中文摘要：

利用PuertoRico站（18．5°N，67．2°w）1997—2013年的Es临界频率数据，对该地区Es特性进行了分析．结果表明：临界频率f0Es日均值具有一定的16天周期性变化特征，其可能是大气行星波引起的；YoEs随地方时的变化基本上呈一个波峰波谷结构，波谷位于06：00LT左右，波峰位于14：00LT左右，同时存在午夜和日落后异常高值现象，其分别与午夜南向风场减弱和夜间电场变化有关；通过分析不同强度Es，发现潮汐风造成较低强度Es发生率的地方时分布与中等和高强度Es明显不同．这些现象的相关物理机制尚需进一步研究论证．

英文摘要：

The Es property in Puerto Rico region （18.5°N, 67.2°W） has been investigated using the Es critical frequency during 1997-2013. Results show that the diurnal average critical frequency presents a weak 16-day period which may be caused by the planetary wave. The critical frequency variation with local time has also been investigated which presents a valley （at about 06：00 LT） and a peak （at about 14：00LT） structure. Two anomalous peaks exist after the midnight and sunset, respectively. The phenomenon is considered as a result of downward electron flux caused by the diedown of southward wind field and variability of electric field during the nighttime. It has also been found that the incidence of Es with lower strength is clearly different from those with medium and higher strength, and it can be attributed to the tidal wind. These physical mechanisms need to be proved by both observations and further mechanical investigation.