Mapping equatorial ionospheric profiles over peninsular Malaysia using gps tomography

Abstract

The ionospheric conditions over Malaysia are profoundly critical not only duea to its location that is near to the equator but also due to the high solar activity that occurred during the 11-years solar cycle. The two-dimensional (2D) single thin layer model (SLM) has been widely used to monitor and model the ionosphere. However, this model only focuses on the height of the maximum densities of the electron, which lies within 300 kilometre (km) to 450 km above the Earth and therefore neglects the information of bottom and topside of the ionosphere. Hence, a three-dimensional (3D) ionospheric structure is proposed to address these limitations. The aim of this study is to model the electron density profile over Peninsular Malaysia using Global Positioning System (GPS) ionospheric tomography method. In doing so, the Malaysian Real-time Kinematic Network (MyRTKnet) over Malaysia was utilized to derive the total electron content (TEC) maps. It was found that the variations of the TEC increase with decreasing of latitude and longitude, and gradually change from East to West direction. The GPS-derived TEC from the years 2009 to 2014 shows that the maximum yearly mean TEC over Malaysia is up to 58 TEC unit (TECU), recorded during the year 2014 which was associated with high sunspot numbers. The maximum yearly mean and the minimum peak of diurnal variations occur at 08 universal time (UT) and 21UT respectively. Next, the receiver code bias (DCBr) was estimated for MyRTKnet stations using the adopted algorithm from IONOLAB-BIAS. For assessment purpose, this method shows a good estimation of DCBr with the International Global Navigation Satellite System (GNSS) Service (IGS) analysis centre compared to with Bernese software. Then, the GPS ionospheric tomography module was developed to reconstruct the electron density profile over Peninsular Malaysia. The results were validated with the nearest ionosonde station and the ionospheric global models such as the International Reference Ionosphere (IRI) model and NeQuick model. It was found that the differences between GPS ionospheric tomography with the models are small during the daytime but large at night-time. It was also found that, the GPS ionospheric tomography appears to be more agreeable with the IRI model than with NeQuick model. For the validation of the NmF2 parameters with the IRI model and ionosonde measurements, the GPS ionospheric tomography is more agreeable with the ionosonde than with the IRI model. The results also show that the GPS ionospheric tomography is capable to show the vertical ionospheric profile over the study area during quiet ionospheric conditions and its irregularities during disturbed conditions of the ionosphere. Overall, it was found that the GPS ionospheric tomography method is suitable for examining and monitoring the ionospheric variations and irregularities in support of the space weather studies in Peninsular Malaysia.