Electric Currents, Atmospheric

the directed motion of charged particles in the atmosphere.

Electric currents in the troposphere and the stratosphere are reduced to convection currents i_v, created by the transport of electric space charges as a result of air flow or gravity; conduction currents i_c, produced by the electric field of the atmosphere; and turbulent-diffusion currents i_t, arising as a result of the density gradient of space charges and turbulent agitation in the atmosphere. In the ionosphere electric currents are also created by the incursion of solar particles and by the motion of the ionospheric plasma in the magnetic field. Convection currents determine the separation of charges; their density j_v, equal to the product of the density of the space charges and the rate of motion of the charges, can vary substantially with time and differ in different regions, undergoing periodic diurnal and seasonal variations.

In zones where the weather is good, the vertical component of j_v is ~ 10^–12 ampere · m^–2 while the horizontal component of j_v may reach 10^–9 to 10^–8 ampere · m^–2; inside thunderstorm clouds, the vertical component of j_v is 10^–6 ampere · m^–2. The currents i_c and i_t limit the charge separation process produced by the convection currents. The density of the conduction current i_c is equal to the product of the field intensity E and the electrical conductivity of the atmosphere λ. In zones of good weather, i_c = (1 – 3) × 10^–12 ampere · m^–2 (see alsoATMOSPHERIC ELECTRICITY). The current density j_t can be an appreciable fraction of j_c. Under steady-state conditions, it can be assumed that up to a considerable height in the atmosphere the electric currents along the vertical are constant; that is j_v + j_c + j_t = const.

The time variations of the total electric currents for the entire earth are very much like the variation of j_v. Considerable currents occur in the atmosphere during corona discharges around pointed objects in a strong atmospheric electric field, giving rise to a glow known as St. Elmo’s fire. Substantial currents, reaching hundreds of thousands of amperes, occur in lightning discharges.