The Earth’s Electric Field: Sources from Sun to Mud

By Michael C. Kelley

The Earth’s Electric Field provides you with an integrated and comprehensive picture of the generation of the terrestrial electric fields, their dynamics and how they couple/propagate through the medium. The Earth’s Electric Field provides basic principles of terrestrial electric field related topics, but also a critical summary of electric field related observations and their significance to the various related phenomena in the atmosphere. For the first time, Kelley brings together information on this topic in a coherent way, making it easy to gain a broad overview of the critical processes in an efficient way. If you conduct research in atmospheric science, physics, atmospheric chemistry, space plasma physics, and solar terrestrial physics, you will find this book to be essential reading.

• The only book on the physics of terrestrial electric fields and their generation mechanisms, propagation and dynamics–making it essential reading for scientists conducting research in upper atmospheric, ionospheric, magnetospheric and space weather
• Covers the processes related to electric field generation and electric field coupling in the upper atmosphere along with providing new insights about electric fields generated by sources from sun to mud
• Focuses on real-world implications—covering topics such as space weather, earthquakes, the effect on power grids, and the effect on GPS and communication devices

• Language: English
• Publisher: Elsevier Science
• Release date: Sep 21, 2013
• ISBN: 9780123978837

Book preview

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Preface

Michael C. Kelley

Robert H. Holzworth

Knowledge of the earth’s electric field has grown greatly during the past few decades. Prior to 1960, the primary interest was in the atmospheric electric field, the fields generated in thunderclouds, and the currents with which they charge the earth worldwide. These studies are most famously linked to Ben Franklin. In the 1960s, the development of scientific radars, scientific rockets, and satellites extended our knowledge into space. Rockets, radars, and low-altitude satellites probed the ionosphere, roughly 100-1000 km in altitude. Satellites extended our knowledge into the magnetosphere, a vast region dominated by the earth’s magnetic field and then into the region called the solar wind, which is dominated by the sun’s upper atmosphere. We treat each of these various areas in this text.

The history of magnetic field studies is much, much longer. For millennia, humankind has known of and used the earth’s magnetic field. Long ago, Chinese and later, European explorers used the earth’s poles as guides into uncharted waters. Today, scientists have developed far more complex models of the earth’s magnetic field and continuously monitor magnetic fluctuations to push our understanding still further. The earth’s magnetic field has been likened to one produced by a bar magnet lying at an 11° angle with the spin axis of the earth. However, while the field lines close to the earth closely resemble those of a bar magnet, field lines are greatly distorted by solar winds at greater distances from the earth’s surface.

While the source of a bar magnet's magnetic field is aligned electron magnetic moments, each contributing to the total magnetic field, this cannot be the cause of the earth’s magnetic field. The temperature in the earth’s core is simply too hot, well above the Curie point for iron. Above this temperature, atoms have so much energy that electron spins no longer align and are more or less random, resulting in a net magnetic field of zero. Scientists believe that telluric currents below the surface of the earth are the source of the earth’s magnetic field. The source of telluric currents has been explained by the dynamo effect. For quite some time, scientists have accepted the dynamo effect as the source of the earth’s magnetic field but now are using complex computer models to provide conclusive proof. Only now are we moving closer to exploring the electric field in models of this sort.

Many books have been written about the earth’s magnetic field. In fact, Epistola de Magnete (1269) by Peregrinus is considered to be the first scientific paper ever written. An additional important text, De Magnete (1600), was written by William Gilbert.

In this text we try, for perhaps the first time, to offer a systematic approach to describing the earth’s electric field. In Chapter 1, we discuss how electric fields are generated using tools common to junior-level electrophysics concepts. Chapter 2 examines the atmospheric electric field sources that dominate below about 90 km. In the 90-1000 km height range, what we term a hydromagnetic generator operates, as discussed in Chapter 3. In this case, the source of energy is the high-altitude atmospheric wind which, when it blows across the magnetic field, generates electric currents. The current system is very complex, charges must build up on boundaries, and electric fields result. Because of the high conductivity along magnetic field lines, which act like electric wires, electric fields are mapped for vast distances, even hemisphere to hemisphere. This generator dominates below about 60° latitude.

In Chapter 4, we turn to fields generated by the sun’s upper atmosphere, which expands at supersonic speeds past the earth, sweeping the earth’s magnetic field into a huge comet-like shape called the magnetosphere. This solar wind is a highly conducting plasma and hence any electric field is shorted out. However, in the earth’s frame of reference, there is quite a large field, called a hydromagnetic generator, that generates up to a few million volts across the magnetosphere. About half of the time, depending on the interplanetary magnetic field direction, roughly 10% of this voltage penetrates into the earth’s polar regions and drives the magnificent aurora, huge electrical currents, and many fascinating atmospheric phenomena. This penetration occurs on the earth’s magnetic field lines that connect to the solar magnetic field above about 75° latitude. Between 60° and 75°, the magnetic field lines connect between the hemispheres but are stretched out of the dipole shape. In Chapter 5, we describe the physical process of a secondary hydromagnetic generator in this latitude region. Again, many interesting phenomena occur due to the currents, electric fields, and plasma boundary phenomena. In Chapter 6, we introduce electromagnetic and electrostatic wave phenomena, which are ubiquitous in the earth’s environs. In Chapter 7, we review the most important electric field measurement techniques.

Hopefully, this unified description will be a useful introduction to the fascinating electrical processes around the earth.

Chapter 1

Electric Field Generation Mechanisms

Abstract

We live in an electromagnetic world, from the chemical bonds that hold us together to the large-scale, powerful effects of thunderstorms and solar flares. Human understanding of electricity began during a time of fascination with and fear of unexplained natural phenomena such as the aurora borealis and lightning. Hundreds of years ago, the earth's magnetic field first became a useful tool for explorers, yet even today, there is no similarly useful large-scale map of the earth's electric field. Indeed, while the earth's magnetic field lines are continuous and penetrate even the depths of the earth, electric fields begin and end with electric charges, which can be anywhere.

