The impact of sunlight on high‐latitude equivalent currents. Issue 3 (18th March 2016)

• Record Type: Journal Article
• Authors: Laundal, K. M.; Gjerloev, J. W.; Østgaard, N.; Reistad, J. P.; Haaland, S.; Snekvik, K.; Tenfjord, P.; Ohtani, S.; Milan, S. E.
• Is Part Of: Journal of geophysical research. Volume 121:Issue 3(2016:Mar.)
• Page Start: 2715
• Page End: 2726
• Publication Date: 2016-03-18
• Subjects: ionospheric currents -- ionospheric conductivity -- ground magnetic perturbations; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1002/2015JA022236
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: 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 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. Key Points: Global equivalent currents are investigated with respect to the position of the sunlight terminator The global equivalent current resembles Hall currents on the dayside but not on the nightside The day‐night contrast increases with solar EUV flux