Electromagnetic instability of partially overlapping flows in the interstellar medium

The stability of the charged particle velocity distributions resulting from intermixing of neighboring flows of interstellar medium are examined, in particular, within the confines of spiral galaxies. It is shown for a specific example that the customary electrostatic instability shows up when the difference in the systematic velocities of both flows is sufficiently large compared to the thermal speed. On the other hand, the electromagnetic instability occurs, in principle, for an arbitrarily small difference in the flow velocities. The characteristic time for this instability to develop is much longer than for the electrostatic instability, but is still short compared to the ordinary evolution time for the interstellar medium.