عنوان مقاله [English]
Electrondensity is one of the significant parameters for monitoring and describing the ionosphere.The ionosphere is a consequential source of errors for the GPS signals that traverse through the ionosphere on their ways to the ground-based receivers, because there is a high concentration of free electrons and ionsreleased by the ionizingaction of solar X-ray and ultraviolet radiation on atmospheric formers. Radio Occultation(RO) is one of the most modern satellite techniques to study on vertical profiles of neutral density, temperature, pressure and water vapor in the stratosphere and troposphere and ionospheric electron density profiles with high vertical resolutions.Since the RO technique using the GPS signals was employed for the first time by the Global Positioning System Meteorology (GPS/MET), the low-earth-orbit-based GPS RO technique has been proven as a successful method in exploring the earth’s lower atmosphere and ionosphere.
Abel transformation is the basic hypothesis made in the retrieval of radio-occulted ionospheric parameters.The Abel inversion is a powerful tool to retrieve high-resolution vertical profiles of electron density from GPS radio occultation collected by satellites into Low Earth Orbit(LEO).
COSMIC satellite records measurements during the whole day and is not limited to the specific times and special atmospheric conditions.It should be noted that the GPS radio occultation techniques provide continuous and useful ionospheric layers information and are not obtained from the point wise measurements by other satellites.
COSMIC satellite also records the altitude for the measurements of the electron density profile. COSMIC satellite provides more than1000 electron density profiles per day with approximately global coverage and also parts of them cover IRAN .In this approach, the LEO-GPS line of sight is occulted by the Earth’s limb with the setting(or rising) motion of the LEO satellite. The GPS-LEO radio connection successively records the atmospheric layers at different altitudes. The ionosphere is highly variable in space and time. Thus, for modeling the electrondensity profile the time changes(diurnaland seasonal) and location changes(geographical position of station), must be considered. In this research, the input space includes the day number (seasonal variation), hour (diurnal variation), latitude, longitude, height and F10.7 index (measure of the solar activity). The output of the model is the ionospheric electron density profile(Ne).The COSMIC observations and IRI-2007-based data of electron density profiles were also analyzed during the solar minimum period. In this research, we used a feedforward Artificial Neural Network (ANN) with 55 neurons in hidden layer for modeling profiles of electron density of COSMIC satellite performance of the ANN models was evaluated using correlation coefficient (R=92%),R-Squared(0.83). It was found that the ANN model could be applied successfully in estimating the electron density profiles retrieved from the FORMOSAT-3/COSMIC.The comparison of the IRI model electron density profile with the COSMIC RO measurements during each month of the year 2007 over IRAN is performed.The electron density profile from all three International Reference Ionosphere (IRI) models, namely IRI-NEQ,IRI-2001, and IRI-01-Corr are used.
The results showed that the results of the IRI2007 model electron density is not satisfactory over IRAN and ANN model electron density profile is in very good agreement with COSMIC RO measurements. It was concluded that IRI-NEQ model is more appropriate thanthe other two models.
The results showed that the differences between the modeled profile electron density and theobserved profile electron density are very lower than the differences between the IRI-2007 models.Maximum changes occurred in January and December at analtitude of about 450 km and minimum changes were recorded in November at the height of 250 Km and in April at the height of 450 Km. The differences also decreased in the summer at higher altitudes and in winter at lower altitudes.