低太阳活动期间极区ELDI的EISCAT/ESR雷达观测

本文利用2009—2011年EISCAT/ESR雷达的场向观测数据,统计研究了低太阳活动期间极区E层占优电离层(ELDI)事件的发生规律及其主要特征.地面雷达观测表明,极区ELDI表现出明显的季节变化特征:在冬季和早春发生率较高.EISCAT雷达(极光椭圆纬度)观测到的ELDI多出现在磁午夜扇区,平均持续30 min;ESR雷达(极尖/极隙区纬度)观测到的ELDI多出现在磁正午附近,平均持续14 min,表现出与之前无线电掩星观测结果不一致的日变化特征.在ELDI事件期间,两处雷达观测到的电离层N_mE/N_mF₂比值和E层厚度都没有表现出显著的空间差异.事例分析证实E层电子增强和F层电子密度耗空都能够独立地导致ELDI,然而,统计分析表明上述两个过程对ELDI的形成都起着不可或缺的作用.

EISCAT/ESR radar observations of ELDI in polar region during solar minimum

Characteristics of E-layer dominated ionosphere (ELDI) were investigated in this paper using the field-aligned measurements of EISCAT/ESR radars during the period of 2009—2011. Measurems of ground-based radars indicate that ELDIs were more often observed in winter and early spring. In winter, ELDI occurrence peaks around geomagnetic mid-night within auroral oval with an average duration of 30 minutes, whilst the maximum occurrence appears around local noon at ESR site with an average duration of 14 minutes which shows some different characteristics of diurnal variations from the radio occultation observations. During presence of ELDI, the ratio of N_mE/N_mF₂ is 1.7 and the thickness of E-layer is 35 km on average, which exhibit undetectable discrepancy at the two sites. Case study confirms that either the ionization enhancement in E-layer probably induced by auroral precipitation or density depletion at F-layer caused by plasma convection alone could lead to presence of ELDI. However, statistical results suggest that the aforementioned two processes seem to be equally important in ELDI formation.