Distributions of the Magnetic Field Fluctuation Characteristics in the Earth's Magnetosheath
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摘要: 本文利用行星际起伏通过激波后的变化的MHD模型, 具体讨论了地球磁鞘中磁场起伏特性在黄道面内的分布.主要结果是:(1)行星际磁场起伏的强度和各向异性在磁鞘中被显著放大;(2)行星际磁场基本位于黄道面内时, 磁鞘中磁场起伏特性(强度、相对起伏和各向异性等)呈现明显的晨一昏不对称性, 早晨侧(准平行激波)显著地高于黄昏侧(准垂直激波);(3)行星际磁场方向对磁场起伏特性在磁鞘中的分布有强烈的控制作用, 早晨侧响应灵敏, 黄昏侧反响不大.相对地讲, 黄昏侧的磁活动较之早晨侧稳定;(4)行星际磁场转南的增强将导致磁鞘中磁场起伏的最大区域自黄道面低纬向北极高纬移动, 南-北不对称性磁活动随之加强, 最强大致出现在磁场与黄道面相交成大约45°时, 而晨-昏不对称性的强弱程度则发生相反变化;(5)行星际磁场的相对起伏增加, 晨-昏不对称性反随之减弱.磁鞘中磁场起伏分布的特性与卫星观测大体符合, 是磁顶、边界层某些晨-管不对称性出现的可能起因之一.Abstract: The distributions of the magnetic field fluctuation characteristics near the ecliptic plane in the magnetosheath are discussed by using the MHDmodel for describing variations of the interplanetary magnetic field fluctuations across a shock wave. The main results are: (1) the fluctuation intensity and fluctuation anisotropy of the interplanetary magnetic field are amplified obviously in the magnetosheath region; (2) when the interplanetary magnetic field is in the ecliptic plane, the magnetic field fluctuation characteristics (intersity, relative fluctuation and anisotropy and so on) show a dawn-dusk asymmetry, namely, the dawn-side (quasi-parallel shock wave) is more remarkable than the dusk-side (quasi-perpendicular shock wave) obviously; (3) the distribution characteristics are controled strongly by the directions of the interplanetary magnetic field, the dawn-side more sensitive than the dusk-side, relatively speaking, the magnetic activity of the dusk-side is more stable than the dawn-side; (4) the turn-southward of the interplanetary magnetic field will results the maximum fluctuation region moving from the ecliptic plane to the north pole-high latitude region, so that the south-north anisotropy of the magnetic activity will increase, which will reach a maximum when the angle between the interplanetary magnetic field and the ecliptic plane equals about 45°; (5) the dawn-dusk anisotropy will decrease with the increase of the relative fluctuation of the magnetic field.The distribution characteristics of the magnetic field fluctuations in the magnetosheath, obtained from our model, are consistent with the satellites observations and are the possible cause of some of the dawn-dusk asymmetry of the magnetopause and the boundary.
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