1. UKM Learning and Research Repository
2. PHD Thesis / Tesis PHD
3. Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

Title:	Investigation of the gravity waves seeding of the equatorial spread F initiation in the South American region
Authors:	Afolayan Abimbola Oluwatosin (P80278)
Supervisor:	Mandeep Singh, Prof. Ir. Dr.
Keywords:	Universiti Kebangsaan Malaysia -- Dissertations Dissertations, Academic -- Malaysia Equatorial F Gravity waves
Issue Date:	10-Jul-2019
Description:	The Equatorial F region is often characterized by the large scale plasma depletion triggered during the post-sunset electrodynamics phenomenon. The subsequent secondary cascading instability processes along the field-aligned bubble wall produces a wide spectrum of irregularities known as the Equatorial spread F (ESF). The resultant ionospheric turbulence could disrupt the radio propagation through an intense phase or amplitude scintillation. The solar terminator-magnetic field alignment control of the Fregion zonal electric field enhancement and the ESF activity is well-known. However, the observation of significant ESF occurrence probability under weak background ionospheric condition highlights the role of externally driven perturbation electric field. This study aims to characterize the complex inter-dependent role of the factors influencing the ESF onset and occurrence probability. Secondly, investigate the seeding effect by the convective gravity waves (GW) on the plasma irregularity features and derive a model for the diurnal variability of the ESF occurrence. The analyzed ionospheric parameters were recorded at the dip-equatorial ionosonde stations (Sao Luiz; 440W, 2.30S and Jicamarca; 75.760W, 8.170S) within the South American longitude sector. The Ionosonde stations in this region were chosen for investigating the ESF occurrence dynamics due to the large data availability. The critical frequency was subjected to the Morlet wavelet transform for days with altitude above the seasonal threshold height. The scale-averaged power spectrum within 30 - 60 minutes period defines the GW-induced density perturbation. The empirical model was derived (validated) using six (three) years data measured at Jicamarca station, which represents the low (LSA), moderate (MSA) and high (HSA) solar activity period. Preliminary results from the Brazilian longitude showed that the magnitude of the density scale length is quite significant to the observed presence/absence of ESF in this region. The comparative analysis of the precursory factors showed that a large seed amplitude becomes requisite for the irregularity initiation during days with an unfavourable background condition. Furthermore, an exponential function was fitted to the altitudinal variation of the requisite seed during each season across the solar epochs. The exponential fit constants were expressed as a function of the solar flux index (F10.7) and an empirical expression was derived for the altitudinal dependence of the requisite seed during each season. The threshold curve generally demonstrated a reduction in the requisite seed for ESF occurrence as the virtual height (h'F) increases but there is observation of a unique altitudinal dependence during each season. While requisite seed amplitude at a given h'F increases with the solar flux due to neutral density effect. For instance, the requisite seed of 0.2698 MHz recorded at ~280 km during the M-equinox of the LSA was increased by a factor of 5 during the MSA. The model showed an accuracy of approximately 80% averaged across the seasons of the HSA, MSA and LSA, while the overall best prediction was recorded during the M-equinox of the LSA. The observed exponential variation of the altitudinal dependence of the seed amplitude is similar with results from previous study in the Indian longitude. The inclusion of scale length effect in this study has further improved the derivation of the model. These results are expected to significantly advance the understanding of the ESF morphology and the day-to-day ESF forecast in the South American region.,Ph.D.
Pages:	159
Publisher:	UKM, Bangi
Appears in Collections:	Faculty of Engineering and Built Environment / Fakulti Kejuruteraan dan Alam Bina

Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.