基于MHD模拟数据的正午午夜子午面磁层顶位形研究

Size and Shape of Magnetopause in the Noon-midnight Meridian Plane Based on the MHD Simulation

通过分析太阳风-磁层-电离层系统的三维全球磁流体力学(MHD)模型的计算数据,给出了正午-午夜子午面磁层顶位形的定量模型.分析表明,正午-午夜子午面磁层顶位形可以用文献[3]提出的基于卫星观测数据的、描述赤道面磁层顶位形的函数来描述.与赤道面磁层顶不同,正午-午夜子午面磁层顶位形更为复杂.在忽略极尖区(cusp)的简化条件下,磁层顶位形仍需利用两条曲线来拟合.太阳风动压D_p与行星际磁场分量B_z是控制磁层顶位形的主要因素.行星际磁场为北向时,磁场增强,日下点距离r_0增大;行星际磁场为南向时,磁场增强,磁层顶日下点距离r_0减小.整体而言,行星际磁场分量B_z由南转北时,r_0增大,且B_z对r_0的影响减弱.太阳风动压D_p是控制磁层顶日下点的主要因素,D_p增大,r_0减小.磁层顶位形的另一个参数磁层顶磁尾张角α,随着行星际磁场南向分量增强而增大,即磁层顶张开程度更加显著,更多的磁通量由向阳侧传输到夜侧;D_p增大,α略增大,这意味着D_p对磁通量由日侧向夜侧的传输也有一定的贡献.

A quantitative model about the location and Shape of Magnetopause（MP） in the noon-midnight meridian plane is given by analyzing the computing data from three-dimensional global MHD simulation of the magnetosphere.Data processing results show the function which was presented by Ref.[3]based on the satellite observational data can also be used to describe MP in noon-midnight meridian plane.The location and shape of the MP in the noon-midnight meridian plane are more complicated than those in the equatorial plane.Although the MP in cusp region is ignored, the location and shape of MP need still be fitted by two different curves.The dynamic pressure of the solar wind（D_p） and the north-south component of the interplanetary magnetic field（IMF B_z） are two main factors determining the size and shape of MP.While for northward IMF B_Z,r_0 increases with increasing northward B_z;for southward IMF B_z,the standoff distance r_0 decreases with increasing southward IMF B_z.On the whole,r_0 increases when B_z turns northward from southward, and the influence of B_z on r_0 becomes weaker,r_0 is mainly affected by the dynamic pressure D_p,r_0 decreases with D_p increasing.The other variable a,the level of tail flaring,increases with southward IMF increasing,which means the magnetopause flares more strongly and more magnetic flux transfers from the dayside to the nightside.The value ofαrises slightly with D_p increasing, which implies that D_p also helps to some extent flux transfer from the dayside to the nightside.