中高度极隙区电磁不稳定性

ELECTROMAGNETIC INSTABILITIES IN THE MID-ALTITUDE CLEFT REGIONS

研究了中高度（离地心３－４个地球半径）极隙区上行电子束流和上行氧离子（ｏ＋）锥引起的沿磁力线传播的电磁不稳定性．采用的物理模型假定：上行电子具有单能束流分布函数，而上行氧离子（ｏ＋）锥可用单能环－束分布函数来描述．结果表明，左旋和右旋圆偏振的低频电磁模是不稳定的，激发不稳定性的自由能源主要由上行电子束流提供，而上行氧离子（Ｏ＋）锥因自由能太小只影响频率色散关系，上行粒子（电子和氧离子）与背景等离子体密度比的变化对电磁不稳定性有重要影响．这些结果对解释权隙区纬度地面站低频电磁波观测资料和理解极隙区动力学过程是很有益的．

Electromagnetic instabilities propagating along the magnetic force lines by the upwelling electron beams and the upwelling oxygen ion (O+) conics in the cleft regions, at altitudes between 2 and 3 Earth radii, are studied in this paper. It has been used some simple assumptions in the physical model:the upwelling electrons prossess a monoenergetical beam distribution and the upwelling oxygen ion (O+)conics prossess a monoenergetical ring-beam distribution. The results show that the circularly polarized electromagnetic modes with left-hand and right-hand are unstable.The free energetic sources are provided mainly from the upwelling electron beams.The upwelling oxygen ion (O+) conics may change the dispersion relations for the unstable waves,because the energy of the oxygen ion(O+)conics is too small. The density ratio of the upwelling electrons to the background plasma has important effects on the electromagnetic instabilities. These results are very helpful to explain the data of low-frequency electromagnetic waves observed at the ground stations in the cusp/cleft latitude range and to understand the dynamic processes in the cleft plasma.