太阳风中小尺度磁通量管边界重联的统计研究

摘要

通过对WIND卫星1995-2005年的数据，利用程序筛选及人工识别两种不同方法确定的小尺度磁通量管进行比较，发现程序筛选法中41％的小尺度磁通量管有边界重联现象，与人工识别法确定的小尺度磁通量管的统计结果接近；通过人工识别和程序筛选两种方法确定的小尺度磁通量管的边界重联特征，包括磁场剪切角、磁场强度以及重联耗散区的持续时间等，也具有相同的统计趋势．结果表明，两种方法确定的小尺度磁通量管在重联特性上没有本质区别，因此采用这两种方法得到的数据作为样本来统计小尺度磁通量管前后边界重联事件．本文共确定了71个重联事件，统计结果显示有50个（70％）重联耗散区磁场的减小超过20％，47个（66％）磁场剪切角大于90°；多数重联事件的磁场剪切角大于90°，表明小尺度磁通量管边界中主要发生的是反平行重联．将小尺度磁通量管的前后边界重联分开进行统计，结果显示其前后边界重联的特征是相似的，与磁云前后边界存在差异的性质不同，这意味着太阳风中的小尺度磁通量管并不具有磁云这种大尺度磁通量管的膨胀特征．%Recently, the improvement on space and time resolution of in-situ instruments makes the small-scale flux ropes the hot point in the study of solar wind. Previous works analyzed the in-situ measurements from WIND between 1995 and 2005, and they reported two different lists of smMl-scMe flux ropes with only 4 cases in common. The two lists were selected by human vision and by computer program, respectively. A recent work surveyed the list from human vision, finding magnetic reconnections in the boundary layers of 42% small-scale flux ropes. In order to reveal the magnetic reconnection properties at small-scale flux ropes＇ boundaries from both lists, we analyzed the computer selected list, finding magnetic reconnection happened at the boundaries in 41% of the flux ropes. Furthermore, other properties also indicate the two flux rope lists have almost thesame trend statistically. These properties include magnetic reconnection shear angle, decrease of magnetic field magnitude and observational time scale of reconnection exhaust. Since more samples give more confident results, we combine the magnetic reconnection events in both lists, totally 71 magnetic reconnection exhausts. Our result shows 70% with a magnetic field strength depression larger than 20%, about 66% of the magnetic reconnection events are associated with a magnetic field shear angle larger than 90°. This indicates that the magnetic reeonnection at the boundaries of the small-scale flux ropes are more likely to be anti-parallel than the reconnection events in the solar wind. Meantime, this result supports the previous work result based on the human vision selecting list. Our discussion first demonstrated the leading edge magnetic reconnections and the trailing ones have similar signatures, implying boundaries of small-scale flux ropes are different from the magnetic clouds. Therefore, the small-scale flux ropes do not expand as their larger scale counterparthe magnetic clouds.