Atmospheric Electricity

Compendium, Chapter XII

Fleagle and Businger, pp 130-145

Wallace and Hobbs, pp 252-259

Elementary Principles

• Coulomb’s Law, electrical point charge (q) placed at a distance (r) from a charge (Q) experiences a force:

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• Electric field- force on a unit test charge due to another charge Q

• Field is proportional to Q, limit of the force per unit test charge

• Using (3.22) electrostatic field may be written as

• Potential energy- work necessary to bring the unit of charge from r = ∞ to r = r0 against electrostatic field.

• Electrostatic potential,

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• (3.24) and (3.25) show that the potential and electrostatic field vector are related by

• Surfaces of equal potential surround charge• Test charge may be moved on a geopotential surface without

work• If charges at rest, surface of a charged conductor must be

equipotential• Potential represents the work done bringing a charge Q from

∞ to the conductor and is proportional to the charge• Capacitance, C

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• Combining (3.25) and (3.26) gives capacitance for a sphere in a vacuum

• Ohm’s law: electrical charge flows along a conductor at a rate proportional to drop in potential

• Where the resistance (R) is characteristic of the conductor

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Earth’s electric field

• Air generally regarded as an insulator• Usually possible to detect a measurable electric field and an

electric current flowing from atmosphere towards earth, even on cloudless days

• Fair weather field- atmosphere is positively charged with respect to the ground

• Local variations• In vicinity of thunderstorms field is reversed (covers less than

1% of earth's surface• Fair weather electric field is normal condition of the

atmosphere

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• Mean electric field• Thunderstorms always active somewhere in the atmosphere;

likened to huge van de Graaff generators providing potential difference between ionosphere and earth ~400kV

• Erath and ionosphere both excellent conductors, charge conducted readily in horizontal, each spherical surface is at uniform potential

• Positive current flows downward through the atmosphere over the earth except for thunderstorms, ~4x10-12Am-2 or 2x103A over entire earth

• Figure 3.23 shows generation of potential difference and the current flow between earth and ionosphere

• Figure 3.24 shows diurnal variation of potential gradient over the sea and number of observed thunderstorms

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Atmospheric Ionization

• Neutral atoms gain or lose electrons, acquire an electric charge to form positive or negative ions

• Elementary electric charge may also attach itself to a molecule, dust particle, cloud droplet etc. to form a large charged particle

• Ions then set in motion by earth's electric field, electric current develops• Cosmic rays from space, radioactive emissions from earth’s surface are

responsible for producing high proportion of atmospheric ionized molecules• Primary cosmic rays- particles of very great energy, mostly protons• Enter atmosphere from all directions, produce other high energy particles by

colliding with neutral air molecules• These are called secondary cosmic rays which produce ions by collisions with

atmospheric gases

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• Near surface, ions produced by decay of radioactive elements in the ground and air

• Over sea, radioactivity is not important for ionization• At high altitudes, cosmic rays less effective ionizers, only

primary particles present• But ions may be produced from absorption of UV and X

radiation from sun• In thermosphere, electrons ejected from neutral O and N

atoms can remain free for a long time• Ionization results in creation of a negative electron and

heavier positive ion• Near sea level, negative and positive ions rapidly attach to

neutral molecules• Size remains molecular and they are called small ions

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• Others attach themselves to aerosols which are larger than molecules and these form large ions (Langevin ions)

• Four categories of atmospheric ions- small, large, positive, negative

• Motion of ion- drift velocity depends on the charge and mass of ion, potential gradient and mean free path

• Mobility of an ion- ratio drift velocity/electric field intensity• Small ions, low mass, higher velocity than large ions• Ionic mobility- average velocity at which an ion drifts through

a specified gas in an electric field of unit strength• Near surface, mobility of small ions ~ 1.3x10-4ms-1Vm-1• Large ions ~4x10-7ms-1Vm-1• Electrical conductivity- quantity of electricity transferred

across a unit area per unit potential gradient per unit timePhysical Meteorology II 13

• Small ions contribute over 95% of total conductivity of atmosphere

• Small ions constantly being destroyed by combination with ions of opposite sign or removed by attachment to large particles

• To determine rate, must know ion density (number of ions per unit volume of air)

• Recombination is proportional to ion density, rate of recombination proportional to square of ion density

• Rate of ion attachment proportional to concentrations of small ions and large particles

• Under equilibrium conditions between ion production and destruction, production rate of small ions, p = αn2 + βnN

• n number of small ions• N number of large particles per unit volume• α recombination coefficient for small ions, 1.6x10 -6cm3s-1

• β combination coefficient for small ions and large particles, 3x10 -6cm3s-1Physical Meteorology II 14

The Ionosphere• At high altitudes, ion destruction proceeds much less rapidly than

at lower altitudes• Spherical shell of high ion density persists in a region known as

ionosphere

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• D-region• Below about 90 km• Reflects low frequency radio waves, absorbs

medium and high frequency waves• Lower limit sbout 70 km• Disappears at sunset• Well developed during periods of enhanced

solar acivity (solar flares) leads to breakdown of medium and high frequency radio communications called sudden ionospheric disturbances (SID)

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• E-region• Lies between ~90 and 140 km• Medium and high frequency waves reflected • Begins to weaken after sunset• Recombination of ions and electrons proceed

most rapidly in its lower levels• Only disappears during the long polar winter

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• F-region• Extends upwards from about 140 km• Upper parts consists almost entirely of protons and

electrons• Characterized by two layers:– F1 layer- distinguished only in the daytime when the sun is

fairly high; merges with the F2 layer at night when sun is low; has important influence on the propagation of medium and high frequency radio waves

– F2 layer-Lies in upper part of F region; important in long distance radio communications; ion density reaches a peak at ~250-500 km

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