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20 April 2009

LOW GEOMAGNETIC LATITUDE IONOSPHERIC MODELING

LOW GEOMAGNETIC LATITUDE
IONOSPHERIC MODELING
OVER INDONESIAN REGION FROM GPS DATA

By
Buldan Muslim
NIM : 35104002
Promotor : Prof. Dr. Ir. Hasanuddin Zaenal Abidin MSc.
Co-Promotor : Prof. The Houw Liong Ph.D.
Co-Promotor : Dr. Ir. Wedyanto Kuntjoro MSc.

Ionospheric total electron content (TEC) determination and low geomagnetic latitude ionospheric modeling over Indonesian region (from -2,77° to -20,65° geomagnetic latitude) are needed for ionospheric correction in high precision point positioning by using single frequency Global Positioning System (GPS) receiver. The data and TEC model are also important for analyze of earthquake precursors in ionosphere several days before earthquakes in effort to understanding coupling mechanisms of lithosphere-atmosphere-ionosphere. In this research, ionospheric TEC determination from GPS data, ionospheric TEC modeling over Indonesian region and revealing method of earthquake precursors from ionospheric TEC data and model have been developed from dual frequency GPS data. Conceptual model of lithosphere-atmosphere-ionosphere has been also developed by involved local earth gravity effect that has not been considered, to describe ionospheric TEC variation several days before earthquakes.

Dual frequency GPS signal observations data in Indonesian region and it’s around have been used to derive ionospheric total electron content (TEC) by using phase leveling method. The algorithm of TEC determination has been implemented by using matlab software that can be used to download GPS data through file transfer protocol (ftp) and then to compute TEC value.

Low geomagnetic latitude ionospheric models over Indonesian region have been developed based on the ionospheric TEC data. Temporally the TEC model can be grouped into daily model that has been developed from daily TEC data and monthly model that has been developed from monthly average of daily TEC data. Spatially the TEC model can be classified into local model based on single GPS reference station and regional model based on multiple GPS reference stations. Daily TEC model formulation is combination of polynomial function that is used to represent spatial variation of ionosphere and harmonic function that is used to represent diurnal variation of polynomial coefficients of spatial model of ionosphere. Order of polynomial and harmonic functions are optimized by minimizing of model errors from observations. In this way the optimum local daily TEC model over West Java from BAKO station obtained is P(12,2,2) where the first number is order of harmonic function, the second number is order of polynomial function to represent ionospheric variation to geographic latitude and the last number is order of polynomial function to represent ionospheric variation to geographic longitude. With same way the optimum regional daily model over Indonesian region obtained is P(12,7,3). The regional monthly TEC model over Indonesia is made by using polynomial function to represent ionospheric variation to geographic latitude, harmonic function to represent diurnal variation of ionosphere and linear function to represent ionospheric response to solar activity. Regional monthly TEC model over Indonesia obtained is P(6,5,0,1), where 6 is order of harmonic function to represent diurnal variation of ionosphere in local time frame, 5 is the order polynomial function to represent latitudinal variation of ionosphere, 0 is order of polynomial function to represent longitudinal variation of ionosphere at same local time and latitude, which means that longitudinal dependence of ionosphere is ignored, and 1 is order of polynomial function to represent ionospheric response to solar activity, which means that ionospheric response to solar activity during increasing and decreasing of solar cycle is linear. The regional monthly TEC model obtained is a simplified low latitude TEC regional model (SLTECRM). The SLTECRM which is based on monthly average of TEC data at certain hour can be used as a reference value of TEC in quite days for a particular month over the Indonesian region so the model can be used for ionospheric correction in positioning by using single frequency GPS receiver during quite days and for revealing of ionospheric anomaly related to solar storm as well as earthquake precursor.

The regional monthly TEC model and TEC data can be combined to derive ionospheric activity index (S index), which can be used as a weather space parameter over Indonesia. Ionospheric diurnal tide activity index (A index) was also derived from daily TEC data. Spatial analysis of A index few days before large earthquakes show that the A index can be used to identify earthquake precursors by using exponential function, to predict large earthquakes by using earthquake preparation zone radius model and to approximate earthquake epicenter by using resection method adopted from GPS point positioning.

Decreasing of day time ionospheic TEC few days before large earthquakes can be described by using coupling model of lithosphere-atmosphere-ionosphere caused by decreasing of local earth gravity that can modify E region dynamo electric field through modification of atmospheric diurnal tide amplitude, can accelerate atmospheric vertical drift, and can reduce diffusion drift of ionosphere. Ionospheric anomaly caused by earth gravity anomaly is a hypothetical alternative. Various theoretical and experimental efforts are necessary to examine the hypothesis.

Key words: GPS, ionosphere, low latitude, Indonesia, model, ionospheric index, anomaly, earthquake precursor.

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