Benjamin Franklin's famous experiments in 1752 "drawing lightning from the cloud" by a kite is generally considered as the beginning of the science of Atmospheric Electricity.The term "Atmospheric Electricity" reflects the earlier efforts to study mainly the electrostatic component of the geo-magnetic field. Since the air is electrically conducting, an electric field in the atmosphere cannot be maintained indefinitely, but must be generated by nonelectric forces. Three sources of low frequency electromagnetic waves are presently known:
1. Thunderstorms and related phenomena in the lower atmosphere.
2. Tidal wind interaction with the ionospheric plasma at dynamo layer heights.
3. Solar wind interaction with the magnetosphere.
These fields map down into the ionosphere and drive electric currents within the dynamo region, the magnetic component of which can be observed on the ground. In addition, instabilities within the magnetosphere are the cause of electromagnetic waves with frequencies ranging from the 0.1 mHz. band to the kHz. band and beyond (geomagnetic substorms, geomagnetic pulsations, and natural ELF, VLF and LF noise).
The atmosphere is an electrically conducting medium so that electric currents can flow due to the electrical ionization around the Earth. The earth's crust contains radioactive materials, mainly uranium, thorium, and their decay products. Beta and gamma rays emitted from the ground can ionize the molecules of the air in the first few meters above ground. The gas radon, which is one of the decay products of uranium 238 can reach greater heights, up to several 100m above ground, before it decays into polonium by emitting alpha particles. Radon is therefore a major ionization source in the first few 100 m above ground over the continents. The second major source of ionization is galactic cosmic rays with a maximum in midlatitudes at about 15 km. height.
Solar X-ray and extreme ultraviolet radiation are the principle sources of ionization above 60 to 70 km. altitude with the rates dependent on latitude, time of day and season, and solar activity. During extremely intense solar flare events, solar protrons of cosmic ray intensity can greatly enhance the ion production rate. The solar radiation is responsible for the formation of various ionospheric layers of intense activity.
The positive and negative ions in the atmosphere are accelerated in opposite directions by the earth electric field. The electric conductivity of the atmosphere below about 70 km. height is mostly in all directions where positive and negative small ions contribute nearly equally to the electric conductivity. There is an increase of conductivity with increasing latitude due to the increase of the ion density with latitude dependent on the cosmic ray intensity.
At an altitude of about 100 km. the electrons drift at right angles to the earth's magnetic field, while the ions move in the direction of this earth field. This area of peak amplitude is called the dynamo region where there is this range of peak parallel conductivity along the geomagnetic field lines of electric equipotential. The direction of the geomagnetic field changes from horizontal at the equator to vertical at the magnetic poles. Since the vertical current cannot flow out of the dynamo region, a polarization charge is built up on both boundaries of the dynamo region causing a vertical electric polarization field with the earth's surface. Amplification of electric current at the geomagnetic equator occurs within this dynamo region.
Thunderstorms behave like batteries which are connected with the highly conducting ionosphere and earth via the barely conducting lower and middle atmosphere. The passive electric contineous current flowing outside the thunderstorm regions down to the earth is part of the global electric circuit. In fair weather regions far away from thunderstorm areas, one measures a downward-directed current density which is believed to be driven by the global thunderstorm activity. This current density is remarkably constant with height. Since the air is electrically conducting, the current is accompanied by a downward-directed electric field of the order of 100 volts per meter on the ground. The average potential between ground and the ionosphere, called the atmospheric electric potential is 240 kV. but varying between about 180 and 400 kV. The electric field on the earth's surface is always at right angles to the ground. This indicates that the earth's surface behaves like an electric equipotential layer for continuous electric fields with greater electric current density at higher altitudes along the surface. One source of the space charge near the ground is due to the electrode effect. The negative ions drift upward and the positive ions drift downward under the influence of the vertical electric field.
Thunderstorms are the main source of electromagnetic energy within the lower atmosphere and are believed to drive the global electric circuit. Typical thunderslouds are convective cumulo-nimbus clouds with vigorous updrafts and downdrafts. Most, but not all accumulate a net positive electric charge in their upper and a net negative charge in their lower regions. The electric fields from these charges become sufficiently intense to reverse the fair weather field over and beneath the clouds and to generate electric currents that can maintain the negative charge of the earth against the fair weather. About 2,000 thunderstorms are active around the earth at all times. The maximum intensity over the respective areas is at mid afternoon local time, but the global activity displays a dependence on the global time with a maximum of activity near 1800 h. UTC when the large continents of Africa and South America are both under the suns influence and therefore both experiencing peak thunderstorm authority.
Telluric currents will flow in the surface layers of the earth. Since thunderstorm maximum events occur at low latitudes in the afternoon and evenings, these telluric currents must be directed equator ward during the daytime and poleward at night. Telluric currents are a relatively shallow surface phenomena, yielding 1/2 day hemispheric telluric currents in the 102 to103 ampere range. The intensity of telluric currents are then adaquate to supply the flow from the global fair weather charge accumulator to the bases of the thunderstorms. Electric currents will flow between hemispheres at all times and the current density at any location is a direct function of the interhemispheric currents and their potential gradients.
