The polar ionosphere at Zhongshan Station on May 11, 1999, the day the solar wind almost disappeared

The solar wind almost disappeared on May 11,1999: the solar wind plasma density and dynamic pressure were less than 1cm-3 and 0.1 nPa respectively, while the interplanetary magnetic field was northward. The polar ionospheric data observed by the multi-instruments at Zhongshan Station in Antarctica on such special event day was compared with those of the control day (May 14). It was shown that geomagnetic activity was very quiet on May 11 at Zhongshan. The magnetic pulsation, which usually occurred at about magnetic noon, did not appear. The ionosphere was steady and stratified, and the F2 layer spread very little. The critical frequency of dayside F2 layer, foF2, was larger than that of control day, and the peak of foF2 appeared 2 hours earlier. The ionospheric drift velocity was less than usual. There were intensive auroral Es appearing at magnetic noon. All this indicates that the polar ionosphere was extremely quiet and geomagnetic field was much more dipolar on May 11. There were some signatures of auroral substorm before midnight, such as the negative deviation of the geomagnetic H component, accompanied with auroral Es and weak Pc3 pulsation.     

南极中山站宁静日午后极光事件分析

对１９９７ 年６ 月２ 日１１２７～１７００ ＵＴ,在南极中山站（不变地磁纬度为７４．５Ｓ,ＭＬＴ＝ ＵＴ＋ ２）利用全天空极光摄像机和三波段极光扫描光度计观测到的午后极光进行了分析,发现宁静日午后极光的形态随磁地方时变化．在午后早些时候,辉度较弱的冕状极光出现在中山站的极向侧和日向侧;随后出现的是来自日侧的带状极光,该结构东向传播,赤道向运动;在午后较晚的时候出现的是来自夜侧的带状极光．同时将午后极光与当地地磁观测、ＤＭＳＰ／Ｆ１１ 卫星的粒子观测以及ＷＩＮＤ卫星的ＩＭＦ观测进行了对照分析,认为宁静日午后极光中的分离结构是由来自磁层顶附近的粒子沉降产生的,其变化和运动与地磁Ｈ 分量和ＩＭＦＢｚ 分量的变化相关．