Statistical properties and possible supply mechanisms of tailward cold O + beams in the lobe/mantle regions
Statistical properties and possible supply mechanisms of tailward cold O + beams in the lobe/mantle regions
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DOI：
10.1029/97ja02137
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发表时间：
1998-03
影响因子：
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通讯作者：
K. Seki;M. Hirahara;T. Terasawa;T. Mukai;Y. Saito;S. Machida;Tetsuo Yamamoto;S. Kokubun
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文献类型：
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作者：
K. Seki;M. Hirahara;T. Terasawa;T. Mukai;Y. Saito;S. Machida;Tetsuo Yamamoto;S. Kokubun
本文研究了1993年10月至1995年3月太阳活动极小期（8 ~ 210 RE（地球半径））在尾叶/地幔区域用Geotail/low-energy particle （LEP）仪器观测到的以接近主H+分量的速度向尾流的冷O+束（COBs）的统计特性。O+的平均密度为~ 1.3×10−3 cm−3，对应于质子组分的~ 1.2%。流速特性表明，并不是磁层对流减弱，而是大的平行速度使O+离子在远叶/地幔区域保持不动。cob的出现频率表明，O+离子倾向于存在于地幔状区域，而不是纤细的“纯叶状”区域，它们的存在与地磁活动有明显的相关性。基于对双峰COB分布沿黎明-黄昏方向的EMF依赖，表明在IMF对日侧重联过程的影响导致鞘层等离子体进入的南北和黎明-黄昏不对称中，COB主要存在于加载象限上。即使在地磁风暴中，也可以看到负载象限上的集中。表明在活动时期COB的频繁发生主要是由于IMF的南向，而不是地磁暴期间动压本身的增加。也就是说，统计表明，cob在日侧重联过程导致的负载象限的地磁活跃时间丰富，其中太阳风起源的离子占主要成分，其场向速度比平时大。这些cob应该起源于日侧磁层和/或极帽区域。从几个keV的COB能量远高于尖/裂离子流出的能量出发，提出需要额外的能量来提高平行速度。通过依赖，一方面提供了COB源的其他可能性，如高能UFI光束和赤道捕获离子。还讨论了每个候选对象的要求，以便成为cob的主要贡献者。
We investigate statistical properties of cold O+ beams (COBs) streaming tailward at the velocity nearly equal to the major H+ component, which were observed by Geotail/low-energy particle (LEP) instrument in the tail lobe/mantle regions at geocentric distance between 8 and 210 RE (Earth radii) during the solar-minimum period (October 1993 to March 1995). The average O+ density is ∼1.3×10−3 cm−3, which corresponds to ∼1.2 % of the proton component. Properties of the flow velocity show that it is not the weakening of the magnetospheric convection but the large parallel velocity which enables the O+ ions to remain still in the distant lobe/mantle regions. The occurrence frequency of COBs suggests that O+ ions tend to exist in the mantle-like region rather than tenuous “pure lobe” and that their existence has a clear correlation with the geomagnetic activity. On the basis of the EMF By dependence of the double-peaked COB distribution along dawn-dusk direction, it is shown that COBs exist mostly on loaded quadrants in the north–south and dawn-dusk asymmetry of sheath plasma entry caused by the IMF By effect on the dayside reconnection process. The concentration on the loaded quadrants can be seen even in geomagnetic storms. It suggests that frequent COB occurrence at active times is mainly due to the southward orientation of IMF rather than the increase of dynamic pressure itself during the geomagnetic storms. That is, the statistics show that COBs are abundant at geomagnetically active times on loaded quadrants resulting from the dayside reconnection process, where the ions of solar wind origin bear the major component, and their field-aligned velocity is larger than usual. These COBs should originate in the dayside magnetosphere and/or the polar cap regions. From the COB energy of several keV, which is rather higher than that of cusp/cleft ion outflows, the necessity of extra energization(s) to elevate parallel velocity is suggested. Clear IMF By dependence, on one hand, provides other possibilities of the COB's source such as the energetic UFI beams and the equatorially trapped ions. The requirements for each candidate so as to be a main contributor to COBs are also discussed.