Investigation of Effects of CME on Ionospheric TEC over Nsukka



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.

Fig. 1.1: The stratified Earth’s atmosphere

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.

Fig. 1.2: The Earth’s ionosphere and reflection of VLF radio waves. © Morris Cohen, Stanford University

According to Okeke (2010), the ionosphere is that region of the atmosphere from about 60 to 1000km above the earth’s surface. In this region, free electrons exist in numbers sufficient to influence the transmission of electromagnetic waves at radio frequencies. Hence, the ionosphere could be regarded as an ionized region of the upper atmosphere, which forms the inner boundary of the magnetosphere. The main reason for the existence of the ionosphere is because solar radiation is absorbed by the upper atmosphere, thereby dissociating the molecules and liberating free electrons. It is important to note that earth’s magnetic field affects the electron density motion at all heights, but with increasing importance at greater heights, where there are few molecules. Another important source of ionization at high latitudes is the precipitation of energetic charged particles. Onwumechili (1997) as in Okeke (2010) ascertained that in any part of the ionosphere, the electron density Ne is determined by the equation of continuity for electrons;

Where  is time rate of change of electron density,

P(Ne) is rate of production of electrons,

L(Ne) is loss of electrons (mostly through chemical processes),

Div(Ne) is rate of outward transportation of electrons (i.e. sink)

V is mean velocity of electrons.

Equation (1) can be written for positive and negative ions respectively as;

In the ionosphere, the basic equation of electric current density in plasma is given as;

Where j is electric current density (Am-3)

          e is charge of electron (C)   are drift of the ions and electrons (ms-1)

          Ne is electron density (pm-3)

1.3.1 Low Latitude Ionosphere

The low latitude ionosphere is a dynamic geophysical system that is difficult to study. Indeed the complex ionospheric and atmospheric dynamics within this region contribute to the formation of the Equatorial Anomaly that extends from the magnetic equator to 300 geomagnetic latitude in both Northern and Southern hemispheres. At low latitudes, unique phenomena such as near-midnight TEC enhancements, TEC depletions or equatorial plasma bubbles and scintillation occur. These phenomena affect Satellite Navigation Systems and they can be studied using ground and space-based GNSS data.

 In the last few years different institutions have started to deploy several experimental instruments of different kinds (e.g. GNSS receivers, ionosondes, magnetometers, etc in low latitude Countries, such as in Africa, South-America and Asia, over which the ionosphere had remained less known because of the scarce distribution of ionospheric sensors. As a consequence the new sets of data now available are expected to make possible improvements in ionospheric modeling efforts particularly considering data assimilation techniques. In addition the possibility to help explain some specific phenomena that take place in this region could be envisaged.

 Moreover, GNSS are used to provide positioning accuracy and safety for navigation on the ground, in the air and on the sea. Since the ionosphere is a major error source for GNSS performance, an improved knowledge of the low latitude ionosphere would help to address mitigation techniques for ionospheric effects on GNSS positioning applications (e.g. precision agriculture, environmental monitoring, civilian aviation) in the same geographic region.

1.4 Coronal Mass Ejections

A coronal mass ejection (CME) is a massive burst of solar wind and magnetic fields rising above the solar corona or being released into space (NASA, 2013). Coronal mass ejections are often associated with other forms of solar activity, most notably solar flares, but a causal relationship has not been established. Most ejections originate from active regions on the Sun’s surface, such as groupings of sunspots associated with frequent flares. Near solar maxima the Sun produces about three CMEs every day, whereas near solar minima there is about one CME every five days (Nicky Fox, 2011).

Coronal Mass Ejections (CMEs) are massive (1014 to 1017 grams) bursts of plasma that are ejected from the sun. CMEs are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours. Although the Sun’s corona has been observed during total eclipses of the Sun for centuries, the existence of coronal mass ejections was unrealized until the space age. The earliest evidence of these dynamical events came from observations made with a coronagraph on the 7th Orbiting Solar Observatory (OSO 7) from 1971 to 1973. A coronagraph produces an artificial eclipse of the Sun by placing an occulting disk over the image of the Sun. During a natural eclipse of the Sun the corona is only visible for a few minutes at most, too short a period of time to notice any changes in coronal features. With ground-based coronagraphs only the innermost corona is visible above the brightness of the sky. From space the corona is visible out to large distances from the Sun and is viewed continuously.

Coronal mass ejections are often associated with solar flares and prominence eruptions but they can also occur in the absence of either of these processes. The frequency of CMEs varies with the sunspot cycle. At solar minimum, about one CME is observed a week. Near solar maximum an average of 2 to 3 CMEs are observed per day (NASA/Marshall Space Flight Center – Solar Physics, 2011).

1.4.1 Description of Coronal Mass Ejections    

Coronal mass ejections release huge quantities of matter and electromagnetic radiation into space above the sun’s surface, either near the corona (sometimes called a solar prominence), or farther into the planet system, or beyond (interplanetary CME). The ejected material is plasma consisting primarily of electrons and protons, but may contain small quantities of heavier elements such as helium, oxygen, and even iron. The theory of heavier element emissions during a CME is speculative information and requires further verification. It is highly unlikely that a CME contains any substantial amount of heavier elements, especially considering that the sun has not yet arrived at the point of helium flash and thus cannot begin to fuse elements heavier than helium. Coronal mass ejections are associated with enormous changes and disturbances in the coronal magnetic field. They are usually observed with a white-light coronagraph.

