Rarefaction of the very slow (<350 km/s) solar wind in Cycle 24 compared with Cycle 23
Rarefaction of the very slow (<350 km/s) solar wind in Cycle 24 compared with Cycle 23
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与周期 23 相比，周期 24 中极慢（<350 km/s）太阳风的稀疏化
DOI：
10.1002/2017ja025014
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发表时间：
2018
期刊：
影响因子：
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通讯作者：
Hakamada Kazuyuki
中科院分区：
文献类型：
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作者：
Tokumaru Munetoshi;Shimoyama Tomoya;Fujiki Ken'ichi;Hakamada Kazuyuki
我们研究了1997年至2015年期间，即第23周和第24周，速度小于350 km/s的极慢太阳风（VSSW）的长期变化，使用行星际闪烁测量，可以确定流速和电子密度波动水平ΔNe的全球分布。结果表明，VSSW的发生率在太阳活动周的最大阶段增加，且主要与第23周的高Δ Ne有关。然而，发现VSSW与第24周的低ΔNein更相关。这一事实与第24周期中原位观察到的低密度VSSW发生率增加一致。这些事实被认为是这个周期中太阳活动微弱的表现，表明VSSW近年来已经变得非常稀薄。我们确定的源区域的VSSW的光球使用的位场分析和检查的磁场特性的VSSW源。我们发现低ΔNeVSSW与较小的膨胀因子相关，并且可能比高ΔNeVSSW的光球场强度略弱。这里得到的结果可能表明，更多的开放的磁场区产生的VSSW在第24周形成在安静的太阳区域，这是与一个较低的质量通量供应到日冕，比那些在第23周。
We investigate long‐term variation of the very slow solar wind (VSSW), whose speed is less than 350 km/s, during the period between 1997 and 2015, that is, Cycles 23 and 24, using interplanetary scintillation measurements, which enable determination of the global distribution of the flow speed and the electron density fluctuation level ΔNe. We find that the occurrence rate of VSSW increases in the maximum phase of the solar cycle, and it was mostly associated with high ΔNein Cycle 23. However, the VSSW is found to be more associated with low ΔNein Cycle 24. This fact is consistent with an increased occurrence of the low‐density VSSW observed in situ in Cycle 24. These facts, which are considered to be a manifestation of weak solar activity in this cycle, suggest that the VSSW has become significantly rarefied in recent years. We identify the source region of the VSSW on the photosphere using a potential field analysis and examine the magnetic field properties of the VSSW source. We find that the low‐ΔNeVSSW is associated with a smaller expansion factor and possibly a slightly weaker photospheric field strength than the high‐ΔNeVSSW. The results obtained here may suggest that more open magnetic field areas producing the VSSW in Cycle 24 are formed in the quiet Sun region, which is associated with a lower mass flux supply into the corona, than those in Cycle 23.