Heliospheric evolution of solar wind small-scale magnetic flux ropes

We present results from the first comprehensive small-scale flux rope survey between 0.3 and 5.5 AU using the Helios 1, Helios 2, IMP 8, Wind, ACE, and Ulysses spacecrafts to examine their occurrence rate, properties, and evolution. Small-scale flux ropes are similar to magnetic clouds and can be modeled as a constant-alpha, force-free, cylindrically symmetric flux rope. They differ from magnetic clouds in that they have durations on the order of tens of minutes up to a few hours, they lack an expansion signature at 1 AU, and they do not have a depressed proton temperature compared to the surrounding solar wind plasma. The occurrence rate of small-scale flux ropes is slightly higher in the inner heliosphere than the outer heliosphere and has a weak dependence on the phase of the solar cycle. The duration of the events as a function of radial distance indicates there is a large, rapid expansion within 1 AU and it becomes constant in the outer heliosphere. This behavior implies small-scale flux ropes are created and nearly complete their evolution within 1 AU.