1.1 BACKGROUND OF THE STUDY
Ozone depletion and geomagnetic storm are among the most severe phenomena that disturb the world today. When there is ozone depletion, harmful ultraviolet (UV) radiations from the sun penetrates into the earth and affect human health and ecosystem in general. Ozone depletion and geomagnetic storm generally contribute to climate change. Some research works have suggested that geomagnetic storm is always associated with ozone variation in the mid-latitude. Until now, no scientific work has been carried out to ascertain this fact. Also, researchers have hitherto considered the zonal wind to be weak at the tropics (where Nigeria is located) hence, dynamical processes around the low and mid-latitude have not been adequately considered.
As a secondary pollutant, ozone is not emitted directly but is generated in the atmosphere through a complex series of chemical reactions initiated by absorption of solar energy (Seinfeld and Pandis, 1998 J.H. Seinfeld and P.J. Pandis, Atmospheric Chemistry and Physics: From Air Pollution to Climate Change, John Wiley, New York (1998).Seinfeld and Pandis, 1998). The atmospheric wind influences the time of occurrence of the daytime ozone maximum. Increase in air temperature as a result of intense solar radiation causes an increase in the variation of total ozone. This is because ozone in the stratosphere is created and destroyed primarily by ultraviolet (UV) radiation from the sun. That is; ozone is formed when oxygen molecules absorb UV radiation and split apart into two oxygen atoms (O), which combine with other oxygen molecules (O2), to form ozone molecules (O3). Ozone is also broken apart as it absorbs UV radiation. In this way, UV helps sustain the natural balance of ozone in the stratosphere, while ozone in turn absorbs UV, protecting life on earth from these harmful radiations.
The solar chromospheric activity in the ultraviolet region is of great importance to our understanding of both the physical properties of the sun as a star, and of the solar influence on the earth’s stratospheric chemistry. Okeke (2012) noted that how sun’s magnetic field connects with the geomagnetic field determines how solar activity affects the earth. The interaction between the solar plasma and the earth’s magnetic field causes a number of current systems to develop in the magnetosphere during a magnetic storm. In other words, compression of the magnetosphere by plasma causes development of magnetopause current and ring current systems which are the main current systems responsible for a typical magnetic storm recorded at mid- and low-latitudes. When the solar activity is high both solar UV radiation and ozone concentrations are high.
The atmospheric activities that cause ozone variations are photochemical processes and dynamical processes. The former refers to the production and destruction of ozone and it is driven by solar ultraviolet (UV) photochemistry in the upper stratosphere. The later refers to how ozone is transported from one location to another by winds and large scale circulation patterns in the atmosphere. Atmospheric angular momentum (AAM) is a fundamental circulation index used in representing and measuring the dynamic state of the whole circulation of the atmosphere and climate (Peixoto and Oort, 1992; Abaraca del Rio et al., 2000; 2003). It has been observed that much of the historic development of modern meteorology is connected with the study of how atmospheric momentum is maintained locally or transported from one region to another and how it is exchanging momentum with the oceans and solid earth (Peixoto and Oort, 1992). The main contributions to the axial AAM components are the global zonal winds in the troposphere and stratosphere. AAM is highly correlated with changes in length of day (LOD), a measure of the earth’s rotation rate (Eubanks, 1993).
Nigeria, being in the tropics and where not much research work in the depletion of ozone and consequences of zonal winds, solar activity phenomenon together with geomagnetic storms on the ozone variation has been investigated, it becomes crucial that this study be carried out in Nigeria. Hence, in our study, an attempt will be made to study the effect of this interaction; zonal winds, solar activity and magnetic storm influence over six stations namely; Sokoto (13.030N 05.270E), Maiduguri (12.000N 13.330E), Abuja (09.080N 07.050E), Ikeja (06.420N 03.450E), Port-Harcourt (04.850N 07.02oE) and Enugu (06.430N 07.480E), in the tropics – Nigeria (West Africa) as a case study; this was selected according to the six (6) geo-political zones in Nigeria to ensure even distribution. Isikwue (2009) suggested that effects of geomagnetic storms on ozone in the tropical latitudes be carried out, since Mitra (1947) noted that increase in the ozone values in the middle atmosphere was always associated with geomagnetic storms. Since then, no research work has been carried out to investigate the extent or validity of Mitra’s finding; it is on this note and on existing controversies and inconsistencies that this work becomes very necessary.
