The source, statistical properties, and geoeffectiveness of long-duration southward interplanetary magnetic field intervals

摘要

Geomagnetic activity is strongly controlled by solar wind and interplanetary magnetic field (IMF) conditions, especially the southward component of IMF (IMF Bs). We analyze the statistical properties of IMF Bs at 1 AU using in situ observations for more than a solar cycle (1995–2010). IMF Bs events are defined as continuous IMF Bs intervals with varying thresholds of Bs magnitude and duration and categorized by different solar wind structures, such as magnetic cloud (MC), interplanetary small-scale magnetic flux rope, interplanetary coronal mass ejection without MC signature (ejecta), stream interacting region, and Shock, as well as events unrelated with well-defined solar wind structures. The statistical properties of IMF Bs events and their geoeffectiveness are investigated in detail based on satellite and ground measurements. We find that the integrated duration and number of Bs events follow the sunspot number when Bz < ?5 nT. We also find that in extreme Bs events (t > 6 h, Bz < ?10 nT), a majority (53%) are related to MC and 10% are related with ejecta, but nearly a quarter are not associated with any well-defined solar wind structure. We find different geomagnetic responses for Bs events with comparable duration and magnitude depending on what type of solar wind structures they are associated with. We also find that great Bs events (t > 3 h, Bz < ?10 nT) do not always trigger magnetic storms.