Abstract
Magnetic clouds (MCs) are transient magnetic structures giving the strongest southward magnetic field (Bz south) in the solar wind. The sheath regions of MCs may also carry a southward magnetic field. The southward magnetic field is responsible for space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions for 1995 to 2017. 85% of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. MCs of the strongest magnetic magnitude and Bz south occur in the declining phase of the solar cycle. Bipolar MCs depend on the solar cycle in their polarity, but not in the occurrence frequency. Unipolar MCs show solar-cycle dependence in their occurrence frequency, but not in their polarity. MCs with the highest speeds, the largest total-B magnitudes, and sheath Bz south originate from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. Combinations of a south Bz in the sheath and south-first MCs in close succession have caused the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017 and now spans 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs that originated from quiescent filaments in decayed active regions and a group of weak MCs of unclear sources, while the polarity of bipolar MCs with active-region flares always has a mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.
Highlights
Coronal mass ejections (CMEs) may carry an enhanced magnetic field and fast-moving plasma into the heliosphere
Detailed analyses of modern era solar-wind and interplanetary magnetic field (IMF) data from 1995 to 2017, spanning the recent two Solar Cycles 23 and 24, have been carried out. We have studied both bipolar and unipolar magnetic clouds (MCs). In addition to their occurrences and orientation polarities, we investigated the solar-cycle and source-region dependence of the MC events’ maximum speed, magnetic magnitude, Bz south component within the MCs, and in the sheath regions ahead of the MCs
The occurrence of bipolar MCs has a less clear solar-cycle dependence, while their polarity does have a clear dependence on the solar cycle
Summary
Coronal mass ejections (CMEs) may carry an enhanced magnetic field and fast-moving plasma into the heliosphere. The Bz field at the leading portion of a bipolar MC is the same as the solar global dipole field ( noted by Mulligan, Russell, and Luhmann, 1998) This finding suggests that MCs preferentially remove the like polarity of the solar dipole field, thereby supporting the idea that CMEs play a role in the solar magnetic cycle. After a brief description of the data source and events selection (Section 2), we present a comprehensive study of MCs encountered at the L1 point from 1995 to 2017, when we have continuous high-quality data We analyze both bipolar and unipolar MCs to investigate the solar-cycle dependence of the Bz field, as well as the south Bz component in the sheath region ahead of the MCs (Section 3). We conclude the article with discussions and conclusions regarding the overall MC characteristics and their implications (Section 7)
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