Alina Marie Hasbi

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.

Climatology of successive equatorial plasma bubbles observed by GPS ROTI over Malaysia

The occurrence rate of the equatorial plasma bubble (EPB) with season, solar activity, and geomagnetic conditions are investigated using long-term data sets of Malaysia Real-Time Kinematics Network (MyRTKnet) from 2008 to 2013. The rate of TEC (total electron content) change index (ROTI) in 5 min was derived from MyRTKnet data to detect the EPB with scale sizes around tens of kilometers. Then, the daily east-west cross sections of 2-D ROTI maps were used to examine the EPB features over 100°E–119°E longitudes. The EPBs tend to occur successively in one night along the observational coverage of MyRTKnet during equinoxes in high solar activity years. The perturbations in a form of wavelike structures along the observed longitudes might be responsible for the development of successive EPBs due to high growth rate of the Rayleigh-Taylor instability (RTI) process. On the contrary, the occurrence of successive EPBs is infrequent and the occurrence day of EPB remains active during equinoctial months in low solar activity years. The small growth rate of the RTI process during low solar activity years might require a strong seed perturbation to generate the EPB structure. The occurrence probability of the EPB was found to be similar during quiet and disturbed geomagnetic conditions. The results imply that the strong perturbations play an important role in the development of the EPB in low solar activity years. Nonetheless, the high growth rate of the RTI could cause the successive occurrence of the EPB in high solar activity years.

The Hurst exponents of the geomagnetic horizontal component during quiet and active periods

The focus in this paper is mainly to characterize the fractal properties denoted by the Hurst exponent of geomagnetic field during quiet and active periods of geomagnetism activity. Analysis is made on the time series of the geomagnetic horizontal component data, H, which were acquired by the Magnetic Data Acquisition System (MAGDAS) developed by the Space Environment Research Center (SERC) of Kyushu University in Japan. The data set covers the quiet and active periods in August 2005 at the equatorial stations of Cebu and Davao in the Philippines. This month had very intense geomagnetic storm with the Dstmin, Kp and Ap indices for the quietest and active days of the month are −20 nT, 2, 6 and −216 nT, 9, 110 respectively. For both stations, the data sampling interval is 1 second for each time periods or one day giving a sample size of 86,400. Using the fast Fourier transform, the power law of the data time series was obtained in the shorter time periods of 10 minutes to 6 hours. This observed power spectrum law exhibit scaling with corresponding Hurst exponents of 0.3–0.5 for quiet periods and 0.5–0.7 for active periods for both stations. For comparison, other fractal techniques were performed which are rescaled range analysis (RS) and detrended fluctuation analysis (DFA) on the data set, and the same ranges of the Hurst exponents were obtained. The work also includes simulation of the similar sized data sets by generating the fractional Brownian motions (FBM) with the Hurst exponents of 0.3–0.7. Our results show that the H component at these equatorial stations is a fractal in nature and become more persistent during active periods of geomagnetism activity. Thus, the Hurst exponent could be used to characterize the geomagnetic time series during quiet and active periods.

Ionospheric TEC and scintillation during the 15–16 May 2005 major storm over equatorial anomaly region at ARAU

The ionospheric TEC and scintillation measurements were investigated at ARAU, Malaysia station based on GPS measurements during major storm on 15-16 May 2005. The major storm was observed for the period from 12 to 17 May 2005 and was accompanied by a moderate solar activity with F10.7 > 100 and SSN > 100. The maximum readings of geomagnetic indices Dst, Kp and Ap recorded by the World Data Centre (WDC) are -263 nT, 9 and 105 respectively. The solar wind index, Bz showed a sharp incursion to the south with a peak reaching -37 nT. The 15-16 May 2005 storm has only one SSC and is the most intense storm recorded during the solar minimum 2005. The results show pronounced phase and amplitude scintillations and sudden increase in TEC is clearly observed during the storm. The peak phase scintillation recorded during the storm is 0.19 m and the measured amplitude scintillation and S4 index on both L1 and L2 bands are > 13 dB-Hz and > 0.7 respectively.

Antennas and propagation around the world: CALLISTO radio spectrometer construction at Universiti Kebangsaan Malaysia

The Compound Astronomical Low-Cost Low-Frequency Instrument for Spectroscopy and Transportable Observatory (CALLISTO) is a worldwide network of radio spectrometers designed for 24-hour solar observation. It is a collaboration among the Swiss Federal Institute of Technology Zurich and numerous local host institutes, spread out around the world. This paper presents an overview of the CALLISTO spectrometer system at the Universiti Kebangsaan Malaysia (UKM). The ?Malaysia-UKM? station is installed inside the campus area, at the Permata Pintar School building. In particular, this paper explains the details of the instruments setup, and reports some of the data observed from this station. The realtime data are available from the e-CALLISTO International Network of Solar Radio Spectrometers.