1.2 THE EARTH’S ATMOSPHERE
The atmosphere that is very essential for all life forms on earth is a mixture of many gases. The three major components of ordinary air near the surface are nitrogen (76.9%) and oxygen (20.7%) with the next largest component being water vapor (1.4%). Many gases in the atmosphere are capable of chemical reactions. Some of those present in trace amounts may form combinations that are commonly considered to be pollutants. These and other potentially harmful gases are monitored in many urban areas by State Health departments or by Environmental Protection Agency, these include nitrogen oxides, sulfur dioxide, carbon monoxide, methane, ozone, and ammonia (Eagleman, 1980). The atmosphere protects life on earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.
1.2.1 TEMPERATURE AND LAYERS
The temperature of the earth’s atmosphere varies with altitude (temperature inversion) as shown in Figure 1. The relationship between temperature and altitude varies among four different atmospheric layers; the ionosphere is part of the thermosphere. The atmosphere is thickest near the surface and thins out with height until it eventually merges with space. There is no definite boundary between the atmosphere and the outer space. It slowly becomes thinner and fades into space. The primary classification is by temperature, composition, and state of mixing. In this system individual temperature regions are named sphere and each upper boundary is called a pause. According to Ahrens, (2003), the stratifications of the earth atmosphere are thus;
The troposphere is the first layer above the surface which extends to between 7 km at the poles and 15 km at the equator, with some variation due to weather factors. This layer contains half of the earth’s atmosphere and weather occurs in it. The troposphere has a great deal of vertical mixing because of solar heating at the area. This heating makes air masses less dense so they rise. When an air mass rises, the pressure upon it decreases so it expands, doing work against the opposing pressure of the surrounding air. To do work is to expand energy, so the temperature of the air mass decreases. As the temperature decreases, water vapor in the air mass may condense or solidify, releasing latent heat that further uplifts the air mass. This process determines the maximum rate of decline of temperature with height, called the adiabatic lapse rate. The average temperature of the atmosphere at the surface of earth is 293 K. Thus, the temperature falls off at a rate of about 10 K km-1 or less. The boundary between the stratosphere and troposphere is known astropopause.
The stratosphere is the region above the troposphere. Stratosphere is a layer located at about 15 – 50 km above sea level. The stratosphere was originally thought to be isothermal; it is a region of increasing temperature. The reason for the temperature inversion in the stratosphere is that the gas ozone plays a major part in heating the air at this altitude. Some of the absorbed energetic ultraviolet (UV) solar energy warms the stratosphere, which explains why there is an inversion. If ozone were not present, the air probably would become colder with height as it does in the troposphere. The ozone is located in the lower portion of the stratosphere from approximately 15–35 km above earth’s surface, though the thickness varies seasonally and geographically and it protects plants, animals, and human beings by shielding them from the most harmful rays of the sun. The level of maximum ozone concentration is near 25km (middle latitudes) and yet the stratospheric air temperature reaches a maximum near 50km. The reason for this is that the air at 50km is less dense than at 25km, and so the absorption of intense solar energy at 50km raises the temperature of fewer molecules to a greater degree. Many jet aircrafts fly in the stratosphere because it is very stable. Stratopauseis the boundary between the mesosphere and the stratosphere, typically 50 – 55 km.
This layer extends from about 50 km to the range of 80 – 85 km. Temperature decreases with height, reaching around 180 K in the upper mesosphere; this is due to the fact that there is little ozone in the air to absorb solar radiation. Consequently, the molecules (most especially those near the top of the mesosphere) are able to lose more energy than they absorb, the result of this process is energy deficit and cooling. The percentage of nitrogen and oxygen in this layer is about the same as at the sea level. Ultraviolet solar energy could easily cause severe burns on exposed parts of the body. Again, given the low air pressure, the blood in one’s veins would begin to boil at normal body temperatures. Meteors or rock fragments burn up in this layer when entering the atmosphere Mesopauseis the coldest place on earth, with a temperature of about 173.1 K and it is at the boundary between the thermosphere and the mesosphere.
