Quasi-Diurnal Lunar Tide O₁ in Ionospheric Total Electron Content at Solar Minimum

For the first time, characteristics of the geographical and seasonal distribution of the quasi-diurnal lunar O₁ tide were derived from a time series of ionospheric total electron content (TEC) maps provided by International Global Navigation Satellite System Service (IGS). The data analysis is focused on solar minimum in 2008 and 2009 where disturbing influences of geomagnetic and solar activity were minimal. We found that the magnitude of the O₁ tide is as strong as the “dominant” semidiurnal lunar M₂ tide. Relative amplitudes of 10% and larger are observed in some regions for the O₁ component in TEC. The O₁ component is particularly strong in northern hemispheric winter over the west coast of South America. There, two maxima occur which are northward and southward of the magnetic equator in the Equatorial Ionization Anomaly (EIA) crest regions. Following Yamazaki et al. (2017, https://doi.org/10.1002/2017ja024601), it might be assumed that a longitudinal anomaly of ionospheric conductivities in the Peruvian sector leads to a stronger modulation of the equatorial electrojet by the lunar tides. Electrodynamic lifting of plasma and transport to the EIA crests may explain the variations of the O₁ component in TEC. Contrary to many studies, we find the O₁ component (period 25.82 hr) more important than the M₁ component (period 24.84 hr, a lunar day). We show that the geographical distribution of the O₁ component is totally different from that of the M₁ component which is smaller. The seasonal variation of O₁ shows maximal amplitudes in northern hemispheric winter and minimal amplitudes in southern hemispheric winter.