Ionospheric dynamo region
In the height region between about 85 and 200 km altitude on Earth, the ionospheric plasma is electrically conducting.
[1] The magnetic manifestation of these electric currents on the ground can be observed during magnetospheric quiet conditions.
[2] Finally, a polar-ring current has been derived from the observations which depends on the polarity of the interplanetary magnetic field.
[4] 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.
Near the geomagnetic dip equator, a west–east directed electric field generates vertical Hall currents which cannot close.
Pedersen and Hall conductivities reach maximum values near 120 to 140 km altitudes with numbers of about 1 mS/m during sunlit conditions.
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.
[5] 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.
[7] 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 atmosphere behaves like a huge waveguide closed at the bottom (the Earth's surface) and open to space at the top.
Because the waveguide is imperfect, however, only modes of lowest degree with large horizontal and vertical scales can develop sufficiently well so that they can be filtered out from the meteorological noise.
[11] Within the thermosphere, however, it becomes the predominant mode, reaching temperature amplitudes at the exosphere of at least 140 K and horizontal winds of the order of 100 m/s and more increasing with geomagnetic activity.
[12] The largest solar semidiurnal wave is mode (2, 2) with maximum pressure amplitudes near the ground of 120 hPa.
[11] Because it is an internal waves, its amplitude increases with altitude, reaching values at 100 km height two orders of magnitude larger than on the ground.
More than 100 geomagnetic observatories around the world measure regularly the variations of the Earth's magnetic field.
The daily variations during selected days of quiet geomagnetic activity are used to determine a monthly mean.
A longitudinal dependence of the Sq current exists which is related to the inclined dipole component of the internal magnetic field, but probably also to nonmigrating tidal waves from below.
This can be attributed to the internal tidal modes which are sensitive to the varying meteorological conditions in the lower and in the middle atmosphere, in part also to solar activity.
A small increase occurs, called geomagnetic solar flare effect or crochet.
In the aftermath of strong magnetospheric disturbances, a current system develops into a quasi anti-Sq-current.
An electric polarization field E is generated by charge separation to enforce the condition of no sources and sinks of the current.
The influence of the mutual coupling between wind and current can immediately be seen if one considers an infinitely large electric conductivity σ.
In the kinematic model, the electric current would become infinitely large, while the wind amplitude remains constant.
In the hydromagnetic model, the current reaches an upper limit, similar to a technical dynamo during short circuit, while the wind amplitude breaks down to a fraction of its original value.
[1] The geomagnetic effect of the L-current can best been seen near the dip equator where the Cowling conductivity strongly enhances that current.
[1] Interaction between solar wind plasma and the polar geomagnetic field produces a global-scale magnetospheric electric convection field directed from dawn to dusk with a potential difference of about 15 kV during quiet magnetospheric conditions, increasing substantially during disturbed conditions.
Therefore, it dominates ionospheric and thermospheric dynamics and causes ionospheric and thermospheric storms [25][26] The magnetospheric electric convection field drives a two cell current within the polar cup with their vortices situated on the morning and on the evening side.
