Three-dimensional tomographic reconstruction of ionospheric sporadic E layers with constraints from GNSS radio occultation data

Ionospheric sporadic E (Es) layers are thin layers with enhanced ionospheric electron densities (IEDs) which occur frequently in ionospheric E region. Previous detecting method based on ground-based global navigation satellite system (GNSS) observations can only obtain the horizontal maps rather than the vertical distributions and structures of Es layers. This study proposes a computerized ionospheric tomography (CIT) method with constraints from GNSS radio occultation data for the reconstruction of three-dimensional (3-D) structure and evolution of Es layers. The strong Es layers that occurred in Australia on January 11, 2021, and in North America on August 4, 2021, are chosen for reconstruction, and the COSMIC-2 IED profiles in the reconstruction region and its surrounding area are used as constraints in the CIT process. The IED distribution in F region is reconstructed at first by using only slant total electron content (STEC) without significant sudden disturbances, and then the E region contribution to STEC is estimated by subtracting the F region contribution, based on which the 3-D structures of Es layers with high spatial and temporal resolutions are reconstructed consequently. The reconstructed results in F and E regions are assessed separately, which show good consistence with GNSS STEC, global ionospheric maps, or ionosonde observations. The evolution of Es layer structures in the reconstructed region is further analyzed, and the large-scale Es structure spanning over more than 10 degrees in longitude and the movement of Es patches are clearly revealed. Particularly, the reconstruction results successfully trace the vertical variation in the altitudes of Es layers.