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 eld, so as to analyze the propagation of hydromagnetic instabilities. In particular we show that the so called Jeans instability is in principle aected by the presence of the magnetic eld. A brief discussion is made attempting to assess this inuence during the stage of the evolution of the Universe when structures were formed. The most logical conclusion is that if magnetic elds existed in those times then their magnitudes were too small to modify the Jeans mass. Our results place limits on the possible values of seed magnetic elds consistent with the formation of structures in the Universe. These values are within the range of the results obtained by other authors.