Above 90 km is the “hot layer” called thermosphere that reaches outer space. In the thermosphere, oxygen molecules (O2) absorb energetic solar rays, warming the air. Because there are relatively few atoms and molecules in the thermosphere, the absorption of a small amount of energetic solar energy can cause a large increase in air temperature usually over 1000 Kelvin. Again, the amount of solar energy that affects this layer depends strongly on solar activity and hence, temperature here varies from day to day. The air density in the upper thermosphere is so low that air temperatures are not measured directly. However, air temperature can be determined by observing the orbital change of satellites caused by the drag of the atmosphere. Enough air molecules strike a satellite to slow it down and making it to drop into a slightly lower orbit. The amount of drag is related to the density of the air which relates to the temperature. Thermopause is the boundary above the thermosphere and it varies in height from 500 – 1,000 kilometers.
This is that region of the atmosphere from about 60 – 1,000 km above the earth’s surface where free electron and ions exist in numbers sufficient to influence the transmission of electromagnetic waves at radio frequencies. Earth’s magnetic field affects the electron motion at all heights but with increasing importance at greater heights. It forms the inner boundary of the magnetosphere. The ionosphere arises because the solar radiation is absorbed by the upper atmosphere, dissociating the molecules, and liberating free electrons. At great heights there are few molecules; at low heights most of the ionizing radiation has been absorbed. The result is a peak in the production of ionization corresponding to a particular absorption wavelength. This region typically overlaps both the exosphere and the thermosphere.
1.3 WINDS IN THE ATMOSPHERE
Some variables that determine the weather and climate systems include: pressure, temperature, wind speed and direction, density, clouds, precipitation, humidity and variable gases (such as ozone). Wind can be defined as any air motion relative to the earth’s surface (horizontal air motion). Wind is very essential and a contributing factor to the total thermodynamic mechanism of the atmosphere; it is a means of transporting heat, moisture and other atmospheric processes from one location of the earth to another.
1.3.1 Solar wind
The solar wind is a stream of energized, charged particles, primarily electrons and protons flowing outward from the sun, through the solar system at speeds as high as 900 km/s and at a temperature of 1 million degrees Celsius. At about this temperature, gases become stripped off of electron by violet collisions and acquire enough speed to escape the gravitational pull of the sun. As these charged particles (ions and electrons) travel through space, they are known as plasma or solar wind. When the solar wind moves close enough to the earth, it interacts with the earth’s magnetic field. Two major ways that solar wind can escape from the gravity of the sun are by gaining enough kinetic energy as it moves away from the sun and also through the high temperature nature of the corona. The solar wind can have a large influence on the earth, particularly in times of the active sun (near sunspot maximum) when the wind is strong and can contain bursts corresponding to flares and coronal mass ejections from the sun. The solar wind has a significant influence on ionosphere, the earth’s magnetic field, earth’s auroras, and telecommunication systems.
1.3.2 Zonal wind
This is a large-scale atmospheric flow of air in which the east-west or vice versa component (i.e., latitudinal) is dominant. The zonal jet stream winds are westerly whenever temperature increases from the pole to the equator. Easterly jet stream winds occur in the stratosphere when the temperature gradient reverses that is decreases from pole to equator. This occurs during summer in the stratosphere. The accompanying meridional (north-south) component is along the local meridian and it is often weaker than normal. The meridional wind is positive if from the south, and negative if from the north. When the wind is blowing straight north or south, the zonal wind will be zero.
In the tropics, the heating provided by the solar radiation is fairly uniform both through the year (temporarily) and at different regions (geographically). Since, the temperature profile is fairly smooth then the zonal winds are fairly weak as well. The most intense winds are found in the winter hemisphere along the edge of the polar night because; the horizontal temperature gradient is also strongest in this region.
1.4 The sun
The sun is an average star, similar to millions of others in the Universe. It is a wonderful energy, manufacturing about 4.0E023 Kilowatts of energy per second. The basic energy source for the sun is nuclear fusion, which uses the high temperatures and densities within the core to fuse hydrogen, producing energy and creating helium as a byproduct. The core is so dense and the size of the sun so great that energy released at the center of the sun takes about 50,000,000 years to make its way to the surface, undergoing countless absorptions and re-emissions in the process. If the sun were to stop producing energy today, it would take 50,000,000 years for significant effects to be felt at earth.
