Comparisons of Characteristics of Magnetic Clouds and Cloud-Like Structures During 1995 – 2012

Abstract

Using eighteen years (1995 – 2012) of solar wind plasma and magnetic field data (observed by the Wind spacecraft), solar activity (e.g. sunspot number: SSN), and the geomagnetic-activity index (Dst), we have identified 168 magnetic clouds (MCs) and 197 magnetic-cloud-like structures (MCLs), and we have made relevant comparisons. The following features are found during seven different periods (TP: total period during 1995 – 2012, P1 and P2: first and second half-period during 1995 – 2003 and 2004 – 2012, Q1 and Q2: quiet periods during 1995 – 1997 and 2007 – 2009, A1 and A2: active periods during 1998 – 2006 and 2010 – 2012). (1) During the total period, the yearly occurrence frequency is 9.3 for MCs and 10.9 for MCLs. (2) In the quiet periods 〈N MCs〉Q1 > 〈N MCLs〉Q1 and 〈N MCs〉Q2 > 〈N MCLs〉Q2, but in the active periods 〈N MCs〉A1 < 〈N MCLs〉A1 and 〈N MCs〉A2 < 〈N MCLs〉A2. (3) The minimum Bz (Bz min) inside of an MC is well correlated with the intensity of geomagnetic activity, Dstmin (minimum Dst found within a storm event) for MCs (with a Pearson correlation coefficient, $\mathrm{c.c.} =0.75$ , and the fitting function is Dstmin=0.90+7.78Bz min), but Bz min for MCLs is not well correlated with the Dst index ( $\mathrm{c.c.} =0.56$ , and the fitting function is Dstmin=−9.40+4.58Bz min). (4) MCs play a major role in producing geomagnetic storms: the absolute value of the average Dstmin (〈Dstmin〉MC=−70 nT) for MCs associated geomagnetic storms is twice as strong as that for MCLs (〈Dstmin〉MCL=−35 nT) because of the difference in the IMF (interplanetary magnetic field) strength. (5) The SSN is uncorrelated with MCs (〈N MCs〉TP, $\mathrm{c.c.} = 0.27$ ), but is well associated with MCLs (〈N MCLs〉TP, $\mathrm{c.c.} = 0.85$ ). Note that the c.c. for SSN vs. 〈N MCs〉P2 is higher than that for SSN vs. 〈N MCLs〉P2. (6) Averages of IMF, solar wind speed, and density inside of the MCs are higher than those inside of the MCLs. (7) The average of MC duration (≈ 18.82 hours) is ≈ 20 % longer than the average of MCL duration (≈ 15.69 hours). (8) There are more MCs than MCLs in the quiet solar period and more MCLs than MCs in the active solar period, probably as a result of the interaction between an MC and another significant interplanetary disturbance (including another MC), which could obviously change the character of an MC, but we speculate that some MCLs are no doubt due to other factors such as complex birth conditions at the Sun.