Observed tidal geomagnetic field variations are due to a combination of electric currents in the ionosphere, ocean, and their induced counterparts. Using these variations to constrain subsurface electrical conductivity in oceanic regions is a promising frontier; however, properly separating the ionospheric and oceanic tidal contributions of the magnetic field is critical for this. We compare semidiurnal lunar tidal magnetic signals (i.e., the signals due to the M-2 tidal mode) estimated from 64 global observatories to physics-based forward models of the ionospheric M-2 magnetic field and the oceanic M-2 magnetic field. At ground level, predicted ionospheric M-2 amplitudes are strongest in the horizontal components, whereas the predicted oceanic amplitudes are strongest in the vertical direction. There is good agreement between the predicted and estimated M-2 phases for the Y component; however, the F and X components experience deviations that may be indicative of unmodeled ionospheric processes or unmodeled coastal effects. Overall, we find that the agreement between the physics-based model predictions and the observations is very encouraging for electromagnetic sensing applications, especially since the predicted ionospheric vertical component is very weak. Plain Language Summary Both Earth's oceans and upper atmosphere produce electric current and electromagnetic ( EM) fields. Tides caused by the gravitational interaction between Earth, the Sun, and Moon create reliable, easily constrained EM signals in both the ocean and upper atmosphere. Geophysicists are interested in using these ocean tidal EM signals to study the electrical conductivity of Earth's interior; however, this requires a solid understanding of the upper atmosphere's contribution to the EM signal. This study uses a physics-based model to estimate the relative EM tidal signals of the ocean versus the upper atmosphere to immediately address this concern. We also compare the model predictions with the EM tidal signals constrained from 64 global observatories. Considering the level of complexity of the upper atmosphere's EM tidal signals, we find remarkable agreement between the observed and modeled EM tidal signals. Some of the discrepancies may be of interest to those in upper atmospheric/space physics because they may be due to unmodeled physical processes. Our results are also very encouraging for geophysicists aiming to use EM tidal signals to study Earth's interior since they suggest a small upper atmosphere contribution to the EM tidal signal generally used.

http://dx.doi.org/10.1029/2018gl078487

Journal Article