The sun emits continuously not only electromagnetic radiations such as X-ray, ultraviolet, visible light, infrared, microwave, radio wave, and magnetic wave, but also a plasma flow consisting of electon and proton plasma, which is called a solar wind. It has recently been recognized that the solar wind is a supersonic flow with a speed of several hundred km per sec emitted from the hot coronal gas surrounding the outermost atmosphere of the sun. The planets of the solar system are immersed in such high speed solar wind plasma, and have a planetary magnetosphere depending on their magnetic moment. The planetary magnetosphere is the outermost magnetic barrier for prevention from the solar wind plasma penetrating into the planetary atmosphere, and is yielded under the electro-magnetic interaction of the planetary magnetic field with the solar wind plasma. The recent studies on the magnetosphere have been devoted to understand our. terrestrial magnetosphere and revealed that the magnetosphere constitutes a system of electrical circuit. Aurora, one of the most beautiful phenomena in our terrestrial magnetosphere, is an electrical discharge of the power generated in the electrical circuit through the solar wind-magnetosphere interaction.
