Mardina Abdullah

Continuous generation and two‐dimensional structure of equatorial plasma bubbles observed by high‐density GPS receivers in Southeast Asia

High-density GPS receivers located in Southeast Asia (SEA) were utilized to study the two-dimensional structure of ionospheric plasma irregularities in the equatorial region. The longitudinal and latitudinal variations of tens of kilometer-scale irregularities associated with equatorial plasma bubbles (EPBs) were investigated using two-dimensional maps of the rate of total electron content change index (ROTI) from 127 GPS receivers with an average spacing of about 50–100 km. The longitudinal variations of the two-dimensional maps of GPS ROTI measurement on 5 April 2011 revealed that 16 striations of EPBs were generated continuously around the passage of the solar terminator. The separation distance between the subsequent onset locations varied from 100 to 550 km with 10 min intervals. The lifetimes of the EPBs observed by GPS ROTI measurement were between 50 min and over 7 h. The EPBs propagated 440–3000 km toward the east with velocities of 83–162 m s−1. The longitudinal variations of EPBs by GPS ROTI keogram coincided with the depletions of 630 nm emission observed using the airglow imager. Six EPBs were observed by GPS ROTI along the meridian of Equatorial Atmosphere Radar (EAR), while only three EPBs were detected by the EAR. The high-density GPS receivers in SEA have an advantage of providing time continuous descriptions of latitudinal/longitudinal variations of EPBs with both high spatial resolution and broad geographical coverage. The spatial periodicity of the EPBs could be associated with a wavelength of the quasiperiodic structures on the bottomside of the F region which initiate the Rayleigh-Taylor instability.

Daytime gigahertz scintillations near magnetic equator: relationship to blanketing sporadic E and gradient-drift instability

Observations made in non-equatorial regions appear to support the hypothesis that the daytime scintillation of radio signals at gigahertz (GHz) frequencies is produced by the gradient-drift instability (GDI) in the presence of a blanketing sporadic E (Esb) layer. However, the only evidence offered, thus far, to validate this notion, has been some observations of Esb in the vicinity of GHz scintillations. A more comprehensive evaluation requires information about electric field, together with the presence of a steep gradient, which is presumed to be that of Esb. In this regard, the region in the vicinity of the equatorial electrojet (EEJ) appears to be an ideal “laboratory” to conduct such experiments. The dominant driver of electron drift there is the same as that of the EEJ, the vertical polarization electric field, and indications are that the presence of Esb in that vicinity is controlled by a balance in horizontal transport of Esb, between the EEJ electric field and the neutral wind, as described in a model by Tsunoda (On blanketing sporadic E and polarization effects near the equatorial electrojet, 2008). In this paper, we present, for the first time, results from a comprehensive study of daytime GHz scintillations near the magnetic equator. The properties, derived from measurements, are shown, for the first time, to be consistent with a scenario in which Esb presence is dictated by the Tsunoda model, and the plasma-density irregularities responsible for GHz scintillations appear to be produced by the GDI.

Improving ambiguity resolution rate with an accurate ionospheric differential correction

Ambiguity resolution is essential for precise range determination. As it is difficult to process, a good ionospheric model is essential to get unambiguous results or to reduce time to solve the ambiguities. In this paper, a developed model to determine the differential ionospheric error to sub-centimetre accuracy is described. As a function of elevation angle and TEC, the model is applicable at any location and only requires a single frequency receiver provided the TEC over the reference station is known. It has been evaluated using real GPS measurements at spaced stations in Glasgow (UK) and Stirling (UK), where the results showed good correlation. It was found that the variance ratio and reference variance of the ambiguity resolution rate and the quality of the differential positioning solution are improved. Significant improvements of more than 50% have also been found by correcting the differential ionospheric delay in the measurements for the estimated positions.

