Three‐dimensional estimation of electron density over Japan using the GEONET GPS network combined with SAC‐C data and ionosonde measurements

Abstract

The work presented here proposes a local tomographic approach to obtain a three‐dimensional description of the ionospheric electron density over Japan. To do this, a combination of GPS and ionosonde data is used in order to take advantage of the strong points of each data type. The GPS data is composed of ground and SAC‐C Low Earth Orbiter observations. The ground observations correspond to measurements of 99 receivers (out of about 1000) from the dense Japanese GPS network (GEONET). SAC‐C GPS limb sounding observations are processed to obtain vertical profiles of electron density. For this purpose, an improved approach of Abel inversion that considers Vertical Total Electron Content data has been used to overcome the assumption of spherical symmetry. The resulting profiles have been averaged to build a set of background profiles, which are rescaled with the ionospheric NmF2 values sounded by the Japanese network of four ionosondes and used by the tomographic algorithm to provide with vertical description of electron density. For the validation of this tomographic approach, the hourly values of NmF2 given by the four Japanese ionosondes and the vertical profiles of electron density measured by the MU Radar in Shigaraki (during the sounding campaign of November 2002) have been used. This validation with independent data shows relative discrepancies about 25% and, moreover, it shows that the use of the profiles obtained from SAC‐C data offers an improvement on the dawn, dusk, and nighttime bottomside and topside estimations of the electron density with respect to the case of using the IRI model.