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 the approach of both simulation and observation. Statistical analysis utilizing various in-situ spacecraft measurements is the main observational approach. In this study, we extend the automated detection of small-scale flux ropes based on the Grad-Shafranov reconstruction to the complete dataset of \emph{Ulysses} spacecraft in-situ measurements. 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 heliographical latitudes and heliocentric radial distances. We then categorize all identified events into three groups based on event distributions in different latitudes separated at 30$^{\circ}$, at different radial distances, and under different solar activities, respectively. By the detailed statistical analysis, we conclude as follows. (1) The properties of flux ropes, such as the duration, scale size, etc., follow the power-law distributions, but with different slope indices, especially for distributions at different radial distances. (2) Also, they are affected by the solar wind speed which has different distributions under the different solar activities, which is manifested as the latitudinal effect. (3) The main difference in flux rope properties between the low and high latitudes is attributed to possible Alfv\'enic structures or waves. (4) Flux ropes with longer duration and larger scale sizes occur more often at larger radial distances. (5) With more strict Wal\'en slope threshold, more events are excluded at higher latitudes. The entire database is published online at \url{http://www.fluxrope.info}.