The impact of sunlight on high-latitude equivalent currents

TL;DR

This study analyzes over 30 years of ground magnetic data to understand how sunlight influences the global ionospheric equivalent current system, revealing significant differences between sunlit and dark conditions and the role of solar EUV flux.

Contribution

It introduces an improved spherical harmonic analysis method to study sunlight effects on equivalent currents, highlighting the impact of solar illumination and EUV flux on current morphology.

Findings

Equivalent currents resemble convection patterns on sunlit side

Dark side currents are dominated by dawn cells with dawnward polar cap flow

Increasing solar EUV flux alters current magnitude and shape

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 $\mathit{F}_{10.7}$ 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.