National Institute for Space Research (INPE), São Paulo, Brazil, 2National Centre for Antarctic and Ocean Research, for intense storms; however, cycle 23 Vasco da Gama, India exhibited equinoctial asymmetry • ICMEs and/or their SH are identified to be the dominant drivers for intense storms during cycle 24 (∼89%) and Abstract The current work investigates the possible solar wind-interplanetary (SW-IP) drivers of cycle 23 (∼64) geomagnetic storms during the longest period (ascending to early descending phases) of the ongoing solar cycle (24). We present a comparative analysis between the two consecutive solar cycles (SCs) 23 and Correspondence to: 24. Both the cycles exhibited dual peak feature as observed in the smoothed sunspot numbers SSNsmoothed. R. Rawat, For both the cycles, second peak in the SSNsmoothed is higher than the first one as exhibited in the revised [email protected] SSNsmoothed version. During the entire interval between the ascending to early descending phases (December 2008 to December 2016) of SC-24, the southward directed Bz and the dawn-dusk electric Citation: field (Ey) were consistently weaker as compared to that during similar interval of SC-23 (May 1996 to July Rawat,R.,Echer,E.,&Gonzalez,W.D. (2018). How different are the solar 2004). The geomagnetic field response represented by Dst index concurrently exhibited similar variation wind-interplanetary conditions patterns during both the periods. A striking reduction in the intense storm occurrence rate by ∼75% and the consequent geomagnetic was observed during the considered period of the current solar cycle in comparison to the previous cycle. activity during the ascending and early descending phases of However, moderate storm occurrence was reduced only by 32% in SC-24 as compared to SC-23, which the solar cycles 23 and 24? could be attributed to the dominance of corotating interaction regions during SC-24. No significant Journal of Geophysical Research: difference is found between the intense storm rates around the vernal and autumnal equinoxes in cycle 24, Space Physics, 123, 6621–6638. https://doi.org/10.1029/2018JA025683 whereas distinct autumnal equinoctial dominance is evident for cycle 23. Further, within each cycle, there is no significant difference in the moderate storm rates around vernal and autumnal equinoxes. Received 17 MAY 2018 Accepted 14 JUL 2018 Accepted article online 24 JUL 2018 1. Introduction Published online 28 AUG 2018 The sunspot numbers (SSNs) and areas are used as proxies to assess the solar activity evolution. On an aver- age, the sunspot cycle and thus the solar activity exhibit a periodicity of 11 years known as the Schwabe cycle (Schwabe, 1844). The solar cycle (SC) is produced by the solar dynamo within the Sun and hence is magnetic in nature. Each sunspot cycle has three typical phases. The period characterized by an increase in SSNs, their groups and areas are designated as the ascending phase, followed by the maximum phase, which is distin- guished by maximization of SSNs, groups, and areas. Eventually, a steady decline in the number of sunspots marks the declining phase of the cycle. The sunspot cycles are known to exhibit double peaked structure during the maximum activity period. First reporting of the dual peak feature was made by Gnevyshev (1963, 1967) for the SC-19, based on the 530.3-nm coronal line observations. The interval between the two peaks when the SSNs diminish is referred as the Gnevyshev Gap (GG; Storini & Pase, 1995). Antalova and Gnevyshev (1965) affirmed that Gnevyshev (1963) conclusions remained unchanged for the other sunspot cycles as well. For this, Antalova and Gnevyshev (1965) superposed the sunspot curves of eight sunspot cycles from 1874 to 1962 and obtained that there are always two maxima in the sunspot cycle. The double peak characteris- tic of the SC is associated with the sunspots in northern and southern solar hemispheres resulting from the global reorganization of the solar magnetic fields (Feminella & Storini, 1997; Norton & Gallagher, 2010; Storini et al., 2003). Different phases of the SC witness diverse solar emissions like coronal mass ejections (CMEs), solar flares, and high-speed streams (HSS) that emanate from the coronal holes. Typically, the maximum phase is dominated ©2018. American Geophysical Union. by CMEs and solar flares. In contrast, the ascending and descending phases are dominated by HSS, which All Rights Reserved. form the corotating interaction regions (CIRs) at their leading edges where they collide with the preceding RAWATETAL. 