A record of the geomagnetic field on the ground sometimes shows smooth daily variations on the order of a few tens of nano teslas. These daily variations, commonly known as Sq, are caused by electric currents of several $\upmu \mbox{A}/\mbox{m}^{2}$
 flowing on the sunlit side of the E-region ionosphere at about 90–150 km heights. We review advances in our understanding of the geomagnetic daily variation and its source ionospheric currents during the past 75 years. Observations and existing theories are first outlined as background knowledge for the non-specialist. Data analysis methods, such as spherical harmonic analysis, are then described in detail. Various aspects of the geomagnetic daily variation are discussed and interpreted using these results. Finally, remaining issues are highlighted to provide possible directions for future work.

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Sq and EEJ—A review on the daily variation of the geomagnetic field caused by ionospheric dynamo currents

Presenting a coherent synthesis of lithosphere studies, this book covers a range of geophysical methods (seismic reflection, refraction, and receiver function methods; elastic and anelastic seismic tomography; electromagnetic and magnetotelluric methods; thermal, gravity and rheological models), complemented by petrologic and laboratory data on rock properties. It also provides a critical discussion of the uncertainties, assumptions, and resolution issues that are inherent in the different methods and models of the lithosphere. Multidisciplinary in scope, global in geographical extent, and covering a wide variety of tectonics settings across 3.5 billion years of Earth history, this book presents a comprehensive overview of lithospheric structure and evolution. It is a core reference for researchers and advanced students in geophysics, geodynamics, tectonics, petrology, and geochemistry, and for petroleum and mining industry professionals.

The Lithosphere: An Interdisciplinary Approach

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Geomagnetic storms, the Dst ring-current myth and lognormal distributions

[1] An empirical model of the quiet daily geomagnetic field variation has been constructed based on geomagnetic data obtained from 21 stations along the 210 Magnetic Meridian of the Circum-pan Pacific Magnetometer Network (CPMN) from 1996 to 2007. Using the least squares fitting method for geomagnetically quiet days (Kp ≤ 2+), the quiet daily geomagnetic field variation at each station was described as a function of solar activity SA, day of year DOY, lunar age LA, and local time LT. After interpolation in latitude, the model can describe solar-activity dependence and seasonal dependence of solar quiet daily variations (S) and lunar quiet daily variations (L). We performed a spherical harmonic analysis (SHA) on these S and L variations to examine average characteristics of the equivalent external current systems. We found three particularly noteworthy results. First, the total current intensity of the S current system is largely controlled by solar activity while its focus position is not significantly affected by solar activity. Second, we found that seasonal variations of the S current intensity exhibit north-south asymmetry; the current intensity of the northern vortex shows a prominent annual variation while the southern vortex shows a clear semi-annual variation as well as annual variation. Thirdly, we found that the total intensity of the L current system changes depending on solar activity and season; seasonal variations of the L current intensity show an enhancement during the December solstice, independent of the level of solar activity.

An empirical model of the quiet daily geomagnetic field variation

SUMMARY

The intrinsic ability of the method of spherical cap harmonic analysis to separate external and internal sources allows the calculation of equivalent ionospheric and induced currents that are able to explain variations of the geomagnetic field over a portion of the earth's surface. Formulations for current densities and current functions are derived and found to be analogous to those derived for the global case from conventional spherical harmonic analysis. Although spherical cap formulations for current density have been given by another worker, they were incorrect because of an error in defining the equivalent current. An example of the use of current functions is given by modelling variations from hourly mean values recorded at 40 geomagnetic observatories over Europe during a very quiet day in 1978. The modelling can be done spatially for each of the 24 hours separately, or spatially and temporally either by expressing each spatial coefficient as a Fourier series or by smoothing the spatial coefficients obtained from the separate hourly models.

Determination of equivalent current sources from spherical cap harmonic models of geomagnetic field variations.