1.4.2 Cause of Coronal Mass Ejections   

Recent scientific research (Science Daily, 2010; Nicky Fox, 2011) has shown that the phenomenon of magnetic reconnection is responsible for CME and solar flares. Magnetic reconnection is the name given to the rearrangement of magnetic field lines when two oppositely directed magnetic fields are brought together. This rearrangement is accompanied with a sudden release of energy stored in the original oppositely directed fields. One of the scientific objectives of LASCO is to understand why these events occur. Their findings revealed that CMEs are caused by instabilities in the solar magnetic field, which is constantly evolving.

 On the sun, magnetic reconnection may happen on solar arcades—a series of closely occurring loops of magnetic lines of force. These lines of force quickly reconnect into a low arcade of loops, leaving a helix of magnetic field unconnected to the rest of the arcade. The sudden release of energy in this reconnection causes the solar flare. The unconnected magnetic helical field and the materials that it contains violently expand outwards forming a CME (Scientific American, 2006). This also explains why CMEs and solar flares typically erupt from the active regions on the sun where magnetic fields are much stronger on average.

1.4.3 Impact of Coronal Mass Ejection on Earth      

CMEs disrupt the flow of solar wind and produce disturbances that strike the Earth with sometimes catastrophic results. A large number of CMEs have been observed at Solar and Heliospheric Observatory (SOHO) using the Large Angle and Spectrometric Coronagraph (LASCO).  CMEs produce halo events that are directed toward the Earth. During halo events, the entire Sun appears to be surrounded by CME.  As they loom larger and larger they appear to envelope the Sun itself.

When the ejection is directed towards the Earth and reaches it as an interplanetary CME (ICME), the shock wave of the traveling mass of Solar Energetic Particles (SEPs) causes a geomagnetic storm that may disrupt the Earth’s magnetosphere, compressing it on the day side and extending the night-side magnetic tail. When the magnetosphere reconnects on the nightside, it releases power on the order of terawatt scale, which is directed back toward the Earth’s upper atmosphere.

 SEPs can cause particularly strong aurorae in large regions around Earth’s magnetic poles. These are known as the Northern Lights (aurora borealis) in the northern hemisphere, and the Southern Lights (aurora australis) in the southern hemisphere. Coronal mass ejections, along with solar flares of other origin, can disrupt radio transmissions and cause damage to satellites and electrical transmission line facilities, resulting in potentially massive and long-lasting power outages (Baker, 2008; Aviation Week & Space Technology, 2013).

 Humans in space or at high altitudes, for example, in airplanes, risk exposure to relatively intense so-called cosmic rays. Cosmic rays are potentially lethal in high quantities. The energy absorbed by the astronaut is not reduced by a typical spacecraft shield design and, if any protection is provided for the astronaut, it would result from changes in the microscopic non-homogeneity of the energy absorption events (Wilson et al., 1993).

1.4.4 Physical Properties of Coronal Mass Ejections 

A typical coronal mass ejection may have any or all of three distinctive features: a cavity of low electron density, a dense core (the prominence, which appears as a bright region on coronagraph images embedded in this cavity), and a bright leading edge.

 Most ejections originate from active regions on the Sun’s surface, such as groupings of sunspots associated with frequent flares. These regions have closed magnetic field lines, in which the magnetic field strength is large enough to contain the plasma. These field lines must be broken or weakened for the ejection to escape from the sun. However, CMEs may also be initiated in quiet surface regions, although in many cases the quiet region was recently active. During solar minimum, CMEs form primarily in the coronal streamer belt near the solar magnetic equator. During solar maximum, they originate from active regions whose latitudinal distribution is more homogeneous.

Coronal mass ejections reach velocities between 20kms-1 to 3200kms-1 with an average speed of 489kms-1, based on SOHO/LASCO measurements between 1996 and 2003. The average mass is 1.6×1012kg. These values are only lower limits, because coronagraph measurements provide only two-dimensional data analysis. The frequency of ejections depends on the phase of the solar cycle; from about once every fifth day during solar minimum to about four times per day near the solar maximum (Carroll et al., 2007). These values are also lower limits because ejections propagating away from Earth (backside or farside CMEs) can usually not be detected by coronagraphs. The backside CMEs are CME events occurring on solar surface not facing the sun.

 Current knowledge of coronal mass ejection kinematics indicates that the ejection starts with an initial pre-acceleration phase characterized by a slow rising motion, followed by a period of rapid acceleration away from the Sun until a near-constant velocity is reached. Some balloon CMEs, usually the slowest ones, lack this three-stage evolution, instead accelerating slowly and continuously throughout their flight. Even for CMEs with a well-defined acceleration stage, the pre-acceleration stage is often absent, or perhaps unobservable.

Investigation of Effects of CME on Ionospheric TEC over Nsukka