Li Y, Luhmann J G, Lynch B J
Space Sciences Laboratory, University of California at Berkeley, 7 Gauss Way, Berkeley, CA 94804 USA.
Sol Phys. 2018;293(10):135. doi: 10.1007/s11207-018-1356-8. Epub 2018 Oct 2.
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- 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.
The online version of this article (10.1007/s11207-018-1356-8) contains supplementary material, which is available to authorized users.
磁云（MCs）是太阳风中具有最强南向磁场（Bz南向）的瞬态磁结构。磁云的鞘区也可能携带南向磁场。南向磁场是造成空间天气扰动的原因。我们报告了对1995年至2017年磁云及其鞘区Bz分量的综合分析。303个磁云中85%包含高达50纳特斯拉的南向Bz。23年期间鞘区Bz可能高达40纳特斯拉。最强磁量级和Bz南向的磁云出现在太阳活动周期的下降阶段。双极磁云在极性上依赖太阳活动周期，但在发生频率上不依赖。单极磁云在发生频率上表现出对太阳活动周期的依赖性，但在极性上不依赖。速度最高、总磁量级最大且鞘区Bz南向的磁云起源于更靠近太阳盘面中心的源区。约80%的大Dst风暴由磁云事件引起。鞘区南向Bz和紧接着的南向优先磁云的组合引发了最大的风暴。双极磁云对太阳活动周期的依赖性扩展到了2017年，现在跨度为42年。我们发现，双极磁云Bz极性对太阳活动周期的依赖性由源自衰退活动区中静止暗条的磁云和一组来源不明的弱磁云给出，而具有活动区耀斑的双极磁云的极性总是具有混合的Bz极性，不依赖太阳活动周期，因此对于Bz预测来说是最不可预测的。
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