A special selection of extremely quiet 1965 geomagnetic daily variation records from North America, Europe, central Asia, and east Asia was used to determine monthly spherical harmonic analysis coefficients representing the Sq fields from these regions. The coefficients permitted a separation of the external and internal contributions to the daily variations. Month-by-month behavior of the equivalent external current system and contours of the daily range of this current showed regional differences in amplitude and location of the equivalent ionospheric currents with seasonal change. Current foci locations reached 35° to 42° geomagnetic latitude in summer and 18° to 35° in winter with a regional dependence. The largest daily range of external current was 14.6×104 A in North America, 15.3×104 A in Europe, 13.7×104 A in central Asia, and 12.3×104 A in east Asia. Continental differences in the behavior of the current seemed to be explained by the relative position of the geographic and geomagnetic poles with respect to these regions.

Quiet ionospheric currents of the southern hemisphere derived from geomagnetic records

Spherical harmonic analysis coefficients of the external and internal parts of the quiet-day geomagnetic field variations (Sq) separated for the seven continental regions of the observatories have been used to determine conductivity profiles to depths of about 600km by the Schmucker equivalent substitute conductor method. The profiles give evidence of increases in conductivity between about 150 and 350km depth, then a general increase in conductivity thereafter. For South America we found a high conductivity at shallow depths. The European profile showed a highly conducting layer near 125km. At the greater depths, Europe, Australia and South America had the lowest values of conductivity. North America and east Asia had intermediate values whereas the African and central Asian profiles both showed the conductivities rising rapidly beyond 450km depth. The regional differences indicate that there may be considerable lateral heterogeneity of electrical conductivity in the Earth's upper mantle.

Upper mantle electrical conductivity for seven subcontinental regions of the Earth

The Sq current systems for quiet days of 1976 and 1977 were studied for observatories established near 76[degrees]E longitude in the northern hemisphere. A special sperical harmonic analysis technique was used to separate the internal and external contributions of the quiet field variations and to determine the equivalent external current contours responsible for the Sq field variations on the sixth and twenty-first of each month. The average ratio of the external to internal spectral power was 6.7 in 1976 and 7.4 in 1977. Focus positions for the India-Siberia region external Sq current vortex, near 22[degrees] to 29[degrees], were found to be at lower geomagnetic latitudes than for Europe and North America. The usual mid-latitude vortex current, reaching 11.0 to 13.2 [times] 10[sup 4] A in summertime, disappeared completely during winter months in both the years. The Sq current position was affected clearly by the off-spin axis position of the north geomagnetic pole. Throughout the 2 years, the authors also found small current vorticies near 70[degrees] to 80[degrees] geomagnetic latitude, which have been ascribed to auroral region activity on quiet days. 19 refs., 11 figs., 1 tab.

External Sq currents in the India-Siberia region

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/JA091iA08p09023

Correction to “Quiet ionospheric currents of the northern hemisphere derived from geomagnetic field records”

The quiet daily variations of the geomagnetic field, called Sq, have been modeled for the extremely quiet (solar-terrestrial disturbance) conditions using the digital values from magnetograms obtained from a worldwide distribution of observatories. This model, WDCA/SQ1, has been deposited at World Data Center A for Solar Terrestrial Physics as a set of data files and a Fortran computer program for retrieving the quiet variation field values at any world location, day of the year, and time of day. The WDCA/SQ1 model does not yet contain a solar-cycle adjustment; it is a best representation for the sunspot minimum period. Presently, the dependence on year is just an adjustment for the geomagnetic coordinate system. Nevertheless, even in active years the model provides an adequate description of the minimum Sq changes.

Edward R. Schiffmacher

Papers

Quiet ionospheric currents of the southern hemisphere derived from geomagnetic records

Upper mantle electrical conductivity for seven subcontinental regions of the Earth

External Sq currents in the India-Siberia region

Correction to “Quiet ionospheric currents of the northern hemisphere derived from geomagnetic field records”

Global quiet day field variation model WDCA/SQ1