Analysis of Small-scale Magnetic Flux Ropes Covering the Whole Ulysses Mission.

Abstract

Small-scale magnetic flux ropes in the solar wind have been studied for decades via both simulation and observation. 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. In this study, we extend the automated detection of small-scale flux ropes based on the Grad-Shafranov reconstruction to the complete data set of in situ measurements of the Ulysses spacecraft. We first discuss the temporal variation of the bulk properties of 22,719 flux ropes found through our approach, namely, the average magnetic field and plasma parameters, etc., as functions of the heliographic latitudes and heliocentric radial distances. We then categorize all identified events into three groups based on event distributions in different latitudes separated by 30°, at different radial distances, and under different solar activities. With the detailed statistical analysis, we conclude the following: (1) the properties of flux ropes, such as the duration, scale size, etc., follow power-law distributions, but with different slope indices, especially for distributions at different radial distances. (2) They are also affected by the solar wind speed, which has different distributions under different solar activities, manifested as a latitudinal effect. (3) The main difference in flux rope properties between the low and high latitudes is attributed to possible Alfvénic structures or waves and to flux ropes with relatively high Alfvénicity. (4) Flux ropes with longer durations and larger scale sizes occur more often at larger radial distances. (5) With a stricter Walén slope threshold, more events are excluded at higher latitudes, which further reduces the latitudinal effects on flux rope properties. The entire database is published online at http://www.fluxrope.info.