page_title: Ionospheric dynamo region
section_title: Morphology
hierarchical_section_title: Ionospheric dynamo region / Electric Currents / Sq-Current / Morphology
caption_attribution_description: English: Variation of Sq current as function of day and latitude
context_page_description: 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 and L-variations of the geomagnetic field.
  Additional electric currents are generated by the varying magnetospheric electric convection field. These are the DP1-currents 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.
context_section_description: 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. From the horizontal component ΔH of such data, one can derive a corresponding overhead equivalent electric sheet current system at dynamo layer heights of strength
  J = 2 ΔH/μ = 1.6 ΔH
  with J (in milliAmpere/meter) the electric overhead sheet current, ΔH (in nanoTesla) the observed horizontal component of the geomagnetic variation, and μ the electric permeability of free space. One can determine the direction of the magnetic field with regard to the current by the simple rule of thumb: if the thumb of the right hand points into the direction of the current, the curved fingers give the direction of the associated magnetic field.
  One has to take into account that this relationship is not unique. In general, electric currents within ionosphere and magnetosphere are three-dimensional, and an infinite number of current configurations fits to the geomagnetic variations observed on the ground.  Magnetic measurement in space are necessary to obtain a realistic picture.
  Figure 1a shows current streamlines of an equivalent Sq current as seen from the sun at noon. This current configuration is fixed to the sun, while the earth rotates beneath it. A total current of about 140 kA flows within one daytime vortex. The rotating Sq current and the electrically conducting earth interior behave like  a huge  transformer with the dynamo region as the primary winding and the earth as the secondary winding. Since the Sq current varies with the basic period of one day, electric currents are induced within the earth's interior. The magnetic field of this secondary current is superimposed on the magnetic field of the primary Sq current. Methods to separate  both components go back to Gauss. The amplitude of the secondary current is about 1/3 of that of the  primary current and slightly shifted in  phase. Figure 1b shows that secondary component. The ratio between sheet current and magnetic component given above has now simply the value one.
  The Sq current depends on season. The summer vortex is intensified compared with the winter vortex and reaches into the winter hemisphere. 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. In the course of the 11-year solar cycle, the amplitude of Sq increases by a factor of more than two from sunspot minimum to sunspot maximum. Two thirds of this increase may result from the enhancement of the electric conductivity with solar activity. The rest is probably due to the increase of the wind speed caused by the temperature increase with increasing solar activity. During the night, the electron density of the ionospheric E-layer diminishes much more strongly than that of the F-layer. Therefore, the height center of the dynamo region shifts upward.
  The main driver of the Sq current is the external (1, -2) tidal wave mode. Due to its constant phase with altitude, is most efficient to drive coherent winds at dynamo layer height, while the currents generated by the internal modes interfere destructively at various heights. A Fourier analysis shows a semidiurnal component with an amplitude of 1 /2 of that of the diurnal component, phase shifted by  180°. This appears to be the result of nonlinear coupling between the product of the diurnally varying wind and the diurnally varying conductivity  
  . 
  The centers of the daytime vortices show a day to day variability. 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.  
  Near the dip equator (where the geomagnetic field lines are horizontal), a strong band of eastward flowing currents can be observed in a range of about 150 km from the equator
  . Such enhancement of the Sq cur
caption: Figure 1. Streamlines of equivalent ionospheric Sq current during equinox (1957 - 1969) at 12 UT separated into primary (a) and secondary (b) part. Between two streamlines flow 20 kA [11].
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