An Island Coalescence Scenario for Near-Earth Current Disruption in the Magnetotail

A current disruption and dipolarization scenario associated with island coalescences in the near-Earth region is proposed. The thin and elongated current-sheet built up during the growth phase is unstable due to a tearing mode instability that leads to formation of multiple magnetic islands （or magnetic flux ropes in the three dimensional case） in the near-Earth region. The growth rate of the tearing mode shoual be different in different locations because the rate is in general determined by the external driving force and the local plasma sheet properties. When the rate of the magnetic reconnection in the mid-tail region around 20RE is much larger than that in other locations, the strong bulk earthward flows resulting from the fast reconnection in the mid-tail drive the earthward convection and the coalescence of the magnetic islands. Consequently, the cross-tail current in the near-Earth region is suddenly disrupted and the geometry of the magnetic field changes from tall-like to dipolar-like in the ideal time scale. This proposed scenario is tested by Hall MHD simulation and is compared with the observations.     

某些非线性三阶常微分方程的几个存在性结论

考察了某些非线性三阶常微分方程的存在性．主要结论的条件涉及非线性项在无穷远处的增长速度．改进和推广了某些现有的结论．     

增长率与增长速度

20 0 2年高考江西、山西、天津卷第 13题 :据新华社 2 0 0 2年 3月 12日电 ,1985年到 2 0 0 0年间 ,我国农村人均居住面积如下图所示 ,其中从年到年的五年间增长最快 .我们的学生在做这一问题时 ,很多人认为 :从1985年到 1990年人均居住面积的增长率高于 1990年到 1995年 ,也高于 1995年到 2 0 0 0年 ,故确定答案为 1985年到 1990年 .还有部分学生根据增长的绝对数 ,认为 1995年到 2 0 0 0年的增加值最多 ,确定答案为 1995年到 2 0 0 0年 .究竟哪个答案正确呢 ?评分标准上是后者 ,而我们的一些教师也认为应是前者 .笔者认为 ,从答案上讲 ,后…     

Topographic effects on polar low and tropical cyclone development in simple theoretical model

The polar low and tropical cyclone type vortices over topography are assumed to be the axisymmetrical and thermal-wind balanced systems, which are solved as an initial value problem of a linearized vortex equation set in cylindrical coordinates. The roles of the sensible and latent heating, friction, and topography in the structure and intensification of the polar low and tropical cyclone type vortices are analyzed. The radial velocity, vertical velocity, azimuthal velocity, and the unstable growth rate including the topography effects are obtained. It is shown that the interaction between the flow and the topography plays a significant role in the structure and intensification of the polar low and tropical cyclone system. The analysis of the topography term indicates that, in the up-slope side of the mountain, the radial inflow and the vertical ascent forced by the mountain can intensify the polar low and tropical cyclone type vortex and increase the unstable growth rate. However, in the lee side of the mountain, the radial inflow and the vertical descent forced by the mountain can weaken the polar low and tropical cyclone type vortex and decrease the unstable growth rate of the polar low and tropical cyclone system. In addition, the evolutionary process and the spatial structure of the polar low observed over the Japan Sea on 19 December 2003 are investigated with the observational data to verify this theoretical result.