Determination of Electron Density Profile in the Magnetosphere Deduced from Propagation Characteristics of VLF Wave

利用Akebono卫星上观测到的Omega信号的波法向矢量方向和延迟时间，以及沿卫星轨道的电子密度，尝试确定等离子体层和电离层中的等离子体密度分布。卫星上的VLF接收器使我们能够连续确定一个多小时的Omega信号的波法向和延迟时间。在已知等离子体层电子密度分布的情况下，还可以用三维射线追踪技术计算卫星位置的波法向和延迟时间。我们的方法是确定代表等离子体层和电离层的假设函数的等离子体参数，以便将上述观测数据与使用假设模型的射线跟踪计算的数据进行拟合。寻找上述几个函数的参数的过程是非线性最小二乘法，以使这些参数的观测值与射线跟踪计算值的差异最小。对于等离子体密度分布，假设离子温度梯度随高度变化的扩散平衡(DE)模型。对于海拔1000公里以下的电子密度剖面，根据IRI模型确定了F2峰和E层上方的谷地的高度。通过与谢菲尔德大学等离子体层和电离层模型(SUPIM)的比较，验证了基于DE模型的等离子体层模型的有效性，可以得出结论：我们的模型足够灵活地描述了等离子体层中电子密度的总体分布。我们的拟合算法的可靠性也通过计算机模拟进行了验证，我们已经证实，从卫星波数据可以确定全球电子密度分布，没有任何显著的误差。

Determination of plasma density distributions in the plasmasphere and ionosphere has been attempted using wave normal vector direction and delay time of Omega signals observed on the Akebono satellite as well as the electron density along the satellite trajectory.The VLF receivers on board the satellite have enabled us to determine the wave normal direction and delay time of Omega signals continuously for more than one hour. The wave normal direction and delay time at the satellite location can also be calculated by three dimensional ray tracing technique, if the electron density distributions in the plasmasphere is known. Our method is to determine plasma parameters of assumed functions which represent the plasmasphere and ionosphere, so as to fit observed data as mentioned above with those calculated by ray tracing by using the assumed model.The procedure to find parameters of several functions mentioned above is the non-linear least squarefitting method so as to minimize the differences of these quantities between the observed and the calculated by ray tracing.For the plasma density distribution a diffusive equilibrium (DE) model with a positive ion temperature gradient with altitude is assumed. For the electron density profile below 1,000km of altitude, the altitudes of the F2 peak and the valley just above the E layr are determined from the IRI model.The validity of our plasmaspheric model based on the DE model was checked by comparing with SUPIM (Sheffield University Plasmaspheric and Ionospheric Model) , and we can conclude that our model is flexible enough to represent the general global distributions of the electron density in the plasmasphere. The reliability of our fitting algorithm was also checked by computer simulations, and we have confirmed that the global electron density distribution can be determined from the satellite wave data, without any significant error.