Scintillation Modelling

Description

Using in-situ data from the Dynamic Explorer 2 (DE2) satellite, an empirical climatological model for the Northern Hemisphere high latitude ionosphere was prepared. This model incorporates B-spline functions together with solar and geomagnetic indices to estimate an amplitude scintillation index, the turbulence strength parameter, Cs, and spectral index. Model input comes from the DE 2 satellite’s retarding potential analyzer plasma density data, combined with the International Reference Ionosphere (IRI) model and a phase screen propagation model.

Using ground station data from GPS GSV4004b receivers at Hornsund (Svalbard) and Warsaw (Poland), similar models were prepared. The model for high latitude scintillation, prepared using in situ data, was compared to the ionospheric scintillation model prepared for the Hornsund site. To study the change in scintillation behaviour between mid-latitudes and high-latitudes we have compared the B-spline model for the Warsaw and Hornsund sites. The comparison was made on the basis of seasonal behaviour and the behaviour of scintillation indices for different geophysical conditions. In addition to these comparisons, we have analyzed GPS data from the period 2007-2011 to determine the nature of scintillation index variation versus satellite elevation both Warsaw and Hornsund. To compare with current theory, the intensity scintillation index was simulated as a function of elevation angle, azimuth, magnetic field inclination, and shape of irregularities, using the phase screen model of scintillation as formulated by Rino (1979). Data analysis was performed for both seasonal and geomagnetic activity dependence of ionospheric scintillation. The scintillation index is a power-law function of the cosecant of the elevation angle. Results show that the power law strongly depends on the form of irregularities, being larger than in isotropic cases for irregularities with dimension along the magnetic field direction smaller than those across the magnetic field. The present work also shows the need to use experimentally derived dependence on elevation.

The main objective of this study was to prepare an empirical model for ionospheric scintillation. Based on the available scintillation data an empirical model of scintillation has been constructed. This model has been compared with other models obtained from in-situ plasma density data, electron density profiles, and utilizing 1) the single phase screen model and 2) multiple phase screen model of radio propagation through random media.

Model inputs:

DE2 retarding potential analyzer measurements, ground-based GPS measurements, and several ionospheric parameters from IRI.

Model outputs:

Maps of two ionospheric parameters, namely the turbulence strength parameter of electron density fluctuation, Cs, and one dimensional spectral index, p, for different geophysical conditions and seasons. Also maps of amplitude scintillation indices.

The retarding potential analyzer onboard the DE2 satellite provides ion density measurements (equivalent to electron density by charge neutrality). The observations were only along the satellite orbit; therefore the WAM (2007) model has significant gaps in its output map coverage. Using our model we have removed all of these gaps, revealing the behaviour of ionospheric parameters and amplitude scintillation indices over previously missing regions. We have modeled both mid and high latitude northern hemisphere scintillation behaviour that demonstrates the changes in ionospheric response to trans-ionospheric communication links when a user moves from mid to high latitude locations. Being empirical, the model is derived from real observations and has certain limitations. However we have shown that the model produces results that agree well with in situ measurements, over a range of different geophysical conditions. DE2 operated during a period of moderate solar activity (sunspot number was between 80 -140), therefore our model is only valid for moderate solar activity conditions. We used the IRI model for the irregularity slab thickness and height of peak electron density. IRI often fails to produce realistic behaviour of ionospheric parameters at high latitudes. There is possibility of erroneous calculations in our model, similar to those reported in the WAM model (Wernik et al. 2007). The third and most significant limitation of our model is the placement and number of B-spline basis function used. The number of data points for high geomagnetic activity conditions are always less than those used during weak geomagnetic conditions.

Sometimes it seems that during weak geomagnetic activity conditions the contour maps are smoother than those produced during high geomagnetic conditions. This may also be considered as a limitation that cannot be addressed since it is natural. As we have already discussed for individual situations we chose different numbers of basis functions and experimented with their placement in order to get more convincing results. It is always possible that different sets of B-spline basis functions could produce better model results than we did here. But, we consider this limitation positively, as there is always potential to improve the model.