On the processes in the terrestrial magnetosheath 1. Scheme development

We propose a new method to study the structure of the magnetosheath and the reby determine the underlying processes that create this structure. This me thod provides a systematic means of separating perturbations due to the sol ar wind variations from those generated within the magnetosheath. As a resu lt, we are able to study the magnetosheath processes as well as the dynamic solar wind-magnetopause interaction. We use the solar wind measurements fr om an upstream monitor as the input to the gasdynamic convected field model and then compare the model output with the in situ magnetosheath observati ons. We introduce three parameters to scale the model prediction to match t he timings of the magnetopause crossing, bow shock crossing, and upstream v ariations. With this procedure the relationship between the upstream measur ements and the magnetosheath observations and the location of the magnetosh eath satellite relative to the magnetopause and bow shock boundaries are hi ghly constrained. We then introduce a series of normalization procedures th at provide the means to remove the effects of the solar wind variations. Th e systematic differences between the model prediction and observation indic ate physical processes that are not included in the gasdynamic model. An ap plication of this approach is presented in a companion paper.