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P.E. Hardee (U. Alabama)
The appearance of helical structures on relativistic jets is studied using a normal mode analysis of the linearized fluid equations. High pressure regions helically twisted around the jet beam may be confined close to the jet surface, penetrate deeply into the jet interior or be confined to the jet interior. The high pressure regions range from thin and ribbon like to thick and tube like. The speed of structures can be significantly different at different wavelengths but are less than the flow speed. A maximum pressure fluctuation criterion is used to estimate the maximum amplitudes of structures. In general, jet distortions and the accompanying velocity fluctuations scale inversely with the Lorentz factor. Emission from high pressure regions can vary significantly as a result of angular variation in the flow direction which is on the order of the beaming angle, \theta=1/\gamma. This angular variation can produce cyclic angular variation and the cyclic variability seen in BL Lac object OJ~287. Helical structure can appear on multiple length scales, e.g., the galactic superluminal GRO~J1655--40.
Supported by the National Science Foundation through grant AST-9802955 to the University of Alabama.