I was hoping someone else would jump in on this because I was also curious how it was done. You can simulate this with steady state data by using stream traces where the time delta is large (like "1"), however, you won't see the effect of the lines increasing in length.
Thus, I conjecture this is time-dependent data and they're releasing streak lines (Analyze -> Calculate Particle Paths and Streaklines) with a Release interval between 0 and 1. (In other words,one particle per fraction of the time steps). The streak lines are depicted by a mesh (the line itself) and use of right triangle scatter symbol.