Abstract
Predicting the arrival time of Coronal Mass Ejections (CMEs) with a lower value of average error between the predicted and the observed transit time is very crucial in space weather forecast. We proposed a modified Empirical Coronal Mass Ejection Arrival (ECA) model to predict the arrival time of twenty eight (28) CMEs from the sun to the earth. We tested the modified model by using data obtained from coronagraph observations of Large Angle Spectrometric coronagraph on aboard the Solar and Heliospheric Observatory (SOHO/LASCO) CME catalogue from the period of 1998-2012. To ascertain the accuracy of our model, we employed the three Empirical Coronal Mass Ejection arrival (ECA) models of Gopalswamy to our data points. The average error obtained between the CMEs transit time and models predicted transit time with their fractional errors were 4.27 hours and 0.10 for the modified model; 10.41hours and 0.23 for the VG2002 model; 12.3hours and 0.29 for G2001 model; 14.46 hours and 0.34 for the G2000 model. Other analyses revealed that Solar wind speed and solar activity cycle have significant influence on CMEs transit time. Solar wind speed and CMEs speed at 1AU have been observed to have contributed to the magnitude of geomagnetic storm. Our modified model has proved to be effective in prediction of arrival time of CMEs. This least average error obtained between CMEs transit time and the model’s predicted transit time and the fractional error was much lower than all the existing models. It is our recommendation that future work be carried out employing our modified model with a view to confirming its accuracy.
CHAPTER ONE
INTRODUCTION
- General Introduction
Coronal mass ejections (CMEs) are massive burst of solar materials (clouds of plasma and magnetic fields) that shoot off the Sun’s surface and released into space. Over a distance of a few solar radii, CMEs may accelerate up to a speed of 300 and subsequently propagate through the solar wind away from the Sun (Mostl et al., 2014 ;Yashiro et al., 2001)
CMEs are known to be the major cause of severe geomagnetic disturbances which is often referred to as space weather (Zhang et al., 2001; Cheng et al., 2014; Cyr et al; 2000; Tripathi et al., 2005). There are several space weather phenomena which tend to be associated with or are caused by geomagnetic storm. These include: Solar Energetic Particles (SEP) events (hazardous to Humans), Geomagnetically Induced Currents (GIC) which cause damages to satellites and electricity grid, ionospheric disturbances which may lead to radio and radar scintillation, disruption of navigation by magnetic compass and aurora displays at much lower latitudes than normal (Baker and John 2008).
Researchers have been able to continuously monitor the Sun using both full disk images and coronagraphs. With the launch of the solar and heliospheric observatory (SOHO), studies of the evolution of CMEs have been carried out which primarily focused on observations by the Large Angle and Spectrometric Coronagraph (LASCO) on board SOHO. Studies were also devoted to the in situ identification of CMEs near the Earth by Advanced Composition Explorer (ACE) that samples particles from the Sun as they stream toward the planet Earth. The Solar Dynamic Observatory (SDO) focuses on solar atmosphere. Other information and the properties of CMEs come from signatures of CMEs in time series of plasma and magnetic field measurement (in situ) in the interplanetary medium especially at 1AU (AU is the astronomical unit of measurement of sun-earth distance) where they are usually referred to as interplanetary CMEs (ICMEs). Combining observations at 1AU with those of the Sun, then permits a close link between solar events and its interplanetary manifestations.
Most of the studies carried out to predict the arrival time of CMEs from the Sun to the Earth have been with a lot of assumptions regarding the geometry and evolution of CMEs in the Inter Planetary (IP) medium. Many works have been done by many researchers in this regard in predicting the arrival time of CMEs to 1AU using different models. Predicting the arrival time of CMEs with a minimal error has been a major issue in the field of Heliophysics, because the average error between the observed transit time and predicted transit time obtained in all the result of the models so far are still very large. There is need for a lower value between the predicted and observed transit time.
