基于CARISMA地磁台链观测的IPDP事件的统计分析

亚暴期间磁尾等离子体片离子注入内磁层能够激发电磁离子回旋(EMIC)波.对应于这种EMIC波,地面磁力仪可观测到周期逐渐减小的地磁脉动(IPDP).利用GOES卫星数据,地磁指数和加拿大CARISMA地磁台站的数据,我们研究了IPDP事件的产生与亚暴磁尾注入的关系.同时利用CARISMA地磁台链中的MCMU和MSTK两个台站,从2005年4月到2014年5月期间的观测数据,统计分析了亚暴期间的IPDP事件,研究了IPDP事件的出现率关于季节和磁地方时的分布特征.我们总共获得128个两个台站同时观测的IPDP事件.该类事件关于季节分布的发生率,冬季最小,为13.28%,春季最大,为32.81%,结果表明IPDP事件关于季节分布的发生率受到电离层电导率及亚暴发生率的影响.两个台站同时观测到的IPDP事件最大出现率出现在15—18 MLT(磁地方时),结果表明IPDP事件主要由亚暴期间产生的能量离子注入内磁层,西向漂移遇到等离子体层羽状结构(Plume)区的高密度等离子体所激发.

Statistical study of IPDP events observed by the CARISMA network of magnetometers

During substorms energetic ions injected from the magnetotail plasma sheet can excite electromagnetic ion cyclotron (EMIC) waves in the inner magnetosphere. As a signature of EMIC waves, the interval of pulsations of diminishing periods (IPDP) is observed by ground-based magnetometers. The generating mechanism of IPDP waves is closely associated with substorm injected energetic ions during substorms. In addition, the occurrence rate of IPDP waves in the inner magnetosphere is very important to study the dynamics of the magnetosphere.Firstly, with observations of magnetometers at MCMU and MSTK stations of CARISMA network in Canada and the GOES 11 satellite for the typical IPDP events, the relationship between IPDP and the energetic ions injected from the magnetotail plasma sheet during substorms was revealed. Then based on the observations of magnetometers at MCMU and MSTK stations of CARISMA network between April 2005 and May 2014, we statistically analyzed the occurrence rate distribution of IPDP events in seasons and magnetic local time (MLT).The results for the typical case show that during substorms, energetic ions injected from the magnetotail plasma sheet can excite IPDP waves in the inner magnetosphere. For the statistics, a total of 128 IPDP wave events have been simultaneously observed by both MCMU and MSTK stations. The maximum occurrence rate of the events is between 15 MLT and 18 MLT, corresponding to the region of plasmasphere plumes. The IPDP events have the least occurrence rate in winter (13.28%), the highest occurrence rate is in spring (32.81%). The least occurrence rate in winter should be related to the ionospheric conductivity while the highest in spring is associated with the spring-autumn asymmetry of substorm and geomagnetic activity.The case study and statistics show that IPDP events are mainly generated by energetic protons, which can be injected into the inner magnetosphere during substorms and drift westward and encounter the plasmaspheric plume. In addition, the occurrence rate of IPDP events is affected by the ionospheric conductivity and the occurrence rate of substorms.