Cloud-Cover Statistics and Cloud Attenuation at Ka- and V-Bands for Satellite Systems Design in Tropical Wet Climate

Cloud-cover statistics, low cloud base height, frequency of precipitation, 0°C isothermal height, and integrated cloud liquid water and cloud attenuation have been obtained for the tropical rain forest climatic zone of Africa. The cumulative distribution of integrated cloud liquid water content shows a departure from the ITU-R model. A comparison of cloud attenuation at Ka-and V-bands show that the ITU-R model underestimates the attenuation up to about 1.7 and 2.3 dB at 30 and 50 GHz, respectively.

Ionospheric mapping function for total electron content (TEC) using global positioning system (GPS) data in Malaysia

The ionosphere layer is very important to the communication system. This research involves the determination of total electron content (TEC) in ionosphere based on height in order to determine the appropriate TEC value for Malaysia and for the equitorial region generally. The ionospheric model used is the single layer model based on the Bernese GPS 5.0 Software. The ionosphere TEC map of Malaysia is produced by using the single layer model which can be found in Bernese GPS 5.0 Software. Results show that the ionospheric variation especially the TEC values are different relative to the height studied. The appropriate TEC value for Malaysia is at the height 450 km and the maximun TEC value is at the height of 150 km. These variations play an important role in understanding the TEC nature in ionosphere and thus will simplify the studies about phenomenon happen in ionosphere especially in Malaysia.

Performance analysis of a free-space terrestrial optical system in the presence of absorption, scattering, and pointing error

Performance analysis has been carried out to evaluate the performance of a free-space optical terrestrial system using on-off keying in the presence of scattering and absorption due to atmospheric gas molecules and aerosols, and pointing error phenomenon due to the link missalignment. It is found that in order to achieve a bit error rate (BER) of 10 −9 at a visibility of 1.9 km, a 13.2 dB power penalty is incurred for atmospheric channel in haze condition and for atmospheric channel in rain condition, a heavy rainfall rate of 25 mm/h produces a high atmospheric attenuation coefficient α, and thus almost 18 dB power penalty is induced to achieve a BER of 10 −9 . However, for the atmospheric channel in a fog condition, the visibility can reach 1.1 km and induces a power penalty of 25 dB to maintain the BER of 10 −9 . The power penalty increases when the visibility is reduced, rainfall intensity increased, and an additional power penalty of more than 25 dB is incurred in the presence of pointing error for atmospheric channel of 1.3 km visibility in a light fog condition compared to clear weather condition. C 2011 Society of Photo-Optical Instrumentation Engineers

GPS Total Electron Content (TEC) Prediction at Ionosphere Layer over the Equatorial Region

Space weather is a fairly new field in science today and has very interesting effects on humans, environment and technology in general. Scientists are now studying space weather with a wide range of tools to try to learn more about the physical and chemical processes taking place in the upper atmosphere and beyond. One of these tools is Global Positioning System (GPS). GPS is currently one of the most popular global satellite positioning systems due to global availability of signal as well as performance. GPS is a satellite-based navigation radio system which is used to verify the position and time in space and on the Earth. GPS nowadays allows to measure positions in real time with an accuracy of few centimetres (Warnant et al., 2007). The advent of GPS has led to technical revolutions in navigation as well as in fields related to surveying. The GPS system - an all-weather satellite-based radio navigation system - can provide users on a world-wide basis with navigation, positioning, and time information which is not possible with conventional navigation and surveying methods. Apart from geodesy and geophysical interest, GPS has great importance in scientific applications. The GPS satellites that are orbiting the Earth, at altitudes of about 20,200 km, transmit signals that propagate through the ionosphere that exists at about 60 –1500 km above the Earth’s surface. The signals from the GPS satellites travel through the ionosphere on their way to receivers on the Earth’s surface. The free electrons populating this region of the atmosphere affect the propagation of the signals, changing their velocity and direction of travel as shown at figure 1. Due to the inhomogeneity of the propagation medium in the ionosphere, the GPS signal does not travel along a perfectly straight line (Ioannides & Strangeways, 2000). The effects of the ionosphere can cause range-rate errors for users of the GPS satellites who require high accuracy measurements (Bradford & Spilker, 1996). Ionosphere is highly variable in space and time (sunspot cycle, seasonal, and diurnal), with geographical location (polar, aurora zones, mid-latitudes and equatorial regions), and with certain solar-related ionospheric disturbances. Ionosphere research attracts significant attention from the GPS community because ionosphere range delay on GPS signals is a major error source in GPS positioning and navigation. The ionosphere has practical importance in GPS applications because it influences the transionospheric radio wave

