Abstract
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.
Highlights
The equatorial anomaly, with its crests either side of the geomagnetic equator, is one of the main features of the quiet ionosphere, and reproducing it correctly is an important challenge for any data analysis system for ionospheric imaging
We studied the possibility of producing Total Electron Content (TEC) maps and electron concentration sections using MIDAS (Multi Instrument Data Assimilation System), a linear inversion technique producing 3D time evolving ionospheric maps from merging of many different kinds of data
The MIDAS technique is based on the use of orthogonal functions (EOFs), an idea suggested in Fremouw et al (1992)
Summary
The equatorial anomaly, with its crests either side of the geomagnetic equator, is one of the main features of the quiet ionosphere, and reproducing it correctly is an important challenge for any data analysis system for ionospheric imaging. We studied the possibility of producing Total Electron Content (TEC) maps and electron concentration sections using MIDAS (Multi Instrument Data Assimilation System), a linear inversion technique producing 3D time evolving ionospheric maps from merging of many different kinds of data. The MIDAS inversions need a selection of functions to define the vertical profile of electron concentration. This kind of inversion problem is highly underdetermined in general (Fehmers, 1995) and this selection of profiles is the requested prior hypothesis. In MIDAS one assumes that the free electron density is a linear combination of a given set of three-dimensional EOFs. Here we apply an optimisation procedure to select the best fitting vertical profile EOFs for imaging the equatorial anomaly as represented by the International Reference Ionosphere (IRI) (Bilitza, 1990)
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