Imaging of the equatorial ionosphere

The equatorial anomaly is the dominant structure in free electron concentration in the tropical ionosphere. Due to its edges (crests) which are characterised by steep latitudinal gradients in TEC and are temporally and spatially variable, it is one of the ionospheric regions most difficult to image with inversion methods. In this paper we reconstruct an International Reference Ionosphere model of the equatorial ionosphere by employing a grid of virtual ground GPS receivers and actual GPS satellite positions. The MIDAS algorithm, an inversion method for reconstructing the ionosphere as a linear composition of given empirical orthogonal functions, is used. Comparing the true model ionosphere with the resulting images a fine tuning of the basis functions (vertical profile contraints) in the inversion is realised. Mailing address: Dr. Massimo Materassi, Istituto dei Sistemi Complessi, CNR, Via Madonna del Piano, 50019 Sesto Fiorentino (FI), Italy; e-mail: [email protected] Fig. 2. Data coverage of the reconstruction region obtained by intersecting the ray paths with a shell at 400 km height. The colour code is associated with the slant TEC values. 478 Massimo Materassi and Cathryn N. Mitchell data along ray paths crossing the region of interest (Mitchell and Spencer, 2003). The essential ingredient of MIDAS inversions is the use of Empirical Orthogonal Functions (Sirovich and Everson, 1992) along which the solution of the inverse problem is assumed to be linearly decomposable (Materassi, 2003). The most general form reads ( , ) ( ) ( , ) . N t X t N t r r recon = a a