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arXiv 1808.04078astro-ph.SRphysics.space-ph
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磁云:太阳周期依赖性、源和地磁影响

Magnetic Clouds: Solar Cycle Dependence, Sources, and Geomagnetic Impacts

Y. Li, J. G. Luhmann, B. J. Lynch

摘要:

磁云(MC)是在太阳风中产生最强南向磁场(Bz south)的瞬态磁性结构。MCs的鞘区也可能携带向南的磁场。向南的磁场是造成空间天气扰动的原因。我们报告了1995年至2017年期间MCs和Bz组件在其鞘区的综合分析。85%的303个MC包含南Bz高达50 nT。鞘层Bz在23年期间可能高达40 nT。最强磁等和Bz南的MC出现在太阳周的下降阶段。双极型MC的极性具有太阳周期依赖性,但其出现频率不具有太阳周期依赖性。单极MCs的出现频率与太阳周期有关,但极性与太阳周期无关。具有最高速度、最大总B星等和最大鞘层Bz南的MC来自更靠近太阳盘中心的源区。大约80%的大Dst风暴是由MC事件引起的。鞘中的南Bz和紧接着的南第一MC的组合产生了最大的风暴。双极MCs的太阳周期依赖性延长到2017年,跨越42年。我们发现,双极MC Bz极性太阳周期的依赖性是由MC起源于静态暗条在衰变的活动区和一组弱MC的来源不明,而极性的双极MC与活动区耀斑总是有混合的Bz极性没有太阳周期的依赖性,因此是最不可预测的Bz预测。

英文摘要:

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 southward magnetic field. Southward magnetic field is responsible for causing space-weather disturbances. We report a comprehensive analysis of MCs and Bz components in their sheath regions during 1995 to 2017. Eighty-five percent of 303 MCs contain a south Bz up to 50 nT. Sheath Bz during the 23 years may reach as high as 40 nT. The MCs of strongest magnetic magnitude and Bz south occur in the declining phase of the solar cycle. The bipolar MCs have solar-cycle dependence 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, largest total B magnitudes and sheath Bz south are from source regions closer to the solar disk center. About 80% of large Dst storms are caused by MC events. The combinations of south Bz in the sheath and the south-first MCs in close succession have given the largest storms. The solar-cycle dependence of bipolar MCs is extended to 2017, spanning 42 years. We find that the bipolar MC Bz polarity solar-cycle dependence is given by MCs 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 mixed Bz polarity without solar-cycle dependence and is therefore the least predictable for Bz forecasting.

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