A global model of the ionospheric F2 peak height based on EOF analysis

M.-L Zhang,C Liu,L Liu,B Ning,W Wan

doi:10.5194/angeo-27-3203-2009

Abstract

Abstract. The ionospheric F2 peak height hmF2 is an important parameter that is much needed in ionospheric research and practical applications. In this paper, an attempt is made to develop a global model of hmF2. The hmF2 data, used to construct the global model, are converted from the monthly median hourly values of the ionospheric propagation factor M(3000)F2 observed by ionosondes/digisondes distributed globally, based on the strong anti-correlation existed between hmF2 and M(3000)F2. The empirical orthogonal function (EOF) analysis method, combined with harmonic function and regression analysis, is used to construct the model. The technique used in the global modelling involves two layers of EOF analysis of the dataset. The first layer EOF analysis is applied to the hmF2 dataset which decomposed the dataset into a series of orthogonal functions (EOF base functions) Ek and their associated EOF coefficients Pk. The base functions Ek represent the intrinsic characteristic variations of the dataset with the modified dip latitude and local time, the coefficients Pk represents the variations of the dataset with the universal time, season as well as solar cycle activity levels. The second layer EOF analysis is applied to the EOF coefficients Pk obtained in the first layer EOF analysis. The coefficients Ak, obtained in the second layer EOF analysis, are then modelled with the harmonic functions representing the seasonal (annual and semi-annual) and solar cycle variations, with their amplitudes changing with the F10.7 index, a proxy of the solar activity level. Thus, the constructed global model incorporates the geographical location, diurnal, seasonal as well as solar cycle variations of hmF2 through the combination of EOF analysis and the harmonic function expressions of the associated EOF coefficients. Comparisons between the model results and observational data were consistent, indicating that the modelling technique used is very promising when used to construct the global model of hmF2 and it has the potential of being used for the global modelling/mapping of other ionospheric parameters. Statistical analysis on model-data comparison showed that our constructed model of hmF2, based on the EOF expansion method, compares better with the observational data than the model currently used in the International Reference Ionosphere (IRI) model.

Highlights

Ionospheric modelling has been one of the leading ways to study the ionosphere
The prepared gridded dataset are decomposed into the empirical orthogonal function (EOF) base functions Ek(μ, LT), which represent the variation of hmF2 with the modified dip latitude μ and the local time (LT), and the associated EOF coefficients Pk (UT,m), which represent the variations of hmF2 with the universal time (UT) and seasonal as well as solar cycle variations as the following hmF2(μ,LT,UT,m) = Ek(μ,LT) · Pk(UT,m)
It is formed as a consequence of the so called “fountain” effect. The influence of this equatorial fountain effects on the F2 peak height hmF2 is that it will produce a latitudinal distribution of hmF2 with higher value near equatorial and low latitudes but with lower value outside

Summary

Introduction

Ionospheric modelling has been one of the leading ways to study the ionosphere. An ionospheric model can be either a theoretical model ( named physical model or first principle model) which is derived from various laws of physics and based on the numerical solution of the equations describing the spatial and temporal distribution of medium parameters or an empirical (or semi-empirical) model which is derived from the observational results. The fact that hmF2 is a parameter which is not easy to obtain from measurement makes it difficulty to have an observational hmF2 dataset with enough spatial coverage and history length of data accumulation that can be used for global modelling It has been shown (Shimazaki, 1955; Wright and Mcduffie, 1960) that hmF2 is strongly anti-correlated to the ionospheric propagation factor M(3000)F2 and, M(3000)F2 can be routinely scaled from ionograms recorded by ionosondes/digisondes distributed globally and its data has already been accumulated for a very long time.

Fundamental of EOF analysis method

Data preprocessing and decomposition

Model construction

Results and discussion

Summary and conclusion