Comparisons of characteristics of magnetic clouds and cloud-like structures during 1995-2012
Thursday, 18 December 2014
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 geomagnetic activity index (Dst), we have identified 168 magnetic clouds (MCs) and 197 magnetic cloud-like structures (MCLs), and we made relevant comparisons. The following features are found during seven different periods [Total Period (TP) during 1995-2012, first and second half period during 1995-2003 (P1) and 2004-2012 (P2), Quiet periods during 1995-1997 (Q1) and 2007-2009 (Q2), Active periods during 1998-2006 (A1) and 2010-2012 (A2)]. (1) During 1995-2012 the yearly occurrence frequency is <NMCs>TP = 9.3 for MCs and <NMCLs>TP =10.9 for MCLs. (2) In the quiet periods, <NMCs>Q1,Q2 is higher than <NMCLs> Q1,Q2 (i.e., <NMCs>Q1 > <NMCLs>Q1 and <NMCs>Q2 > <NMCLs>Q2), but <NMCs> Q1,Q2 is lower than <NMCLs> during the active periods (i.e., <NMCs>A1 < <NMCLs>A1 and <NMCs>A2 < <NMCLs>A2). This is probably due to the lower interaction rate between MCs/MCLs and the quiet background solar wind during lower solar active periods in Q1, Q2, and higher interaction rate and highly disturbed background solar wind during the active periods in A1 and A2. (3) The minimum Bz (Bzmin) inside of a MC is well correlated with the intensity of geomagnetic activity, Dstmin (minimum Dst found within a storm event) for MCs (correlation coefficient, c.c. = 0.75 and the fitting function is Dstmin = -1.74+ 7.23 Bzmin), but Bzmin is not well correlated with MCLs (c.c. = 0.57). (4) MCs play a major role in producing geomagnetic storms: the absolute value of the average Dstmin for MCs (<Dstmin>MC = -70 nT) associated geomagnetic storms is two times stronger than that for MCLs (<Dstmin>MCL = -35 nT) due to the difference in the IMF (interplanetary magnetic field) strength. (5) Over the Total Period the SSN is not correlated with <NMCs>TP (c.c. = 0.27), but is well associated with <NMCLs>TP (c.c. = 0.85). Note that the c.c. for SSN vs. <NMCs>P2 is better than that for SSN vs. <NMCLs>P2. (6) Averages of IMF, solar wind speed, and density inside of the MCs are larger than those inside of the MCLs. (7) Average of MC duration (~18.82 hours) is ~20% longer than the average of MCL duration (~15.69 hours).
*Work of CCW was supported by ONR 6.1 program