Lightning discharges are breakdown electric currents in regions where the electric field locally exceeds about 400 kV/meter. Ground discharges normally transport negative charge (electrons) from the lower part of the cloud to the ground and are therefore part of the global circuit. Each ground discharge is generally made up of one or more intermittent partial discharges. A total lightning discharge, the time duration of which is about 1/3 second, is called a flash. Each component discharge, lasting of the order of tens of milliseconds, is called a stroke. Each lightning stroke is preceded by a barely luminous predischarge, the leader which produces a negatively charged ionized path between cloud and ground for the return stroke to follow. A corona sheath with a diameter of about one meter envelops the highly conductive leader channel. When the leader has approached to within 5 to 50 meters of the ground, a positively charged streamer from some point on the earth comes up to meet it, and then commences the return stroke which travels up the ionized path established by the leader. The negative charge in the channel is lowered to the ground. The return stroke is an upward movement of a luminous wave front with a velocity that reaches 0.1 to 0.3 of the speed of light with peak current amplitudes of 10 to 100 kA. within a few tens of microseconds. Subsequent return strokes follow, on the average of two or three occuring during one flash, but as many as 26 strokes have been detected in a single flash. A contineous current of 100 Amperes or more flows from the ground to the negatively charged portion of the cloud in the interval of the return strokes.
Luminous events occur at temperatures exceeding 10,000 degrees Kelvin. The visible spectrum of the lightning stroke consists mainly of neutral nitrogen and oxygen emissions. The temperature in return strokes can reach 30,000 K. and the pressure in the channel can increase to a peak of more than 106Pa. A pressure shock wave is thus generated which propagates away from the channel with a speed of about 3km/sec. This speed decreases rapidly as the shock front expands. The acoustic signal of the shock front is heard as thunder, which travels with the speed of sound (330 m/s). The maximum spectral amplitude of a thunder signal is near 100 Hz. The mean flash rate of lightning over the whole earth is estimated to be of the order of 100 flashes per second. These lightning flashes generate radio noise at frequencies greater than above 100kHz.
Lightning channels behave like huge antenna systems which radiate electromagnetic energy. These natural impulsive signals at frequencies below 100 kHz arise from coherent electric currents in lightning channels during return strokes and are called atmospherics. These signals are radiated throughout the earth and atmosphere, and propagate within the waveguide of the earth-ionosphere cavity. The waveguide thus has two windows of peak activity, one in the ELF (Extremely Low Frequency) range, and the other one in the VLF (Very Low Frequency) range. The phase velocity of the mode decreases with increasing frequency and is larger than the speed of light within the VLF range. The transmission function of the terrestrial waveguide has a window at extremely low frequencies and pulses containing significant spectral amplitudes in that range may be received around the earth where the waveguide behaves as a natural resonator. At these ELF ranges, resonances occurs for waves with horizontal wavelengths which are an integral multiple of the earth's circumference; and the spectral signals from lightning are amplified at these frequencies. The activity of this Schumann Resonance reflects the global lightning with maximum activity occuring between 1600 h and 200 h UTC.
The second generator of electromagnetic energy in the atmosphere is the ionospheric dynamo. The dynamo coil is the electrically conducting air within the dynamo region between about 80 and 200 km. height. The driving force is the tidal wind which moves the ionospheric plasma against the geomagnetic field and induces electric fields and currents. Manifestations of the electric currents on the ground are regular variations of the geomagnetic field depending on solar day and lunar day.
The solar tidal winds are excited by solar differential heating of the atmosphere which is accompanied by day-night differences in atmospheric pressure and temperature. The basic period of the solar tides is one solar day. The basic wavelength is the size of the earth. The tidal wave modes therefore depend on the spherical and rotating earth. Within the lower and middle atmosphere traveling waves transport energy and drive the ionospheric plasma at dynamo layer heights so that a horizontal electric current flows. Electric charge separation sets up an electrically polarized field and the magnetic field generated by this current can be measured on the ground as variations superimposed on the geomagnetic field from the earth's interior. In addition, atmospheric lunar tides are excited by the gravitational force which the moon exerts on the earth. Its pressure amplitude on the ground is 20 times smaller than the solar tide but the lunar tide does induce electric currents at dynamo region heights.