Two-dimensional structure of equatorial plasma bubble observed using GPS networks in South East Asia region

Malaysia is located at equatorial region, which is from 3° to 8° South of the magnetic equator where equatorial plasma bubbles (EPBs) frequently occur. EPBs could disrupt radio communication and navigational systems in this region. We present the two-dimensional structure of EPBs observed using GPS networks over South East Asia (SEA) near the equatorial region. 59 days with EPB structure were observed using GPS networks in 2011. The results show that; 1) the initial EPBs onset time occurs mostly after post-sunset between 1900 to 2100 local time (LT); 2) the duration of the occurrences of EPB is mostly from 3 to 6 hours; 3) the GPS networks in SEA region are able to observe east-west size of EPBs from 50 km to 650 km with accuracy +/- 50 km.

Development of a VLF receiver system for Sudden Ionospheric Disturbances (SID) detection

Sudden Ionospheric Disturbances (SID) are transient changes in the ionosphere caused by enhancement in X-ray and EUV fluxes during solar flare events. The SuperSID space weather monitor has been developed by Stanford University Solar Center to detect SID via VLF remote sensing. In this paper, a receiver system named as UKM-SuperSID has been developed to detect SID in the equatorial region. The system, which consists of loop antenna, preamplifier and a computer, is able to detect VLF signals with frequency of 19.8 kHz transmitted from North West Cape station (NWC), Australia. The data obtained showed repetitive diurnal variation. During the observation period from 11 December 2011 until 17 January 2012, M and C class solar flares events have been detected, with a class M1.5 flare observed as the largest detected flare. The results showed that the effectiveness of the UKM-SuperSID in detecting SID in the equatorial region thus enables it to be part of the global space weather sensor network.

Improving ionospheric forecasting using statistical method for accurate GPS positioning over Malaysia

Total Electron Content (TEC) is the main parameter in the ionosphere that has significant effect on the propagation of radio waves causing delay errors on the GPS signals. Therefore, identifying an effective forecasting method is important to reduce the GPS position error. This research aimed to use the statistical Holt-Winter method to forecast the ionospheric vertical TEC (VTEC) over Malaysia. The data were derived from the Department of Survey and Mapping Malaysia (DSMM) dual frequency GPS receiver located in three stations over Malaysia, namely the Kedah station at geographic coordinates of 6.46°N-100.50°E and geomagnetic coordinates of 3.32°S-172.99°E, Johor station (south of Peninsular Malaysia) at geographic coordinates of 1.36°N-104.10°E and geomagnetic coordinates of 8.43°S–176.53°E, and Terengganu station at geographic coordinates of 4.62°N-103.21°E and geomagnetic coordinates of 5.64°N-174.98°E, during March of 2013. Based on the result, the peak of the ionospheric delay for the diurnal hourly variation was at 17:00 LT, while the minimum was from 6:00–7:00 LT. There was no significant difference in the VTEC trend among the three stations. The Holt-Winter method provided an accurate forecast during the quiet and disturbed period. The maximum MAPE was 4.5% during the quiet day and 9.1% during the disturbed day.

Performance of Multi tone Code Division Multiple Access (MT-CDMA) in an AWGN Channel and in Presence of Narrowband Jamming

The main limits of the conventional single carrier modulation techniques are the restrictions imposed by the multipath channel, and the receiver complexity. On other hand the multicarrier techniques such as multi-carrier (MC) and multi-tone (MT) can provide high data rates at reasonable receiver complexities. Jamming on the other hand is of interest in some communication applications, and in military anti-jam systems. In this work, we study the performance of multi-tone code division multiple access (MT-CDMA) in presence of narrowband jamming. We investigate bit error rate (BER) performance as a function of some system parameters such as number of sub-carriers, processing gain, and as a function of channel conditions such as jamming type, number of jammers, jamming center frequency, jamming power, and jamming to signal power ratio. We compare the performance with the analytical performance in absence of any jamming. The simulation results show, that the performance can be enhanced by increasing the processing gain, number of jammers or by decreasing the number of sub-carriers, the jamming to power ratio or jamming power. Furthermore the effect of jamming center frequency on the performance is negligible, and good agreement with the analytical performance in absence of jamming is achieved.