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

Abstract

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.

Phase Changes of Electromagnetic Oceanic Tidal Signals

“…In addition, other tidal systems, for example, the ionosphere, have nonnegligible vertical transports (Schnepf et al, ; Yamazaki & Maute, ). There, the described phase change mechanism will also arise in and (if U x and U z , respectively, U y and U z differ in their phases, cf., equation ).…”

“…Especially since the launch of the Swarm satellite magnetometers (Friis‐Christensen et al, ), the number of EMOTS‐related studies is ever growing. Alone in the last few years, many papers studying or using EMOTS were published (Grayver & Olsen, ; Guzavina et al, , ; Petereit et al, , ; Šachl et al, ; Schnepf et al, ; Saynisch et al, ; Velimsky et al, ). EMOTS are used to infer information about the Earth's lithosphere and mantle (Guzavina et al, ; Grayver et al, ; Kuvshinov et al, ; Schnepf et al, ).…”

“…Likewise, EMOTS are proposed to infer information about the oceans (Irrgang et al, ; Petereit et al, ; Saynisch et al, , ). In addition, EMOTS influence on other EM observations, and EMOTS errors are important topics (Guzavina et al, ; Saynisch et al, ; Schnepf et al, , ). Most of the mentioned studies focus only on the amplitudes of EMOTS and their sensitivities.…”

“…However, only amplitude and phase together describe a harmonic oscillation to its full extent. A few of the mentioned studies compare amplitude and phase of EMOTS between models and observations (e.g., Grayver & Olsen, ; Sabaka et al, ; Schnepf et al, ). However, studies of EMOTS phase sensitivities are absent from the literature.…”

Constraints on the resistivity of the oceanic lithosphere and asthenosphere from seafloor ocean tidal electromagnetic measurements

“…Recently, the use of ocean tidal EM (OTEM) fields to image electrical resistivity beneath the ocean basins has drawn significant attention. Generated by the gravitationally-forced tide that drives conductive seawater through Earth's ambient magnetic field, the OTEM signal has been detected in ground (Junge 1988;Maus & Kuvshinov 2004;Schnepf et al 2018), seafloor (Larsen 1968;Luther et al 1991;Schnepf et al 2014) and satellite (Tyler et al 2003;Sabaka et al 2015) measurements. The OTEM approach has two potential advantages over other natural source induction methods, such as MT and Geomagnetic Deep Sounding (GDS).…”