Observational Analysis of Small-scale Magnetic Flux Ropes from Ulysses In-situ Measurements

Abstract

Small-scale magnetic flux ropes, which have similar magnetic field configuration as their large-scale counterparts (i.e., magnetic clouds), but with different sizes and origin, constitute an important element of solar wind structures. They are also considered to be associated with local particle energization and other related processes. In this report, we apply the Grad-Shafranov (GS) reconstruction method to detect these small-scale flux ropes with a set of quantitative criteria by utilizing data from the Ulysses spacecraft measurements for the first time. We conduct full range automatic detection for years 1994, 1996, 2004 and 2005 during the solar minimum periods. Based on solar wind speed/helio-latitude ranges, these periods are categorized into two groups: one with high solar wind speeds at high latitudes (1994 and 1996) and the other with low solar wind speeds at low latitudes (2004 and 2005). Through mainly statistical analysis of the results from these four years worth of Ulysses data, we have obtained the following findings: (1) Alfvénic structures occur more frequently at higher latitudes or in high speed solar wind (1994 and 1996). (2) Small-scale flux ropes at lower latitudes tend to align with the nominal Parker spiral direction. (3) The scale sizes of small-scale flux ropes are in the same range for different heliocentric distances. Both scale size and duration distributions seem to obey power laws, similar to the analysis results at 1 astronomical unit (AU). (4) The waiting time distribution (WTD) is fitted well by an exponential function rather than a power law. (5) The power law fitting is applied to the wall-to-wall time distribution with the break point at ∼ 200 min which is 3∼4 times the result at 1 AU.