A simple model for hydromagnetic instabilities in the presence of a constant magnetic field

In this paper we study a simple model consisting of a dilute fully ionized plasma in the presence of the gravitational and a constant magnetic field to analyze the propagation of hydromagnetic instabilities. In particular we show that the so called Jeans instability is in principle affected by the presence of the magnetic field. A brief discussion is made attempting to assess this influence in the stage of the evolution of the Universe where structures were formed. The most logical conclusion is that if magnetic fields existed in those times their magnitudes were too small to modify Jeans' mass. Our results places limits of the possible values of seed magnetic fields consistent with the formation structures in the Universe. These values are within the range of the results obtained by other authors.