Statistical properties of southward IMF and its geomagnetic effectiveness X. Zhang, M. B. Moldwin

1. Statistical properties of southward IMF and its geomagnetic effectiveness X. Zhang, M. B. Moldwin Department of Atmospheric, Oceanic, and Space Sciences, University of Michigan SM11D-2318 Abstract The Interplanetary Magnetic Field (IMF) z- component at 1 AU and its geoeffectiveness have been studied using in-situ observations. It is shown that the duration and number of strong southward IMF ( Bz < -5 nT) presents an obvious trend as solar activity. It is also found that there are less Bs intervals during the well-defined Solar Wind (SW) structures, e.g. MC, SMFR, SIR, Shock, than the others. The statistical results indicate that the distributions of the properties of IMF Bs are different for SW structures. It is interesting to know that prolonged and strong Bs intervals are not always correlated with any structures, or even trigger geomagnetic storms. Methodology We use WIND MFI data to select the Bs-event in the following steps: (1). Set up the lower threshold of the magnitude and duration of IMF southward component; (2). Automatically scan the data to find the intervals satisfying the conditions in (1), allowing single unqualified data point; (3). Select out the Bs-events from (2) that are related to the Solar Wind (SW) structures identified in previous studies; (4). Find the corresponding intervals in OMNI data and analyze the geomagnetic activity indices, e.g. Dst, SYM-H, PC, AE. Discontinuity Bs-event w/ intense storm Other Bs-event w/ moderate storm Other Bs-event w/o storm • Bs-event examples not related to well-defined SW structures • Discontinuity (excluding Shock) are often observed in the other Bs-events. • Prolonged and strong Bs-events not guarantee to induce geomagnetic storms. • Distribution of minimum Bz, duration and SYM-H for Bs-events in different categories. • Magnetic Clouds have the largest average of Bs magnitude with a long tail in the distribution. • There are more cases with minimum Bz less than -10 nT in the other Bs-events than the ones in SMFR. • Bs-events related with SMFR is unlikely to induce geomagnetic storms. • There are intense storms triggered by Bs-events not identified as SW structures. • Yearly trend of total duration and count of Bs-events • Strong Bs-events (Bz < -5 nT) show significant trend as solar activity. • Bs-events (Bz < 0 nT) is not obviously following the solar cycle. • Conclusion • The duration of strong southward Bz (less than -5 nT) is significantly correlated with solar activity, showing the signature of a solar cycle. • More Bs-events are not related to any type of well-defined Solar Wind (SW) structures, the properties of Bs show different among these structures. • There are Bs-events ( t > 5 hrs, Bz < -5 nT) not included in any SW structures, or triggering geomagnetic storms. • Future work • Investigate data from other spacecraft in the region between the Sun and the Earth to seek for the sources of the Bs-events unrelated with SW structures. • Use SWMF to model the response of Earth’s magnetosphere to strong Bs-events without storm occurrence, and compare with the observations for model validation and more knowledge about the Sun-Earth coupling theory. • Contact Information: zhangxy@umich.edu • Distribution of Bs-events in different SW structures and other • The undefined type dominates in the shorter and weaker Bs-events. • For strong and prolonged Bs-events, Magnetic Cloud increases and shares the same portion as the undefined ones. MC Bs-event SIR Bs-event Shock Bs-event • Bs-event examples in different solar wind structures. • The other parameters in SW and IMF are different among different structures in these events, but all of them trigger moderate and intense geomagnetic storms.