CHAPTER ONE: INTRODUCTION
1.1 General Introduction
Modern society depends heavily on a variety of technologies that are susceptible to the extremes of space weather — severe disturbances of the upper atmosphere and of the near-Earth space environment that are driven by the magnetic activity of the Sun. for instance, strong electrical currents driven in the Earth’s surface during auroral events can disrupt and damage modern electric power grids and may contribute to the corrosion of oil and gas pipelines. Changes in the ionosphere during geomagnetic storms driven by magnetic activity of the Sun interfere with high-frequency radio communications and GPS navigation. During polar cap absorption events caused by solar protons, radio communications can be severely compromised for commercial airliners on transpolar crossing routes. Exposure of spacecrafts to energetic particles during solar energetic particle events and radiation belt enhancements can cause temporary operational anomalies, damage critical electronics, degrade solar arrays, and blind optical systems such as imagers and star trackers used on commercial and government satellites. Space explorers must be constantly aware of the current space weather and be prepared to handle the most extreme conditions that might be encountered.
Thus, this work aims at making valuable contribution to space weather monitoring since much of the dynamics of storm-time ionosphere originates in the equatorial region.
1.2 The Earth’s Atmosphere
The Earth’s atmosphere is stratified into layers based on temperature variation as shown in Fig.1.1. The stratification in increasing order of altitude from sea surface are; the troposphere (0 – 18 Km), Stratosphere (18 – 90 Km), Mesosphere (90 -350 Km), thermosphere (350 – 1000km). The Ionosphere is overlapped by both mesosphere and thermosphere.
Each layer is characterized by different density of atmospheric constituents and experience different weather phenomenon. Due to its apparent proximity to the Sun, the ionosphere is specifically characterized by ionized particles and thus is greatly influenced by space weather.
1.3 The Ionosphere
The ionosphere has several layers created at different altitudes and made up of different densities of ionization. Each layer has its own properties, and the existence and number of layers change daily under the influence of the Sun. During the day, the ionosphere is heavily ionized by the Sun. During the night hours the cosmic rays dominate because there is no ionization caused by the Sun, which has set below the horizon. Thus there is a daily cycle associated with the ionizations.
In addition to the daily fluctuations, activity on the Sun can cause dramatic sudden changes to the ionosphere. The Sun can unexpectedly erupt with a solar flare (Okeke and Soon, 2004), which is a violent explosion in the Sun’s atmosphere caused by huge magnetic activity. These sudden flares produce large amounts of X-rays and EUV energy that travel to the Earth and other planets at the speed of light.
The Sun spews out a constant stream of X-ray and extreme ultraviolet (EUV) radiation. This energy, along with that from cosmic rays, affects the Earth’s ionosphere. When solar energy in the form of solar wind blows across the ionosphere, electrons are precipitated from neutral molecules, resulting in ionization. These free electrons in the ionosphere strongly influence propagation of radio signals.
As illustrated in Fig. 1.2, radio frequencies of very long wavelength and otherwise very low frequency (VLF) reflect off these free electrons in the ionosphere thus, allowing radio communication over the horizon and around the curved Earth. The strength of the received radio signal changes according to how much ionization has occurred and from which level of the ionosphere the VLF wave has reflected.