Dipole Tilt Effect on Magnetopause Reconnection and the Steady-State Magnetosphere-Ionosphere System: Global MHD Simulations
Dipole Tilt Effect on Magnetopause Reconnection and the Steady-State Magnetosphere-Ionosphere System: Global MHD Simulations
复制标题
DOI：
10.1029/2019ja027510
复制
发表时间：
2020-07-01
影响因子：
2.8
通讯作者：
Chittenden, J. P.
中科院分区：
文献类型：
--
作者：
Eggington, J. W. B.;Eastwood, J. P.;Chittenden, J. P.
地球偶极子倾角的日变化和季节变化在磁层-电离层耦合系统中引入了许多变化。通过改变磁场重联的位置和强度，偶极子倾斜影响了全球范围内的对流。然而，由于系统的非线性性质，各种其他效应，如偶极旋转、变化的行星际磁场(IMF)方向和不均匀的电离层电导，可能会抹杀纯粹由与太阳风耦合的变化引起的倾斜效应。为了解释潜在的倾角依赖关系，我们对稳态磁层-电离层系统进行了磁流体动力学(MHD)模拟，在0-90度倾角的纯向南IMF条件下。我们确定了磁选机在每种情况下的位置，发现由于磁剪切角的变化，随着倾角的增加，磁层顶上的三维X线向南移动。该分离器在50度倾角以上高度不稳定，这是磁层顶上规则通量转移事件(FTE)产生的特征。重联率随着倾角变大而下降，但在白天保持连续，以至于磁层即使在90度内也是开放的。这些趋势向下映射到电离层，极帽随着倾斜角的增加而收缩，I区场向电流(FAC)迁移到更高纬度，形态发生变化。倾斜导致磁层对流的南北不对称，从而在北半球(朝阳)比南半球驱动更多的FAC以获得大倾角，而不受电导的影响。这些结果突出表明，在严重空间天气事件的潜在影响中，对发生时间的敏感性很强。
The Earth's dipole tilt angle changes both diurnally and seasonally and introduces numerous variabilities in the coupled magnetosphere-ionosphere system. By altering the location and intensity of magnetic reconnection, the dipole tilt influences convection on a global scale. However, due to the nonlinear nature of the system, various other effects like dipole rotation, varying interplanetary magnetic field (IMF) orientation, and nonuniform ionospheric conductance can smear tilt effects arising purely from changes in coupling with the solar wind. To elucidate the underlying tilt angle dependence, we perform magnetohydrodynamic (MHD) simulations of the steady-state magnetosphere-ionosphere system under purely southward IMF conditions for tilt angles from 0-90 degrees. We identify the location of the magnetic separator in each case and find that an increasing tilt angle shifts the 3-D X line southward on the magnetopause due to changes in magnetic shear angle. The separator is highly unsteady above 50 degrees tilt angle, characteristic of regular flux transfer event (FTE) generation on the magnetopause. The reconnection rate drops as the tilt angle becomes large, but remains continuous across the dayside such that the magnetosphere is open even for 90 degrees. These trends map down to the ionosphere, with the polar cap contracting as the tilt angle increases, and region I field-aligned current (FAC) migrating to higher latitudes with changing morphology. The tilt introduces a north-south asymmetry in magnetospheric convection, thus driving more FAC in the Northern (sunward facing) hemisphere for large tilt angles than in the Southern independent of conductance. These results highlight the strong sensitivity to onset time in the potential impact of a severe space weather event.