Dawn-dusk asymmetry and spongy nature of the Jovian magnetosphere.

Being based on the existence of the disc plasma flowing outward in the localized flow region with the super magnetosonic velocity called Jovian disc wind, a dynamical feature of the Jovian magnetosphere has been studied in the equatorial plane. Using a concept that the pressures inside and outside of the magnetopause are balanced by each other, a pressure balance equation has been obtained and calculated numerically to express the shape of the Jovian magnetosphere. The main pressure supporting the solar wind pressure is not the planetary magnetic pressure but the dynamic pressure of the Jovian disc wind. The nature of the Jovian disc wind with large azimuthal component in its bulk flow, therefore, has a great influence on the shape of the Jovian magnetosphere. The results of our numerical calculation of the pressure balance equation indicate a clear dawn-dusk asymmetry of the Jovian magnetosphere; i. e., Jupiter has a large scale magnetosphere in the dawn side and has a relatively small scale magnetosphere in the dusk side. Our calculation also indicates a sensitive response of the location of the Jovian magnetopause for the variation of the solar wind pressure. This feature coincides with the in-situ observations by Pioneer 10, 11, Voyager 1 and 2, and is called the “spongy nature” of the Jovian magnetosphere. In addition to the calculation for the super magnetosonic wind solution, we have obtained the shape of the magnetosphere for the case of sub-magnetosonic breeze. The results for the breeze case indicate that the stable magnetopause is able to exist only in the limited region in the day side.