Keywords

Electric field; Magnetic field; Aurora; Lightning; Charge density

1.1 Introduction

1.2 Visual Effects of Electrical Phenomena

1.3 Generation of Electric Fields

1.4 Relativistic Effects

1.5 Electric Field Mapping

1.6 An Energy Theorem

1.7 Summary

1.1 Introduction

We live in an electromagnetic world, from the chemical bonds that hold us together to the large-scale, powerful effects of thunderstorms and solar flares. Human understanding of electricity began during a time of fascination with and fear of unexplained natural phenomena such as the aurora borealis and lightning. Hundreds of years ago, the earth's magnetic field first became a useful tool for explorers, yet even today, there is no similarly useful large-scale map of the earth's electric field. Indeed, while the earth's magnetic field lines are continuous and penetrate even the depths of the earth, electric fields begin and end with electric charges, which can be anywhere.

Lacking an accurate global electric field model to complement the one we do have for the earth's magnetic field, we still can gain critical insight into our electrical environment by applying basic electromagnetic theory to our geophysical environment. At the most fundamental level, charged particles, such as electrons and ions, cannot gain energy unless an electric field exists in their frame of reference. Acceleration in circular motion, such as an orbiting satellite or an electron encountering a static magnetic field, does not result in energization of the satellite or the electron. Rather, circular motion causes only a continuous change in the object’s direction of motion, not the magnitude of the velocity. Thus, imparting energy to a charged particle fundamentally requires electric fields and forms the most basic reason for studying geophysical electric fields.

A major complicating factor in studying the earth's electrical environment is that strong electric fields can exist in a region with practically no evidence of these strong fields outside that localized region. For instance, we now know that large-scale electric fields dominate the dynamics of near space, including the ionosphere and magnetosphere. Yet, these electric fields are nearly impossible to detect near the ground, just 100 km from the global-scale sources in the ionosphere. It was not until the 1960s, shortly after Sputnik and the beginning of the Space Age, that we began to learn anything about these electric fields. Compared to magnetic field research, environmental electric field research in our near-space region is hundreds of years behind. Of course, back in the 1750s, Ben Franklin first identified the large electric fields associated with thunderstorms and lightning. Within decades, this new knowledge changed our world view (c.f. Krider), removing a layer of superstition about the nature of lightning. Even today, the electrical potentials found inside thunderstorms, which can exceed a billion volts (1,000,000,000 V) across a few kilometers (Tom Marshall article), remain hard to predict and model accurately because measurements inside thunderstorms are extremely difficult to make.

Knowledge of electric fields in the atmosphere and space has become very important to our understanding of the dynamics of both the neutral and charged atmosphere at all altitudes, from the clouds to outer space. Even short distances away from the huge fields in thunderstorms, the surface electric field may bear little or no resemblance to fields in the charged cloud. Thus, we depend on in situ measurements of the vector electric field by balloon, rocket, and satellite platforms to enhance our understanding of the earth's electric field.

1.2 Visual Effects of Electrical Phenomena

When the electrons of an atom or molecule relax into lower energy states, they emit electromagnetic energy in the form of photons. These photons often have wavelengths in the visible spectrum, allowing us to see the electrical processes in action. Similarly, when an ion recombines with an electron, it also emits a photon, allowing charged gas dynamics to become visible. A familiar example is the little spark one receives when touching a doorknob on a low-humidity day. The process of spark discharge occurs when the electric field between your finger and the doorknob exceeds 30,000 V/cm! This is the electric field required to break down the air, allowing a cascade of electrons to flow and neutralize the electric charges that caused the large electric field in the first place. When energetic electrons pass through neutral air, they can cause ionization (as for the spark) or simply excitation of the air molecules. The visible aurora borealis is an example of excited molecules or atoms energized by the impact of magnetospheric electrons and ions, which subsequently release that energy as visible photons (Fig. 1.1).

Fig. 1.1 A nighttime photograph of multiple cloud-to-ground and cloud-to-cloud lightning strokes. Thousands of flashes, each consisting of several strikes, occur every day around the world. Figure courtesy of NOAA.

In this section, we examine visible evidence of the earth's large-scale electric fields to set the stage and motivate the technical study found in this book. Lightning is primarily a land-based phenomenon. Each cloud-to-ground (CG) flash actually consists of several strokes that follow the same channel. However, each stroke lasts less than a millisecond and the whole flash typically less than a second. This time frame is usually too small for the human eye to clearly resolve each flash into multiple events. A typical multi-stroke flash lasts several hundred milliseconds but can last longer than 1 s. The majority of CG lightning strokes bring negative charge to the earth, leaving net positive charge behind in the cloud. A few percent of all CG strokes have the opposite polarity, depositing positive charge on the earth. (For more information about the physics of lightning, see Rakov and Uman, 2003.)

Recently, it was discovered that the electric fields inside clouds can be large enough to accelerate particles so high that, when they collide with atmospheric particles, they not only ionize but also create antimatter (positrons). Using data from the NASA Fermi satellite, a team under Briggs studying high-energy photon bursts associated with lightning, called terrestrial gamma-ray flashes (TGFs), revealed the characteristic decay signature of positrons during high-altitude lightning events (Briggs et al., 2010, 2011). Using lightning data from the World Wide Lightning Location Network, the Fermi satellite group demonstrated that these TGFs are associated with in-cloud discharges, producing extremely high currents composed of both electrons and positrons streaming oppositely to make electric currents that are orders of magnitude larger than ever thought possible with just electrons (Connaughton et al., 2013) (Fig.