A Comparison of Model‐Based Ionospheric and Ocean Tidal Magnetic Signals With Observatory Data

• Published in: Geophysical research letters Vol. 45; no. 15; pp. 7257 - 7267
• Main Authors: Schnepf, N. R., Nair, M., Maute, A., Pedatella, N. M., Kuvshinov, A., Richmond, A. D.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.08.2018
• Subjects: Amplitudes; Atmosphere; Atmospheric models; Coastal effects; Coastal processes; Components; Diurnal variations; Earth; Earth gravitation; Electric currents; Electrical conductivity; Electrical resistivity; Geomagnetic field; Geomagnetic field variations; Geomagnetism; Geophysics; Gravity; Ground level; Ionosphere; Ionospheric models; ionospheric tides; Magnetic field; Magnetic fields; Magnetic signals; marine electromagnetism; Moon; Observatories; Ocean currents; Ocean models; Oceans; Physics; physics‐based modeling; Predictions; Tidal energy; Tidal power; Tides; Upper atmosphere
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2018GL078487

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 M2 tidal mode) estimated from 64 global observatories to physics‐based forward models of the ionospheric M2 magnetic field and the oceanic M2 magnetic field. At ground level, predicted ionospheric M2 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 M2 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. Key Points We compare tidal magnetic signals (amplitudes and phases) from observatories compared to those from physics‐based numerical models We present the first comparison of magnetic fields induced by TIME‐GCM's physics‐based ionospheric M2 tidal electric current We find that predicted ionospheric M2 Z component is very small; this is encouraging for electromagnetic sensing