عنوان مقاله [English]
نویسندگان [English]چکیده [English]
To understand the physics of the F2 ionospheric layer and for ionospheric radio-wave propagation studies, accurate values of the peak height of the F2 ionospheric layer (hmF2) are needed. The peak height of the F2 ionospheric layer is the most important parameter after peak density in high frequency (HF) propagation prediction. Diurnal variation of hmF2 is controlled by solar radiation and thermospheric winds. Thermospheric winds cause pressures that change the ionosphere at different altitudes. Therefore, hmF2 is a key parameter in studying ionospheric dynamics. In addition, the interaction between radio waves and charged particles of ionospheric plasma can cause significant changes in the phase, amplitude and polarization of navigation signals. All of these changes depend on the value of the total electron content (TEC) in the ionosphere.
Data measured by IPS-71 at the ionospheric station at the Institute of Geophysicsof the University of Tehran from July 2006 to June 2007 were used to perform the calculations. As the real height is difficult to acquire directly from ionograms, the Shimazaki and Bilitza formulations are used to calculate the peak height of the F2 ionospheric layer for the period from July 2006 to June 2007.
The most commonly employed mathematical function that depicts the electron density profile in the ionosphere is the Chapman function. In this work, we have used the method presented by Ezquer et.al (1992), in which two Chapman profile expressions are utilized for topside ionosphere and bottomside ionosphere, to obtain TEC from ionograms.
Furthermore, in the present work, the international reference ionosphere (IRI-2007) model with CCIR coefficients was used to predict the peak height of the F2 ionospheric layer and the TEC for the period from July 2006 to June 2007. The IRI-2007 model is an empirical models that is widely used to show ionospheric changes. The IRI model is a standard model of the ionosphere supported by the Committee on Space Research (COSPAR) and International Union of Radio Science (URSI). The IRI model has many practical applications in High Frequency (HF) predictions. The IRI model is based on a mathematical description of the ionosphere as a function of location, time, altitude, solar activity and geomagnetic activity. It has two options for the prediction of the F2, F1 and E layer critical frequencies (CCIR and URSI coefficients).
The integration of two Chapman profiles shows that almost 2/3 of the TEC of the ionosphere is in the topside ionosphere and almost 1/3 is in the bottomside of ionosphere. Additionally, the daytime values of the TEC show intense fluctuation, while they becomes smoother in the nighttime. Furthermore, during the daytime values of hmF2 computed by the Bilitza method are lower than those calculated by the Shimazaki method. However, during the nighttime the values of hmF2 computed by Bilitza method are the as same as those computed with Shimazaki method. In addition, our study shows that around midnight, the values of hmF2 are greater than those of the daytime. Conversely, there is a downward shift in the F2 peak near sunrise. From morning to afternoon, the hmF2 increases again. In the evening an upward drift of the F2-peak is clear.