Ocean effect

The Earth's oceans are made of an electrically conducting fluid with relatively high conductivity (an average conductivity about 3.0 S/m) and the large-scale salty water moving through the geodynamo-produced field can induce electric currents, which generate the induced magnetic fields measurable by modern high-precision geomagnetic satellites. The high-precision geomagnetic satellites like Swarm are capable of identifying magnetic signatures less than O(1) nT at satellite altitude about 400-500km or less, representing a really remarkable achievement of the modern geomagnetic satellites. There exist two different components in the oceans-flow-induced magnetic field: an invisible toroidal field whose amplitude is about O(100) nT and a visible poloidal field whose amplitude is about O(10)nT in oceans. Since the oceans-flow-induced magnetic signals are sensitive to the oceans bottom structure and conductance, the signals of the induced magnetic field can be employed to probe the geochemical and geological structure below the oceans floor. Extracting contributions from the the oceans-flow-induced magnetic field is also crucial to isolating the magnetic field that originates from the core geodynamo.

It is now well established that the gravitational tidal phenomena in oceans can induce weak but measurable magnetic signals in the form of fast periodic magnetic perturbations, in contrast to the slow and chaotic secular variations of the core geodynamo field. Existing studies have mainly focused on the oceans-induced magnetic field generated by the well-known lunar semidiurnal tide, M2, with a period of about 12.2 hours. This discrete frequency in connection with the motion of Sun and Moon helps distinguish the induced field from other sources such as the core geodynamo field and the lithospheric field. Magnetohydrodynamic equations describing the oceans-induced magnetic field via the tidal flow can be substantially simplified by assuming that the Lorentz force is negligibly small on the fluid motion of oceans. The induced magnetic field measured by Galileo spacecraft was successfully employed to predict the existence of a global ocean in Europa from its response to the Jupiter’s magnetic field.

Below is a brief description of our progress on the tide effect. Electromagnetic fields are induced in the interior of the Earth by a current source that generated by the interaction of moving conductive seawater (tide) with the Earth's main magnetic field. Extracting data of different principal components or periods from tides can obtain a variety of tidal models: such as the lunar semidiurnal(M2) tide model, with a period of 12.421 hours, and the diurnal(O1) tide model, with a period of 25.819 hours.

Figure 1 shows the magnetic signals produced by the M2 tide model observed at an altitude of 450 km. The real part of the radial component generated by the M2 tide in the interior of the earth produces a large positive response in the Indian Ocean, and in the Pacific region, the response value in the latitude direction has the characteristics of alternating positive and negative, in southern Africa, southern South America, and North America. The imaginary part of the radial component retains a positive response in the Indian Ocean. The maximum magnitudes of the north component and the east component of the magnetic induction are smaller than those of the radial components. The real and imaginary parts of the northward component signal have similar characteristics to the radial component in the Indian Ocean, except that the real part of the northward component signal exhibits a negative response in the western Indian Ocean, while it has a larger range of positive response characteristics in the eastern Pacific Ocean. The imaginary part of the component signal has a large negative response in the New Zealand region, and the amplitude is large. The northward component signal has similar response characteristics to the radial component in the Pacific region, but has a large-scale positive response in the southern Indian Ocean, and a negative response in the northern Indian Ocean near the Arabian Sea.

Figure 1. the magnetic signals of M2 tide model at 450km satellite altitude. The first column is the real part (top) and the imaginary part (bottom) of the northward component magnetic field signal, the second column is the real part (top) and the imaginary part (bottom) of the eastern component magnetic field signal, and the third column is the real part (top) ) and the imaginary part (bottom) of the radial component magnetic field signal. Note: The color scale values of the radial component and the horizontal component are different in size.

Figure 2 illustrates the magnetic signals produced by the O1 tide model observed at an altitude of 450 km. For the O1 tidal signal, the real part of the radial component has a strong negative response feature near the Bering Strait and the waters of southern Australia, and has a significant positive response in the ocean between Antarctica and Australia, but has a significant positive response in the Pacific Ocean close to Antarctica. Negative response. The other two components have the characteristics of alternating positive and negative responses near Antarctica. The author believes that this is likely to be affected by the Antarctic circulation. The radial component has a larger magnitude than the horizontal component.

Figure 2 illustrates the magnetic signals produced by the O1 tide model observed at an altitude of 450 km. For the O1 tidal signal, the real part of the radial component has a strong negative response feature near the Bering Strait and the waters of southern Australia, and has a significant positive response in the ocean between Antarctica and Australia, but has a significant positive response in the Pacific Ocean close to Antarctica. Negative response. The other two components have the characteristics of alternating positive and negative responses near Antarctica. The author believes that this is likely to be affected by the Antarctic circulation. The radial component has a larger magnitude than the horizontal component.