Forecasting of ionospheric delay over Parit Raja Station, Johor, using statistical method

GPS is of great importance in precise positioning, however, the accuracy is marked by error sources, i.e. ionospheric effects. The signal information is delayed and the carrier phase experiences an advance due to the dispersive character of the ionosphere. The delay can be more than 100 meters in the worst case scenario and tends to increase with increasing solar activity. This error can be corrected by processing the GPS data if it is known. This paper describes the possibility of using statistical methods to forecast the ionospheric delay which shows repeatable patterns in time series. The statistical method used is the Holt-Winter method due to its ability to forecast time series with repeated trends and seasonal patterns. Based on the TEC data collected during a period of a month, ionospheric delay forecast is generated for the following month, which is then compared with the real data. Results show that there is a 6% error between the forecast and the real ionospheric delay and the error correction for the delay can be more than 90%.

Effect of Leadframe Oxidation on the Reliability of a Quad Flat No-Lead Package

This study analyzed the reliability of two condition types of copper leadframe, namely good and oxidized. Both types of leadframe were used in the fabricating process of a quad flat no-Lead (QFN) package. Determining the adhesiveness of die strength and determining the strength of the leadframe are important in order for the package to obtain higher reliability. Commonly an epoxy material is used in the die attachment process for die adherence onto the leadframe. A statistical analysis of the die shear test and the cyclic strain test of a QFN package was performed in this paper. From the application of the t-test method, the p-value shows a significant difference in die shear stress depending on leadframe condition. Based on a process capability ratio (C pk ) above 1, the leadframe in good condition showed better processing capability than the oxidized leadframe. The Coffin-Manson approach was used in the cyclic loading of a QFN package to predict the package reliability in terms of cyclic strain. The leadframe in good condition showed a higher die strength value and lower microstrain compared with the oxidized leadframe. The oxidized leadframe provided a poor surface to attach adhesive and a higher microstrain on cyclic load, resulting in a negative effect on package reliability, such as a crack phenomenon at the epoxy interface between the die and the leadframe. This occurrence may ultimately cause delamination, which occurs between the die and the leadframe die pad.

On the evolution of a magnetic flux rope: Two‐dimensional MHD simulation results

We use the time-dependent, two-dimensional (2-D), ideal MHD equations to simulate and investigate the evolution of magnetic field and plasma profiles of the typical (T) and crater (C) magnetic flux ropes (FRs). The T-FR has a magnetic pressure peak at the center of the flux rope while the C-FR has a local dip instead. The simulation starts with a 2-D magnetic flux rope in magnetohydrostatic equilibrium, where pressure gradient forces are balanced by Lorentz forces. The magnetic field and plasma pressure profiles for the initial flux rope are derived from the analytical solutions by Zhang et al. (2010). The initial flux rope starts to evolve when the force balance is broken by imposing pressure or magnetic field perturbations onto the equilibrium system. The pressure perturbations are produced by increasing/decreasing the internal plasma pressure of the flux rope, while the magnetic field perturbations are produced by increasing/decreasing the transverse magnetic fields across the flux rope. We conclude that a T-FR can be evolved into a C-FR and vice versa, if the perturbation strength is sufficient, and that the plasma pressure and density in the new equilibrium state could be either increased or decreased for the evolution of C-FR to T-FR and also for the evolution of T-FR to C-FR.