  Ground magnetic field measurements can be mathematically related to an
overhead ionospheric equivalent current. In this study we look in detail at how
the global equivalent current, calculated using more than 30 years of SuperMAG
magnetometer data, changes with sunlight conditions. The calculations are done
using spherical harmonic analysis in quasi-dipole coordinates, a technique
which leads to improved accuracy compared to previous studies. Sorting the data
according to the location of the sunlight terminator and orientation of the
interplanetary magnetic field (IMF), we find that the equivalent current
resembles ionospheric convection patterns on the sunlit side of the terminator
but not on the dark side. On the dark side, with southward IMF, the current is
strongly dominated by a dawn cell and the current across the polar cap has a
strong dawnward component. The contrast between the sunlit and dark side
increases with increasing values of the $\mathit{F}_{10.7}$ index, showing that
increasing solar EUV flux changes not only the magnitude but also the
morphology of the equivalent current system. The results are consistent with a
recent study showing that Birkeland currents indirectly determine the
equivalent current in darkness and that Hall currents dominate in sunlight.
This has implication for the interpretation of ground magnetic field
measurements and suggests that the magnetic disturbances at conjugate points
will be asymmetrical when the solar illumination is different.
