Electric Field of the Earth

the natural electric field of the earth as a planet, observed in the solid body of the earth, in the oceans, and in the atmosphere and magnetosphere. It is caused by a complex aggregate of geophysical phenomena. The potential distribution of the field provides some information about the structure of the earth and the processes that occur in the lower layers of the atmosphere, in the ionosphere, and in the magnetosphere, as well as in the near-earth space and in the sun.

The methods for measuring the earth’s electric field depend on the medium in which it is being observed. The most universal method involves finding the potential difference between electrodes arrayed in space, and it is used to record the earth’s currents (seeTELLURIC CURRENT), to measure the electric field of the atmosphere from aircraft, and to determine the electric fields in the magnetosphere and in space from spacecraft (in this case, the distance between electrodes should exceed the Debye shielding distance in the space plasma, that is, it should be hundreds of meters).

The existence of an electric field in the atmosphere is associated mainly with the ionization processes in the air and with the spatial separation of the positive and negative electric charges arising in the course of ionization. The air is ionized by the action of cosmic rays, by ultraviolet radiation from the sun, by radiation from radioactive substances found on the earth’s surface and in the air, and by electric discharges in the atmosphere. Many atmospheric processes, such as convection, the formation of clouds, and precipitation, lead to the partial separation of unlike electric charges and the formation of atmospheric electric fields (seeATMOSPHERIC ELECTRICITY). The earth’s surface is charged negatively with respect to the atmosphere.

The presence of an electric field in the atmosphere generates currents that discharge the electric “capacitor” formed by the atmosphere and earth. A significant part in the exchange of charges between the earth’s surface and the atmosphere is played by precipitation. On the average, precipitation brings 1.1 to 1.4 times more positive charges than negative charges. Other factors that contribute to the dissipation of charges from the atmosphere are the charges associated with lightning and the leakage of charges from sharply pointed objects (points). The balance of the electric charges brought to the earth’s surface per sq km per year can be characterized by the data in Table 1.

Over a large part of the earth’s surface, namely, the oceans, there are no points, and the balance here will be positive. The existence of a static negative charge on the earth’s surface (about 5.7 × 10⁵ coulombs) indicates that these currents, on the average, are balanced.

The electric field in the ionosphere is caused by processes that occur both in the upper layers of the atmosphere and in the magnetosphere. The tidal movements of the air, the winds, and turbulence all generate an electric field in the ionosphere owing to the effect of the hydromagnetic dynamo (seeTERRESTRIAL MAGNETISM). An example of this is the solar-diurnal electric system of currents, which produces a diurnal variation in the magnetic field at the earth’s surface. The value of the electric field intensity in the ionosphere is a function of the location of the point of observation, the time of day, the overall state of the magnetosphere and ionosphere, and the solar activity. It varies from several units to tens of millivolts/m (mV/m) and reaches a hundred or more mV/m in the high-latitude ionosphere. In this case, the current reaches hundreds of thousands of amperes. Owing to the high electrical conductivity of the ionospheric and magnetospheric plasma along the lines of force of the earth’s magnetic field, the electric fields of the ionosphere drift into the magnetosphere and the magnetospheric fields drift into the ionosphere.

One of the direct sources of the electric field in the magnetosphere is the solar wind. When the solar wind flows around the magnetosphere, an emf E = v × b⊥ is produced, where b⊥ is the normal component of the magnetic field at the surface of the magnetosphere and v is the average velocity of the particles of the solar wind. This emf gives rise to electric currents that are completed by the reverse currents flowing across the tail of the magnetosphere (seeEARTH). The latter are generated by positive space charges on the morning side of the magnetosphere’s tail and negative charges on the evening side. The electric field intensity across the tail reaches a value of 1 mV/m. The potential difference across the polar cap is 20 to 100 kilovolts.

Another mechanism for the production of an emf in the magnetosphere involves the collapse of oppositely directed lines of force of the magnetic field in the tail portion of the magnetosphere; the energy thus liberated causes the magnetospheric plasma to be shifted violently toward the earth. In this case, the electrons drift around the earth toward the morning side, while the protons drift toward the evening side. The potential difference between the centers of the equivalent space charges reaches tens of kilovolts. This field is opposite in direction to the field of the tail portion of the magnetosphere.

The presence of a ring of magnetospheric current around the earth is directly associated with the drift of particles. When there are magnetic storms and auroras, the electric fields and currents in the magnetosphere and the ionosphere undergo marked changes (seeMAGNETIC STORMS and AURORA).

In addition to the quasistatic electric fields already mentioned, there exist in the magnetosphere and the ionosphere changing electric fields that involve a different kind of plasma fluctuation (seeMAGNETOHYDRODYNAMICS).

Magnetohydrodynamic waves, generated in the magnetosphere, propagate in natural wave-guide channels along the lines of force of the earth’s magnetic field. Upon entering the ionosphere, they are converted into electromagnetic waves, some of which reach the earth’s surface and some of which propagate in the ionospheric wave guide and are attenuated. At the earth’s surface, these waves are registered according to the frequency of fluctuations as either magnetic pulsations (10^–2 to 10 hertz) or very low-frequency waves (oscillations at frequencies of 10² to 10⁴Hz).

The earth’s changing magnetic field, whose sources are localized in the ionosphere and magnetosphere, induces an electric field in the earth’s crust. The intensity of the electric field in the surface layers of the crust varies with the locality and the electrical resistance of the rocks from several mV/km to several hundred mV/km, while during magnetic storms it increases to several V/km and even tens of V/km. The interrelated changing magnetic and electric fields of the earth are used for electromagnetic sounding in geophysical prospecting, as well as for depth sounding of the earth.

The contact potential difference between rocks having different electrical conductivities (the thermoelectric, electrochemical, and piezoelectric effects) makes a specific contribution to the earth’s electric field. Volcanic and seismic processes play a special part in this.

Electric fields are induced in the oceans by the earth’s changing magnetic field; they also occur during the movement of the conducting seawater (ocean waves and currents) in the magnetic field. The densities of electric currents in oceans reach 10^–6ampere/m². These currents can be used as natural sources of a changing magnetic field for magnetic-variation sounding on the shelf and in the ocean.

The problem of the electric charge of the earth as the source of the electric field in interplanetary space has not yet been fully resolved. It is assumed that the earth as a planet is electrically neutral. However, this hypothesis requires experimental confirmation. The first measurements have indicated that the intensity of the electric field in near-earth space varies from tenths of a mV/m to several tens of mV/m.