In the height region between about 85 and 200 km altitude on Earth, the ionospheric plasma is electrically conducting. Atmospheric tidal winds due to differential solar heating or due to gravitational lunar forcing move the ionospheric plasma against the geomagnetic field lines thus generating electric fields and currents just like a dynamo coil moving against magnetic field lines. That region is therefore called ionospheric dynamo region. The magnetic manifestation of these electric currents on the ground can be observed during magnetospheric quiet conditions. They are called Sq-variations (S=solar; q=quiet) and L-variations (L=lunar) of the geomagnetic field. Additional electric currents are generated by the varying magnetospheric electric convection field. These are the DP1-currents (the auroral electrojets) and the polar DP2-currents. Finally, a polar-ring current has been derived from the observations which depends on the polarity of the interplanetary magnetic field. These geomagnetic variations belong to the so-called external part of the geomagnetic field. Their amplitudes reach at most about 1% of the main internal geomagnetic field Bo.
Atmospheric electric conductivity Radioactive material from the ground and galactic cosmic rays ionize a small fraction of the atmospheric gas within the lower and middle atmosphere and make the gas electrically conducting. Electrons quickly attach to neutral particles forming negative ions. The positive ions are mostly singly charged. The electric conductivity depends on the mobility of the ions . That mobility is proportional to the reciprocal air density. Thus, the electric conductivity increases almost exponentially with altitude. The ions move with the neutral gas making the conductivity isotropic. At heights between about 85 and 200 km however -the dynamo region-, solar X- and extreme ultraviolet radiation (XUV) is almost completely absorbed generating the ionospheric D-, E-, and F-layers. Here, the electrons are already bound to the geomagnetic field gyrating several times about these lines before they collide with the neutrals, while the positive ions still essentially move with the neutral gas. Thus, the electric conductivity becomes anisotropic. The conductivity parallel to an electric field E is called Pedersen conductivity. The conductivity orthogonal to E and the geomagnetic field Bo is the Hall conductivity. Ohmic losses and thus Joule heating occur when Pedersen currents flow. The component parallel to Bo still increases with altitude. Near the geomagnetic dip equator, a west–east directed electric field generates vertical Hall currents which cannot close. Therefore, a vertical polarization field builds up generating a horizontal Hall current which adds to the Pedersen current. Such enhancement is described by the Cowling conductivity. Pedersen and Hall conductivities reach maximum values near 120 to 140 km altitudes with numbers of about 1 mS/m during sunlit conditions. During the night, these numbers may decrease by a factor of ten or more. The values of these conductivities depend on local time, latitude, season and solar 11- year cycle. The height integrated conductivities become of the order of 50 S, or a total resistance of the dynamo region of about 1/50 = 0.02 Ohm during daytime conditions. In the auroral regions which lie between about 15° and 20° geomagnetic co-latitude and the corresponding latitudes in the southern hemisphere, precipitating high energy particles from the magnetosphere ionize the neutral gas, in particular at heights around 110 to 120 km, and increase the electric conductivity substantially. During magnetospheric disturbed conditions, this conductivity enhancement becomes much larger, and the auroral regions move equatorward. Under low electric conductivity conditions, e.g., during night time and during winter, the dynamo region becomes too resistive, leading to the ionospheric feedback, which is responsible for higher occurrence of aurora under low sunlight. At heights above about 200 km, collisions between neutrals and plasma become rare so that both ions and electrons can only gyrate about the geomagnetic lines of force, or drift orthogonal to E and Bo. The parallel conductivity is so large that the geomagnetic lines of force become electric potential lines, and only electric fields orthogonal to Bo can exist (see magnetosphere).
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