Predicting the arrival time of CMEs with minimal average error will help serve as a practical way of getting advanced warning of solar disturbances heading towards the earth, save billions of currency that would have been used to repair or replace damaged satellites and power grids, identify communication problems, help high altitude flight management and make provisions for renewable energy sources to protect the Earth against a black out.
1.2 The Earth’s Atmosphere.
The Atmosphere of the Earth is a layer of gases surrounding the planet Earth that is retained by Earth’s gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention (greenhouse effect) and reducing temperature extremes between day and night (the diurnal temperature variation).
The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and outer space. Although air content and atmospheric pressure vary at different layers, air suitable for the survival of terrestrial plants and animals is found in the Earth’s troposphere.
1.2.1 The Thermal structure of the Earth’s Atmosphere.
In general, air pressure and density decrease with altitude in the atmosphere. The general pattern of the temperature/altitude profile is constant and recognizable through means such as balloon soundings. The temperature behavior provides a useful metric to distinguish between atmospheric layers.
The Earth’s atmosphere is stratified into five main layers, namely; the troposphere, stratosphere, mesosphere, thermosphere and Exosphere.
The troposphere is the lowest layer of the Earth’s atmosphere. It extends from the Earth’s surface to an average height of about 12km, although this altitude actually varies from about 9km at the poles to 17km at the equator. The temperature in the troposphere decreases as altitude increases, dropping from about 17oC to -52oC (Subbiondo, 2004). This is because it is mostly heated through energy transfer from the surface. Thus, the lowest part of the troposphere is typically the warmest section of the troposphere. This promotes vertical mixing the troposphere is denser than all its overlying atmospheric layers because a larger atmospheric weight sits on top of the troposphere and causes it to be most severely compressed. It is primarily composed of Nitrogen (78%) and Oxygen (21%) with only small concentrations of other trace gases. Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most conventional aviation activity takes place.
The stratosphere is the second –lowest layer of the Earth’s atmosphere. It is bounded by the stratopause. The stratosphere contains the ozone layer. In this layer, the temperature rises with increasing altitude. The rise in temperature is caused by the absorption of ultraviolet radiation (UV) radiation from the Sun by the ozone layer which restricts turbulence and mixing. The troposphere is bounded above by the tropopause, a boundary marked by stable temperature. The stratospheric temperature profile creates very stable atmospheric conditions so that the stratosphere lacks the weather-producing air turbulence that is so prevalent in the troposphere.
The mesosphere is the third highest layer of the Earth’s atmosphere, occupying the region above the stratosphere and below the thermosphere. It extends from the stratosphere an attitude of about 50km to the menopause at 80-85km above the sea level. Temperature drops in increasing attitude to the mesopause; it is the coldest region on Earth. According to State and Gardner (2010), the average temperature of the mesopause is around 85oC (-120oF or 190K).The mesosphere is also the layer where most meteors burn up upon atmospheric entrance. It is too high above the Earth to be accessible by jet-powered aircraft and too low to support satellites and orbital or sub-orbital spacecraft. It is mainly accessed by rocket-powered aircraft and unmanned sounding rockets.
The Thermosphere is the second-highest layer of Earth’s atmosphere, it extends from the mesopause (which separates it from the mesosphere) at an altitude of about 80km up to the thermopause at an altitude range of 500-l,000km.The lower part of the thermosphere from 80km to 550km above the Earth’s surface contains the atmosphere. In this region temperature increases with altitude. This is due to the extremely low density of its molecules. The temperature of this layer can rise as high as l,727°C (Subbiondo,2004).The air is rarefied that an individual molecule travels an average of 1 kilometer between collisions with other molecules.
The exosphere is the outermost layer of the Earth’s atmosphere. It extends from the exobase, which is located at the top of the thermosphere at an altitude of about 700km above sea level to about 10,000km. The exosphere merges with the emptiness of outer space, where there is no atmosphere. The layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase. The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Since the exosphere no longer behaves like a gas, the particles constantly escape into space. The free moving particles follow ballistic trajectories and may migrate in and out the magnetosphere or the solar wind.
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