Weibel Instability in a Foreshock Region*

Collisional shocks, especially the relativistic ones, in weakly-magnetized plasmas are accompanied by an extended magnetized region in front of it -- the foreshock. The foreshock is produced by outgoing particles (cosmic rays) interacting with the ambient medium via plasma instabilities. Weibel Instability (WI) is known to be particularly important in GRB and early SN shocks. Here we discuss how WI is proceeding in the 'pre-conditioned' medium of the foreshock, when 'fresh' cosmic rays are continuously ejected from the shock front into the turbulent plasma with fairly strong and small-scale magnetic fields.