6621 Journal of Geophysical Research: Space Physics 10.1029/2018JA025683 slower solar wind (Tsurutani & Gonzalez, 1997). The interplanetary counterparts of CMEs are known as inter- planetary coronal mass ejections (ICMEs). ICMEs with flux rope structures are called as magnetic clouds (MCs). The MCs are characterized by high magnetic field, abnormally low proton temperature, large rotation of field, and low plasma beta (Burlaga et al., 1981). Dominance of these different solar wind-interplanetary (SW-IP) drivers during various phases of SC leads to varying intensity of geomagnetic activity. The geoeffectiveness of these interplanetary structures is measured in terms of the intensity of geomagnetic activity in correspon- dence to the solar wind and interplanetary magnetic field (IMF) behavior exhibited by the structures. It is well established that IMF Bz plays a crucial role for the efficient energy transfer from the solar wind into the magnetosphere through the magnetic reconnection process (Dungey, 1961; Gonzalez et al., 1994). Further, dawn-to-dusk component of the interplanetary electric field (Ey = Vx × Bz), where Vx is the x component of Vsw, plays a pivotal role for the ring current injection during geomagnetic storms (Burton et al., 1975; Kan & Lee, 1979; Rawat et al., 2010). ICMEs are predominant drivers of nonrecurrent intense to severe geomagnetic activity, primarily due to the presence of large southward directed Bz within them (Echer et al., 2008; Gonzalez et al., 1999, 2007; Zhang et al., 2007). CIRs, on the other hand, generally lead to recurrent moderate (Alves et al., 2006; Yermolaev et al., 2012) and sometimes intense activity (Richardson et al., 2006) generally driven by the southward fields asso- ciated with Alfvenic fluctuations in the IMF. The current SC is 24th that commenced in December 2008 after a deep solar minimum following the SC-23 (Tsurutani et al., 2011). It would be extremely interesting to com- pare the various activity levels during both the SCs. Several studies have been carried out discussing the deep solar minimum and the ongoing SC (e.g., Gopalswamy et al., 2015; Kilpua et al., 2014; Lepping et al., 2015; Richardson, 2013; Richardson & Cane, 2012b; Shanmugaraju et al., 2015). Richardson and Cane (2012b) and Richardson (2013) discussed the solar wind and geomagnetic activity over several cycles (1964–2011) and SC-24, in particular (2008–2012). Kilpua et al. (2014) and Selvakumaran et al. (2016) examined the inter- planetary causes of low geomagnetic activity during the current SC. In more recent studies, Shanmugaraju et al. (2015) analyzed halo CMEs and their geoeffective parameters during 2011–2013 and Gopalswamy et al. (2015) discussed the MC properties during first 73 months of SCs 23 and 24 in order to ascertain the cause of the weaker geomagnetic activity during the current cycle. Shen et al. (2017) discussed the comparison of geo- effectiveness of several types of ICMEs during different solar phases for the period between 1995 and 2014. They also concluded that the probabilities of ICMEs in causing geomagnetic storms in cycle 24 are much lower compared to cycle 23. We investigate the possible SW-IP drivers of the moderate and intense geomagnetic storms during the longest span of the current SC (24), covering ascending to early descending phases (EDPs) between December 2008 and December 2016. Therefore, our present work is the most updated study of the ongoing SC. Current study delineates the differences between the ascending to EDPs of the two consecutive SCs 23 and 24 in terms of storm occurrence distribution during both the periods, seasonal variation of storms, SW-IP conditions, and their corresponding geomagnetic response. Section 2 explains the method of determining the periods under study, the database used and selection criteria of the storm events, and identification method of their possible SW-IP drivers. Statistical results and their discussion are described in section 3, followed by the conclusion in section 4. 2. Data and Method 2.1. Sunspot Numbers For the purpose of examining the SC variation we use SSN, which is the commonly used solar activity index. We utilize the widely used International smoothed monthly mean SSNs (SSNsmoothed), which contain the averaged over fast variations like random surges and 27-day rotational modulation. The revised version of SSNsmoothed data is available between 1749 and May 2017 at the time of writing of this manuscript from Sunspot Index and Long-term Solar Observations (SILSO). However, after December 2016, the SSNsmoothed data are provisional and subject to a possible revision. SSNsmoothed value is obtained by computing a 13-month running mean of SSNs, centered at a base month and using half weights for the start and end months (Hathaway, 2010). Figure 1 portrays the SILSO SSNsmoothed data for the two SCs 23 and 24. It can be distinctly observed that both the cycles exhibited dual peaks. The two peaks in SSNsmoothed for SC-23 were observed in April 2000 and November 2001 (marked by red circles), while for SC-24 were noticed in March 2012 and April 2014 (marked by blue circles).
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