Effects of Radial Distances on Small-scale Magnetic Flux Ropes in the Solar Wind

Abstract

Small-scale magnetic flux ropes (SFRs), in the solar wind, have been studied for decades. Statistical analysis utilizing various in situ spacecraft measurements is the main observational approach which helps investigate the generation and evolution of these small-scale structures. Based on the Grad-Shafranov (GS) reconstruction technique, we use the automated detection algorithm to build the databases of these small-scale structures via various spacecraft measurements at different heliocentric distances. We present the SFR properties including the magnetic field and plasma parameters at different radial distances from the sun near the ecliptic plane. It is found that the event occurrence rate is still in the order of a few hundreds per month, the duration and scale size distributions follow power laws, and the flux rope axis orientations are approximately centered around the local Parker spiral directions. In general, most SFR properties exhibit radial decays. In addition, with various databases established, we derive scaling laws for the changes of average field magnitude, event counts, and SFR scale sizes, with respect to the radial distances, ranging from $\sim$ 0.3 au for Helios to $\sim$ 7 au for the Voyager spacecraft. The implications of our results for comparisons with the relevant theoretical works and for the application to the Parker Solar Probe (PSP) mission are discussed.