The sun has been producing its radiant and thermal energies for the past four or five billion years; it has enough hydrogen to continue producing for another hundred billon years. However, in about ten to twenty billion years the surface of the sun will begin to expand, enveloping the inner planets (including earth). At that time, our sun will be known as a red giant star. If the sun were massive, it would collapse and re-ignite as a helium-burning star. Due to its average size, however, the sun is expected to merely contract into a relatively small, cool star known as a white dwarf (Prialnik, 2009). Sun is the source of light and heat for the planets in the solar system; sun shines, life thrives – in other words, sun sustains life on earth and controls our climate and weather. Ultraviolet radiation from the sun continuously strikes the upper atmosphere. These harmful ultraviolet rays would make life on earth impossible if it were not for the ozone layer that absorbs the radiation.
1.4.1 Some solar activity indices
It has long been known that the sun is neither featureless nor steady. These are some phenomenon during high solar activity:
Sunspots are dark areas on the solar surface that contain transient concentrated magnetic fields. Hence, sunspots on solar surface show that the sun is magnetically active. They are the most prominent visible features on the sun.When the solar activity is high (high number of sunspot) both solar UV radiation and ozone concentrations are high. Measurements in the tropic suggest a change of ~ 6% of the total ozone column during the 11 year solar cycle from solar low to high (Labitzke and Van loon, 1997). Hence, the total ozone column enhances during magnetically disturbed conditions which are associated with peak solar activity period. During periods of maximum sunspots the sun emits more energy (about 0.1% more) than during the periods of sunspot minima (Ahrens, 2003).
1.4.3 Solar flares
Solar flares are intense, temporary, releases of energy. They are seen at ground-based observatories as bright areas on the sun in optical wavelengths and as bursts of noise at radio wavelengths; they can last from minutes to hours. Flares are our solar system’s largest explosive events, which can be equivalent to approximately 40 billion Hiroshima-size atomic bombs. The primary energy source for flares appears to be the tearing and reconnection of strong magnetic fields. They radiate throughout the electromagnetic spectrum, from gamma rays to x-rays, through visible light out to kilometer-long radio waves.
1.4.4 Coronal Mass Ejections
The outer solar atmosphere, the corona, is structured by strong magnetic fields. Where these fields are closed, often above sunspot groups, the confined solar atmosphere can suddenly and violently release bubbles or tongues of gas and magnetic fields called Coronal Mass Ejections (CMEs). A large Coronal Mass Ejection can contain 10.0E16 grams of matter that can be accelerated to several million miles per hour in a spectacular explosion. Solar material streaks out through the interplanetary medium, impacting any planets or spacecraft in its path. Coronal Mass Ejections are sometimes associated with flares but usually occur independently.
1.4.5 Mg II core-to-wing ratio
The Mg II core-to-wing ratio is a measure of solar chromospheric variability. The Mg II core-to-wing index is a ratio of the Mg II chromospheric emission at 280 nm to the photospheric radiation in the line wings (de Toma, et al., 1997). In the UV region, the Mg II core-to-wing ratio introduced by Heath and Schlesinger, (1986) is one of the most widely used indices of solar activity.
1.4.6 Solar flux (F10.7)
A measure known as the solar fluxis used as the basic indicator of solar activity, and to determine the level of radiation being received from the sun. The solar flux is measured in solar flux units (SFU) and is the amount of radio noise or flux that is emitted at a frequency of 2800 MHz (10.7 cm). The solar flux is closely related to the amount of ionization and hence the electron concentration in the F2 region. As a result it gives a very good indication of conditions for long-distance communication. The F10.7 index is a measure of the noise level generated by the sun at a wavelength of 10.7 cm at the earth’s orbit. It represents a measure of diffuse, non-radiative heating of the coronal plasma trapped by magnetic fields over active regions, and is an excellent indicator of overall solar activity levels. It has been proposed that 10.7 cm solar flux can interfere with point-to-point terrestrial communications (Poole, 2002).
1.5 Geomagnetic storm
When the earth’s magnetic field is severely disturbed, a “magnetic storm” is said to occur as depicted in Figure 2. A magnetic storm is caused by the electric current systems set up in the magnetosphere when an enhanced solar wind consisting of a stream of protons and electrons from the sun strikes the magnetosphere and, also major magnetic storms are caused by intense, long duration southward interplanetary magnetic fields. Often storms begin abruptly and they are called the “sudden commencement” (SC) storms. A sudden commencement storm (SC) has three typical phases – initial, main and recovery phases.