Knot structures in jets formed by a two-mode ejection velocity time-variability

We present a 1D analytic model for the formation of working surfaces along a jet produced by an outflow source with a double-sinusoidal mode ejection velocity variability. We succeed in obtaining simple expressions for the distance of formation and for the initial velocity jump across the 'fast-mode' working surfaces, which aremodulated by the phase of the 'slow mode'. These results can be straightforwardly applied to interpret observations of Herbig-Haro jets showing a chain of aligned knots close to the source, and large 'heads' at greater distances, for which a two-mode ejection variability is likely to be applicable. We use our new analytic model to interpret observations of the externally photoionized HH 333 jet.