Abstract
The hourly values of the F-layer critical frequency from the ionospheric sounder in Dourbes (50.1°N, 4.6°E) during the time interval from 1957 to 2010, comprising five solar cycles, were analyzed for the effects of the solar activity. The hourly time series were reduced to hourly monthly medians which in turn were used for fitting a single station foF2 monthly median model. Two functional approaches have been investigated: a statistical approach and a spectral approach. The solar flux F10.7 is used to model the dependence of foF2 on the solar activity and is incorporated into both models by a polynomial expression. The statistical model employs polynomial functions to fit the F-layer critical frequency while the spectral model is based on spectral decomposition of the measured data and offers a better physical interpretation of the fitting parameters. The daytime and nighttime foF2 values calculated by both approaches are compared during high and low solar activity. In general, the statistical model has a slightly lower uncertainty at the expense of the larger number of fitting parameters. However, the spectral approach is superior for modeling the periodic effects and performs better when comparing the results for high and low solar activity. Comparison with the International Reference Ionosphere (IRI 2012) shows that both local models are better at describing the local values of the F-layer critical frequency.
Highlights
An operational system for local ionospheric nowcast and forecast is being developed to aid the monitoring of the ionospheric effects on the Global Navigation Satellite System (GNSS) applications (Stankov et al 2012)
For the modeled foF2 values, the uncertainties are calculated by propagating the errors of the different coefficients and parameters in the polynomial model and for the spectral model the uncertainty is calculated from the remaining terms in the Fourier transformation plus the error propagation whose dependence is modeled by polynomials
Two approaches were investigated for fitting the F-layer critical frequency as a function of the F10.7: a polynomial fitting model using first and seconddegree polynomials to model the available measured monthly median models
Summary
An operational system for local ionospheric nowcast and forecast is being developed to aid the monitoring of the ionospheric effects on the Global Navigation Satellite System (GNSS) applications (Stankov et al 2012). We have opted for a single-station model due to the following reasons: the global models tend to smooth out ionospheric features that are typical for a particular location and, a single-station model would yield better accuracy for use in local modeling, nowcast and forecast; it would be much easier to update the coefficients/statistical parameters of the model when new measurement data are incorporated; such a model would offer possibilities for a direct application in regional (such as over Europe) ionospheric mapping. In Europe, Stanisáawska (1994) proposed an autocovariance prediction in which the predicted value (for instance foF2) is calculated in such a way that the covariance remains unchanged in the sense of the “minimum least squares” criterion An advantage of this approach is that it requires only the existence of (sufficiently) long time series of observations of the modeled parameter í the nature of (geo-)physical processes and conditions governing the variations of this parameter may not necessarily be known and may not be (explicitly) used. A geomagnetically correlated statistical model (GCSM) was developed, incorporating the auto- and cross-correlation between the ionospheric char-
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