This current configuration is fixed to the sun, while the earth rotates beneath it with a current of about 140kA flowing. The rotating current and the conducting earth behave like a huge transformer with the dynamo region as the primary winding and the electrically conducting earth as the secondary winding. Since the current in the primary varies with the basic earth rotation period, electric currents are induced in the earth's interior and these secondary currents are superposed on the magnetic field of the primary dynamo current. The secondary current has an amplitude of about 1/3 of that of the primary current. Remarkable peak amplitude currents appear at the geomagnetic equator which are produced by this current band equatorial electrojet. The electric polarization of the field current and tidal winds have primarily an east-west component at the equator. These polarization changes produce a vertical electric field at the equator, thus enhancing the total current strength in the immediate environment and therefore producing a strong band of eastward flowing currents between the morning and early afternoon hours.
The third main source of electromagnetic energy within the atmosphere is the magnetospheric hydromagnetic dynamo. A steady flux of low energetic particles flows from the sun in a radial direction called the solar wind. This solar wind velocity varies between about 300 and 900 km/second. The number density of the solar wind particles decreases as the inverse square of the heliocentric distance and gives values of 106 to 107 per cubic meter in the vicinity of the earth. Magnetic fields from the sun offer periodic fluctuations and reversals of the polarities within this solar wind, with the well known 22 year sunspot cycle being dominant. The area of separation between field lines directed away from the sun and those directed toward the sun are called the interplanetary neutral sheet. This neutral sheet has a wavy configuration which the earth orbits through at the solar ecliptic plane. The earth passes through two or four sectors of opposite polarity of the interplanetary magnetic field during one synodic rotation of the sun, a period of about 27 days.
The plasma of the solar wind impinging on the earth's magnetic field cannot penetrate directly into the earth's atmosphere. It by-passes the geomagnetic field and creates a cavity--the magnetosphere. The boundry of this magnetospheric cavity is called the magnetopause which is the outermost region of the geomagnetic field. Since the solar wind is supersonic, a shock front forms at a distance of several earth radii in front of the subsolar point. The solar wind is a homogeneous, fully ionized, weakly magnetic, unidirectional plasma. The geomagnetic field surrounded by the solar wind plasma is therefore compressed and confined to a cavity, and electric currents flow on the border of this cavity. Two neutral magnetic points called cusps, form on the dayside magnetopause near =/-75degrees geomagnetic latitude. Here, the solar wind has direct access to the magnetosphere. The solar wind also stretches the geomagnetic field out into a long magnetic tail on the downward night time side, several hundred earth radii long.In the neutral sheet the electrons move toward dawn while the protrons move toward dusk, thus establishing a cross-tail current in the sheet directed toward dusk.
Several belts of highly energetic particles (the van Allen belts) are formed within the magnetosphere and surrounding the earth as intense toriodal fields that are formed of the spiraling and drifting protron/electron flows. The current via the dynamo region of the ionosphere is related to the dawn-dusk cycle. The electric power of the two magnetohydrodynamic generators in both hemispheres is in the order of billions of watts of energy. Nearly 10% of the solar wind energy moving past the earth magnetosphere is converted into electric energy. The superposition of magnetic fields originating from all fluctuating magnetospheric electric currents measured on the ground determines the degree of geomagnetic activity. The electric conductivity in the dynamo region varies with time of day, season, and latitude. Solar wind-magnetospheric interaction causes a never-ceasing, but highly fluctuating energy input into the polar fields, which leads to an almost permanent enhancement of the earth electrical conductivity.
Electromagnetic and hydromagnetic waves can be excited within the magnetosphere either by solar wind-magnetospheric interaction or by internal instabilities. Electromagnetic pulses generated by lightning events can propagate through the magnetosphere along the geomagnetic field lines and can be observed on the ground and within the magnetosphere. The magnetosphere behaves like a resonant cavity for waves with wavelengths comparable with the dimensions of the magnetosphere. The largest resonant wavelength being around 105km. which yields a time period of about 140 seconds and a frequency of around 7 mHz. Many resonances of this frequency and higher are detected within the magnetosphere and earth. Waves present within the magnetosphere can be amplified by energized particles, mainly near the magnetospheric equator. These enhanced waves in the VLF range can penetrate the magnetosphere and propagate along the geomagnetic field lines.
Fluctuations of the geomagnetic field with periods ranging from 0.2 seconds to more than 10 minutes are called geomagnetic pulsations. These pulsations are observed at all geographic latitudes with maximum occurances at the subaurora latitudes. There is a clear dependence of the average period of geomagnetic activity where solar disturbed conditions occur more frequently. Sudden pulses are a worldwide phenomena occurring nearly simultaneously on the day side and on the night side.
The low frequency spectrum of the electromagnetic energy of lightning signals is ducted within the terrestrial waveguide. Part of this energy can tunnel through the ionosphere and can propagate through the magnetosphere. The broad spectrum of a lightning pulse with maximum spectral amplitude near 5 to 10 kHz. is dispersed during its propagation through the magnetospheric plasma. The magnetospheric regions are copious sources of radio emmission, which radiate low frequency wave power of about 109 Watts into space at maximum spectral amplitudes near 200 kHz.
The general global electrical circuitry combine to a powerful unified geo-electrical structure.
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