Peculiarities of VLF wave propagation in the Earth's magnetosphere in the presence of artificial large-scale inhomogeneity

摘要

We study specific features of VLF waves propagation in the Earth's ionosphere and magnetosphere in the presence of large-scale field-aligned plasma inhomogeneities (ducts). These inhomogeneities can, e.g., be formed during the ionosphere heating by high-power HF facilities such as High Frequency Active Auroral Research Program and Sura. They can extend up to altitudes of several thousand kilometers along geomagnetic field lines and have transverse scales of about 1 degrees determined by the heated region scale. We analyze ray trajectories of VLF waves with frequencies of 1 to 15kHz starting from about 100km altitude and use the plasma parameters obtained within the framework of the SAMI2 simulation model (Huba et al., 2000). This model employs MHD equations for thermal plasma and allows one to obtain the plasma parameters along the entire magnetic flux tube in a fixed meridional plane. By knowing the ray trajectories, we calculate and compare the amplitude variation along the ray paths for the cases of unperturbed and heated ionosphere. We show that the presence of a large-scale density disturbance produced by the HF heating can lead to significant changes of wave propagation trajectories. In particular, efficient guiding of VLF waves in this region can take place, which in turn can result in the appearance of several wave focusing regions and a drastic local increase of the VLF wave amplitude in these regions (up to approximate to 10 times as compared to the case of unperturbed plasma). The number of focusing regions is determined by an extension of an artificial duct along the geomagnetic field.