Total Electron Content (Tec) Variability of Low Latitude Ionosphere and Role of Dynamical Coupling: Quiet and Storm-Time Characteristics

Abraha, Gebregiorgis (2014) Total Electron Content (Tec) Variability of Low Latitude Ionosphere and Role of Dynamical Coupling: Quiet and Storm-Time Characteristics. PhD thesis, Addis Ababa University.

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The low latitude ionosphere and its coupling to lower atmosphere such as (mesosphere, stratosphere and thermosphere)and high latitude ionosphere are investigated using observational and model data. The dominant modes of variability of low latitude ionosphere are linked to Equatorial Electrojet (EEJ), Counter Electrojet(CEJ) and Prereversal Enhancement(PRE). These are investigated using magnetometer data and EEJ models. The results of the analysis reveal that EEJ undergoes day and seasonal variations. From the daily variation analysis, the EEJ value at Addis Ababa sharply increases to its peak value from 9.00-13.00 LT and dies out around 18.00 LT. EEJ versus time graphs shows that the H-components of the magnetic field at Addis Ababa location varies by a factor of 2-4 times larger compared to Adigrat. Seasonally as both measurements and model data revealed, EEJ has significantly increased during E season compared to D and J seasons. Our observation also shows that counter CEJ are more frequent on solstices than equinoxes and on the contrary Morning Counter Electrojets (MCEJ) occurs more frequently on equinoxes than solstices. Apart from that the localized EEJ, CEJ and PRE, low latitude ionosphere is dynamically coupled to lower atmosphere and high latitude ionosphere that control both low latitude ionospheric dynamics and chemistry during both quiet and storm-time. In this regard, the storm-time low latitude ionospheric total electron content (TEC) variability and the influences of lower and high latitude ionsophere are investigated. The TEC variation is characterized in relation to diurnal, seasonal, temporal, spatial and longitudinal behaviors. The diurnal characteristics of TEC can be explained in terms of the Diurnal, Semidiurnal,Terdiurnal and Quterdiurnal tides. The TEC climatology from NeQuick and ionosonde is also compared during both quiet and geomagnetically disturbed days. Day time enhanced TEC value followed by night time depilation is a typical behavior of the ionosphere. A geomagnetic storm occurred during January 22-25, 2012. The Disturbance storm time (Dst) values for the period shows that it is a moderate intensity geomagnetic storm. The geomagnetic storm is characterized by different indices and indicators. The Symmetric H component of ring current (SY M−H) value on January 22 shows a sudden increase to more than 50 nT at the Sudden Storm Commencement (SSC) and followed by sharp decrease to a value of -100 nT after which a recovering started. A second SSC on January 24 followed by a shock on January 25. These SSCs before the main storms on January 22 and 25 are evidences for the occurrence of Coronal Mass Ejection (CME). Moreover, the short recovery period, unlike that of Co-rotating Interaction Regions (CIR) driven geomagnetic storms, implies the geomagnetic storm is a CME-driven. The sudden jump of the solar wind dynamic pressure and the The southward Interplanetary Magnetic Field (IMF Bz) are also consistent with occurrence of CME. Our observational evidences of Proton fluxes of high energy ranges and increase in proton density are also other indicators for occurrence of CME. The high values Auroral Electrojet (AE) index on these dates implies ionospheric perturbation in response to the storm. This is also reflected in the Total Electron Content (TEC) change during the storm relative to quiet day TEC over the polar regions. The response of the ionospheric to geomagnetic storms is also investigated from amplitude modulation of wave components that account for the majority of TEC variance during the period. The diurnal and semidiurnal TEC variances account upto a maximum of 83% and 30% of the TEC variance over fairly exclusive ionospheric regions respectively. The diurnal variability dominates the subtropical latitude where the solar cycle is an important factor while the semidiurnal variance is limited to higher latitudes. The diurnal and semidiurnal TEC variances show hemispheric asymmetries. The stationary planetary waves also account for TEC variances that exceed semidiurnal TEC variance and exhibit hemispheric asymmetry of opposite sign to diurnal TEC variance. These features of TEC variance are climatology of TEC variability irrespective of the storms. However, the impact of the geomagnetic storms are distinctly marked in the daily time series of amplitudes of diurnal and semidiurnal migrating tides and stationary planetary waves. The abrupt changes in amplitudes of diurnal (upto 5 TECU) and semidiurnal (upto 2 TECU) migrating tides are observed within the 20oS-20oN latitude band and along 20oN respectively while that of stationary planetary wavenumber 1 is in the order of 3 TECU and is mainly localized along 20oS.

Item Type: Thesis (PhD)
Subjects: Q Science > Q Science (General)
Q Science > QC Physics
Divisions: Africana
Depositing User: Selom Ghislain
Date Deposited: 19 Nov 2018 13:14
Last Modified: 19 Nov 2018 13:14

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