RELATIONSHIPs among Geomagnetic storms, interplanetary shocks, magnetic clouds, and SUNSPOT NUMBER during 1995-2012

Abstract:

During 1995-2012 Wind recorded 168 magnetic clouds (MCs), 197 magnetic cloud-like structures (MCLs), and 358 interplanetary (IP) shocks. Ninety four MCs and 56 MCLs had upstream shock waves. The following features are found: (i) Averages of solar wind speed, interplanetary magnetic field (IMF), duration (<Δt>), strength of Bz_min, and intensity of the associated geomagnetic storm/activity (Dst_min) for MCs with upstream shock waves (MC_SHOCK) are higher (or stronger) than those averages for the MCs without upstream shock waves (MC_NO-SHOCK). (ii) The <Δt> of MC_SHOCK events (≈19.6 hr) is 9% longer than that for MC_NO-SHOCK events (≈17.9 hr). (iii) For the MC_SHOCK events, the average duration of the sheath (<Δt_SHEATH>) is 12.1 hrs. These findings could be very useful for space weather predictions, i.e. IP shocks driven by MCs are expected to arrive at Wind (or at 1 AU) about ~12 hours ahead of the front of the MCs on average. (iv) The occurrence frequency of IP shocks is well associated with sunspot number (SSN). The average intensity of geomagnetic storms measured by <Dst_min> for MC_SHOCK and MC_NOSHOCK events is -102 and -31 nT, respectively. The <Dst_min> is -78, -70, and -35 nT for the 358 IP shocks, 168 MCs, and 197 MCLs, respectively. These results imply that IP shocks, when they occur with MCs/MCLs, must play an important role in the strength of geomagnetic storms. We speculate as to why this is so. Yearly occurrence frequencies of MC_SHOCK and IP shocks are well correlated with solar activity (e.g., SSN). Choosing the right Dst_min estimating formula for predicting the intensity of MC-associated geomagnetic storms is crucial for space weather predictions.