[en] The ion distribution function in a thermal barrier is modeled by counter streaming Maxwellian distribution. Its stability with respect to parallel propagating electrostatic and electromagnetic modes is investigated both analytically and numerically. It is found that the electrostatic mode is stable if T/sub e//T/sub i/ greater than or equal to 3.5 and the electromagnetic mode is stable if the streaming velocity is less than the Alfven speed. The latter condition implies the barrier potential must be less than the average magnetic energy per particle