Tidal Composition Analysis of Global Sq Current System

Abstract

In this study, we examine the spatiotemporal variability of the global equivalent ionospheric current system estimated from geomagnetic Solar‐quiet (Sq) variations at 124 mid‐ to low‐latitude ground‐based magnetometer stations during 1–31 May 2020. In this period, geomagnetic activity was particularly low, that is, the hourly geomagnetic activity index Hp60 did not exceed 4o. The spherical harmonic analysis is performed on the Sq variations to estimate the equivalent current function at each universal time hour. Hourly maps of the global Sq current system are used to evaluate, for the first time, the tidal composition of mid‐latitude Sq currents and its temporal evolution. Although tides are known to be an important driver of Sq currents, the tidal composition analysis was difficult in previous studies that focused on Sq currents at particular longitude sectors. Our results show that the migrating tidal components (DW1, SW2, and TW3) are predominant in both hemispheres. This is as expected from the strong day‐night contrast in ionospheric conductivities. The day‐to‐day variations of the migrating tidal components are, however, not strongly correlated between the two hemispheres, suggesting that these variations arise not only from the global effect of solar radiation on ionospheric conductivities but also from the local effect of neutral winds. Variations associated with non‐migrating tides are also found. Especially, eastward‐propagating diurnal and semidiurnal tides with zonal wavenumber 1 (DE1 and SE1) are the largest non‐migrating components. Their production mechanisms remain to be understood.