Relationships among magnetic clouds, CMEs and geomagnetic storms
Wu, C.-C, R.P. Lepping, N. Gopalswamy
During solar cycle 23, 82 interplanetary magnetic clouds (MCs) were identified by the MFI team using Wind solar wind plasma and magnetic field data for the interval January 1995 - August 2003. The average occurrence rate is 9.5 MCs per year for the overall period. It is found that some of the anomalies in the frequency of occurrence were during the early part of solar cycle 23: (i) only 4 MCs were observed in 1999, and (ii) an unusually large number of MCs (17 events) were observed in 1997, when the Sun was starting to leave solar minimum. We also discuss the relationship between MCs, coronal mass ejections (CMEs) and geomagnetic storms. During the period, 1996-2003, almost eight thousand CMEs were observed by SOHO LASCO. The occurrence frequency of MCs appears to be related neither to the occurrence of CMEs as observed by SOHO LASCO nor to the sunspot number. When we included "magnetic cloud-like structures" (MCLs, defined by Lepping et al. [2005b]), we found that the occurrence of the joint set (MCs+MCLs) is correlated with both sunspot number and the occurrence rate of CMEs. The average duration of the MCL structures is (~40%) shorter than that of the MCs. In addition, the MCs are typically more geoffective than the MCLs. Since on average |B| is greater in MCs than in MCLs, then it is expected that |Bz | at Bzmin is generally greater. Also typically the duration of a MC is greater than that for MCL strusture, and, therefore, the interval of negative Bz is usually longer. Both of these factors (|Bz | magnitude and duration of negative Bz) apparantly play a role in the difference in geomagnetic effectiveness between MCs and MCLs.
Solar Physics, submitted, 2006.