Observations of Small-Scale Magnetic Flux Ropes Associated With Significant Values of Partial Variance Increment Indices

Abstract

We report recent findings for the magnetic field configurations of small-scale magnetic flux ropes (SFRs) broadly defined and identified by using the Grad-Shafranov-based techniques for in situ measurements via the Parker Solar Probe (PSP), Solar Orbiter (SolO), and two Helios spacecraft. Since the current sheets were found to occur at boundaries of SFRs and/or inside SFRs at 1 AU via the partial variance increment (PVI) and the Grad-Shafranov (GS) reconstruction technique by Pecora et al. (2019), https://doi.org/10.3847/2041-8213/ab32d9, we further examine such a co-existence in this study by assessing the maximum PVI indices within SFR intervals using the above four spacecraft observations throughout the inner heliosphere ($⩽$1 AU). Less than 15% of SFRs have maximum PVI indices exceeding a threshold value of 6 that is used to indicate a current sheet structure. Three representative events are selected to explain the most common situations. (a) Current sheets occur at SFR boundaries and near the center. Each could be a weak switchback feature in the time-series profile of the gradually bipolar magnetic field rotations. (b) An SFR configuration is confirmed by both the measurement of counterstreaming electrons and the GS reconstruction result, despite that a large PVI value occurs near the SFR center which is due to an arbitrary kink instead of a current sheet. (c) A current sheet is falsely identified as an SFR where a significant PVI value ($\sim$ 7) occurs near the center. In the end, we discuss the necessity of using multi-point spacecraft measurements to discern the structures associated with SFRs.