ACE飞船发射于1997年8月15日,将维持到2024年,位于日地连线重力平衡点L1。本文采用ACE飞船行星际磁场观测数据,基于GSE坐标系统,确定L1点的行星际磁场主体方位,进而将行星际磁场观测数据转换为极性符号序列。对于序列中的任一时间点,通过改变以它为中心的子序列探测行星际磁场的准27d重现周期和结构,再应用模板匹配方法识别重现结构,最后采用分段常值函数拟合获得分段稳定的周期和结构。得到的主要结果如下：(1) 1999—2008年间,IMF重现周期绝大多数在(27±2)d范围内,只有1个例外,即2001年10月27日10:00—11月28日12:00时间内,重现周期为21d14h;(2) 2001年1月8日12:00—2004年7月17日04:00,IMF为2扇瓣结构,2004年7月17日05:00—2007年6月30日23:00,IMF为4扇瓣结构,2007年7月1日00:00—2008年12月31日23:00,IMF为2扇瓣结构。

ACE was launched on August 15, 1997 and placed into an orbit about the L1 point between the Earth and the Sun. The L1 point is one of several points where the gravitational attraction of the Sun and Earth are equal and in the opposite direction. The Parker spiral angle at L1 point with ACE MAG data is calculated and the data is converted to the polarity symbolic sequence. For any point in the sequence, the quasi 27-day reoccurrence period and the structure of IMF are detected through altering the sub-sequence centered on this point. And then, the reoccurrence structure is identified with the template matching method. Finally, the piecewise constant function fitting is used to obtain the piecewise constant period and structure. The obtained results are as follows: (1) in the period of 1999—2008, the reoccurrence period is mostly in the range of (27±2) days, with only one exception: a month from 2001-10-27 10:00 to 2001-11-28 12:00, the reoccurrence period is 21 days and 14 hours; (2) from 2001-01-08 12:00 to 2004-07-17 04:00, IMF is in a two fan disc structure; from 2004-07-17 05:00 to 2007-06-30 23:00, IMF is in a four fan disc structure; from 2007-07-01 00:00 to 2008-12-31 23:00, IMF is in a two fan disc structure.