We adopt a one-fluid description (Magnetohydrodynamics or MHD) of the solar wind plasma. A fluid description describes the plasma in terms of the evolution of its macroscopic variables: density, velocity, pressure, heat flux, ...and the evolution of the magnetic and electric fields, \(\mathbf B \) and \(\mathbf E \), given by Maxwell’s equations. In this chapter we detail the MHD equations and the limitations of this model. We then introduced the Expanding Box Model equations, a modified version of the MHD equations that accounts for the radial expansion of the solar wind.
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