The agreement between Rankine-Hugnoniot equations and the simulation of interplanetary MHD shock that can arrive 1AU or further is investigated. A method to locate the position of the forward fast shock in the shock pair structure is proposed in this paper. The method is composed of two main steps. First, by analyzing topology of the simulated MHD shock and its evolution during its propagating forward, it is identified that the discontinuity structure in the computational domain is in fact shock pair structure. According to this judgment, a method to locate the shock in the computation domain is developed. The second step is to select the proper time levels where the space between the shock and the grids is minimal, so as to minimize the errors that the shock is not exactly on the grids. By using this method to locate the shock and the method that was proposed in Reference [14] to determine the local parameters of the shock, it is found that the forward fast shock in the shock pair structure matches the Rankine-Hugoniot relations perfectly with the relative difference limited in the scale, when the shock propagates to or farther, and that the discrepancy between them is limited in the scale when the shock propagates to or farther. This result confirms that the finite-difference numerical solutions can describe the shock accurately. And as to clear out the cause of the discrepancy, the relationship between the discrepancy and the evolution of the shock structure as well as the relationship between the discrepancy and the polytropic index is discussed.
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