The theoretical linear stability of a shock wave moving in an unlimited homogeneous
environment has been widely studied during the last fifty years. Important results
have been obtained by Dýakov (1954), Landau & Lifchitz (1959) and then by Swan
& Fowles (1975) where the fluctuating quantities are written as normal modes. More
recently, numerical studies on upwind finite difference schemes have shown some
instabilities in the case of the motion of an inviscid perfect gas in a rectangular channel.
The purpose of this paper is first to specify a mathematical formulation for the
eigenmodes and to exhibit a new mode which was not found by the previous stability
analysis of shock waves. Then, this mode is confirmed by numerical simulations which
may lead to a new understanding of the so-called carbuncle phenomenon.