Simulation Of Ocean Background Magnetic Field And It's Distribution In East China Sea

As a weak conductive medium, sea water can induce electric current with the motion across the earth's magnetic field, hence generates the ocean background magnetic field, that its strength about one hundred thousandth of earth's magnetic field. In airborne based ocean surveillance, the moving ocean can generate magnetic noise that is not ignorable when the noise magnitude is comparable to normal measurement. In underwater target detection and identification, i.e. submarines, by measuring the magnetic anomaly, ocean background magnetic noise is not ignorable either. Therefore it is necessary to establish numerical model of ocean background magnetic field to calculate and analyze its distribution to provide theoretical reference for ocean target detection and identification.In general, the magnetic field in the ocean has four components: (1) the field generated by Earth's conducting, fluid outer core.; (2) the field arising from magnetized crustal rocks; (3) the field arising from currents flowing in the ionosphere and magnetosphere; (4) the field induced by sea water movement (ocean waves, ocean current, internal wave, etc.). The first three parts composes the earth magnetic field, and the fourth part is generally referred as the ocean background magnetic field.The motion of seawater with velocity V across magnetic field B produces conduction current J in the sea. Together with the distribution of V , the induced magnetic field H is obtained in terms of Ampere ring circuit theorem and Faraday electromagnetic induction theorem. Therefore, simulating the seawater velocityV is the key point for the induced magnetic field calculation. For different ocean phenomena, i.e., ocean waves, ocean currents and internal waves, their distribution of V is different, so that their corresponding induced magnetic field need to be treat separately.For a simple surface wave, one of most remarkable characteristics of water surface is the harmonics form fluctuation along with the time, and can be treat as its surface oscillates in the form of sine wave or cosine wave. In this thesis, we treat ocean waves as sine waves to calculate the magnetic field. Internal waves are gravity waves that oscillate within, rather than on the surface of, a fluid medium. They arise from perturbations to hydrostatic equilibrium, where balance is maintained between the force of gravity and the buoyant restoring force, so that the internal wave induced magnetic field can be calculated from finding the internal wave velocity distribution. For ocean current, the following model can be assumed and used: First we divide can ocean current block into different thin layers and every layer is combined as many long and thin ocean current wire. Just like a wire moving in earth magnetic field, thin ocean current wire induces electric current hence generates magnetic field. The integral of the magnetic field in the vertical direction and horizontal direction of all these kinds of thin wires results the ocean currents generated magnetic field. The summation of the magnetic filed generated by ocean waves, ocean currents and internal waves becomes the ocean background magnetic field.In this thesis, the induced magnetic field of the three typical ocean moving components, i.e., ocean waves, ocean currents and internal waves, is introduced and summarized. An ocean background magnetic field simulation model with its software coded in C is provided and tested in which the World Magnetic Model 2005 is embedded. Meanwhile, the monthly magnetic field distribution in East China Sea is calculated by the application of the simulation software to the circulation current data, simulated from the Princeton Ocean Model (POM) driven by multi-satellites sensor data. Some areas are selected and their ocean background magnetic field is calculated and yearly variation of magnetic field is derived. Finally, the magnetic field induced by waves and currents in Qingdao Yachting Center is also calculated using the in-site measured data.