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The Coupling of Atmospheric Electromagnetic Fields

3.3.1 Lightning

Te lightning that streaks across the sky with deafening thunder and lightning bolts, accompanied by

a violent evening shower, is a wonderful summer tradition in Japan and the most important source of

electric felds generated near the earth’s surface.

Summer thunderclouds occur at a height of 0.5–1 km above the ground and ofen consist of several

parallel cloud masses that are about 10 km wide and 10–13 km high. Tunderclouds go through three

stages of development, maturity, and weakening. Each individual cloud mass is called a thundercloud

cell, and within each cell, convection occurs. Tese convection currents create violent updrafs with

wind speeds of up to 30 m/s and these currents carry water vapor up to 10–15 km in the air. Te water

vapor is condensed by the cold air above, but is quickly cooled to super-cooled water droplets. In addi­

tion, if there is a nucleus of dust or other particles, it will freeze into ice fakes. Te super-cooled water

droplets collide with the ice fakes and crystallize, growing into large powdery ice particles. As the ice

particles grow and become heavier and cannot be supported by the updraf, they begin to fall, dragging

the surrounding air with them and causing downward airfow. Tis is how violent convection occurs.

In this convection storm, water droplets and ice fragments collide with each other, splitting and becom­

ing electrically charged. Tus, positive charges are distributed above the cell and negative charges are

distributed below. As a result, a large potential diference of tens of volts to 100 million volts is generated

between the cloud base and the ground surface. A lightning strike is when the electric charges accumu­

lated in the thundercloud discharge toward the earth, and the current of a single lightning strike reaches

several thousand amperes to several tens of thousands of amperes.

Lightning discharges are considered as a source of electric currents, and it is believed that light­

ning discharges can generate from several kA to several hundred kA. Te path of a lightning stroke

(Figure 3.1), which can be of various lengths, acts as a huge antenna. Electromagnetic waves, of frequen­

cies determined by the length of the lightning stroke path, are emitted. Te length of the discharge path

can exceed several kilometers, and the frequencies range from several Hz to the GHz band. Tese are

naturally emitted electromagnetic felds.

When observing the occurrence of thunderclouds on the earth with satellites, we can see that there

are always 1,000–2,000 thunderclouds of various sizes developing simultaneously on the earth. Te aver­

age frequency of lightning strikes is about 160 times per second, and the current fowing into the earth

due to lightning strikes is estimated to be about 200,000 A/s.

Electrical potential is negative on earth and positive in the upper atmosphere. Tunderclouds are the

origin of a large global electric feld, which is created by pumping electricity near the earth’s surface far

up into the sky. Te earth’s electric feld is about 0.1 kV/m when it is highest and calmest near the surface,

0.03 kV/m at an altitude of 1 km, and about 0.01 kV/m at an altitude of 10 km. As a result, the potential

diference between the earth and the ionosphere is 200–400 kV. When a well-developed thundercloud

is overhead, the electric feld at the earth’s surface is about 3–20 kV/m. Tus, a large current circuit is

thought to be formed between the earth and the ionosphere. Te potential at the earth’s surface and

ionosphere is 20–50 kV (or 0.1–0.15 kV/m), and the return current in good weather is about 1–1.5 kA (or

3 × 10⁻¹² A/m²). Te electric feld at the ground surface is about 1.5 kV/m in the presence of atmospheric

disturbances and 3–20 kV/m during thunderstorms, depending on meteorological conditions such as

humidity, temperature, wind, and fog, and on the ion concentration in the atmosphere.

Lightning strikes have been known to afect biological systems directly by causing injury, or death

in trees, cattle, and humans. In this way, direct efects of lightning on biological systems are obvious.

Indirect efects have also been observed.

Te natural sources of electromagnetic processes are associated with lightning discharges, and the

resultant signals are called “atmospherics” or “sferics.” About 100 lightning discharges per second occur

globally. For example, one cloud-to-ground fash occurs about every second, averaged over the year in

the USA (Home page of GHRC: https://ghrc.nsstc.nasa.gov/home/). Tey vary with time and location,

and they have waves in ELF and very low frequency (VLF) ranges (ending at 300 kHz). Atmospheric