Fast Computerized Ionospheric Tomography Using GNSS-TEC in Densely Observed Regions

In recent years, the range of technologies affected by the ionosphere has been steadily expanding. Technologies such as Global Positioning System, which have become indispensable in modern society, can suffer from degraded accuracy due to ionospheric structures. To meet these needs, it is essential to estimate ionospheric structures with both low latency and high resolution. However, research addressing this issue has been limited. In this study, we propose an algorithm for effective ionospheric tomography over large regions where high-density observation data are available. The algorithm exploits the fact that ionospheric electron density remains relatively stable over the 30-s sampling interval of Total Electron Content (TEC) measurements, using the most recent known distribution as an initial guess for the unknown distribution and a conjugate gradient method with diagonal scaling preprocessing to significantly accelerate the convergence of the solution. When applied to actual data, the proposed algorithm successfully estimated electron density distributions over the entire region of Japan with a spatial resolution of 0.25° mesh, using TEC data sampled at 1-min intervals. This result demonstrates the effectiveness of the proposed method. Additionally, the algorithm was used to evaluate seasonal variations in the structure of Medium-Scale Traveling Ionospheric Disturbances. The findings indicate that high-resolution and high-accuracy tomography enables detailed analysis of ionospheric structures.