In order to explain the apparent alternation in VLA jet structures, we appeal to evaporation from an outer disk to switch the direction of the central energetic flow by a process of 'optical depth modulated' choking, That is as the radiation from a central engine heats the surface of an extended disk surrounding the engine, the disk surface will evaporate thermal gas, expanding the disk atmosphere and creating a disk wind. We argue that its dynamic pressure plays a very important role in confining the central energetic flow. If the disk has initially an asymmetric density distribution in the upper and lower atmospheres (i. e., asymmetric distribution of optical depth), then this can initiate the alternating jets(flip-flop jets), A flip-flop time scale is calculated from the rate of optical depth variation in the disk atmosphere(e.g., the optically thin atmosphere becomes optically thick due to the mass influx from the central engine), This gives a time scale of 3×10^7 yrs, which value, however, is strongly dependent on the size of the jet cavity where in the density(optical depth) is varied. We compare the theoretically derived flip-flop time scale to those estimated observationally for the standard straight, as well as the rotationally and mirror symmetric radio sources. The prediction is found to be reasonably consistent with the observationally derived time scales of the standard straight radio sources, however it is likely to be longer than those in the rotationally and mirror symmetric radio sources.

• 최승언
• R.N. Henriksen